Chicken fillets are not meat. If you're a cat.

I've just had a very near miss with feeding one of my cats.

Mini came to us as a kitten around 2012. Back in those days I fed raw meaty bones and the only food available with bone sizes appropriate for a cat easily available in the UK was chicken wings. During bone growth I wanted to ensure adequate calcium and phosphate and nutritional grade bone meal had gone from routine use to virtually unavailable. I'd not, at that time, delved deep in to the problems of PUFA which are plentiful in the chicken wings.

Over the years she had low grade on-going gingivitis problems, with hindsight probably related to the high linoleic acid content of her diet. As I  came to realise that the omega 6 PUFA are a problem I transitioned most of our other cats to 12% fat beef mince and that's what they all eat nowadays.

Not Mini. She routinely threw up on beef mince but was fine on the chicken wings, so there she stayed.

Then she broke two teeth (on a chicken wing!) and when I did her dentistry it was obvious that her low grade gingivitis was not low grade and that all of her teeth were in a bad way, so I took them all out.

No cat is going to eat raw chicken wings with her gums. I tried her on beef mince again, she threw up again. Eventually I decided to try her on chicken breast meat, well cut up and relatively low in fat so low in PUFA. She liked it, maintained weight and had no suggestion of ammonia toxicity from an almost all protein diet. She's a cat after all.

Then last November she hid up under one of the children's beds and when I managed to get her out she was very distressed and had laboured breathing. A quick trip to the practice and 30 seconds of ultrasound by someone better at it than me confirmed heart failure.

Heart failure in cats is common. It's usually hypertrophic cardiomyopathy and is essentially untreatable. I assumed that if HCM was present then high protein + high glucose (cats really do convert protein to glucose continuously) -> high GH -> IGF-1 -> cardiac hypertrophy. Probably wrong but who knows? My colleague of the ultrasound machine was uncertain if she could see hypertrophic or dilated cardiomyopathy so I booked a scan with a cardiologist. A few frusemide tablets stopped her drowning and she had no suggestion of a thrombus or pre-thrombus in either atrium.

It was dilated cardiomyopathy. Or end stage hypertrophic cardiomyopathy where replacement of cardiac myocytes with fibrous tissue looks, on ultrasound, just like the dilated form.

DCM is genetic or caused by taurine deficiency. So I PubMed-ed:

The taurine content of common foodstuffs

and found this:

Poultry breast meat is special when compared to poultry leg/wing meat. It has the lowest taurine content of any meat. If you feed your cat on chicken fillets she will develop dilated cardiomyopathy. If the cardiomyopathy allows the generation of an atrial thrombus which breaks off the syndrome produced (iliac thrombosis) is terminal, whatever hope your vet might cautiously suggest re "treatment".

On the plus side DCM from taurine deficiency is completely reversible. Mini was re-scanned recently by the same cardiologist and now has a normal heart. She has been off of frusemide for months.

Taurine comes by the kilo as a sports supplement. I put a pinch of it on her food each night.

Due to my initial thoughts before getting the diagnosis I'd changed her on to beef mince yet again and was going to put up with paddling in cat vomit on the kitchen floor occasionally. Didn't happen. No more vomiting on beef. Huh?

So now all of our cats are on beef mince.

Moral of the story:

Don't feed you cat on chicken fillets.

That was a very, very near miss and (another) lesson to me that I do not have all of the answers! Occasionally reality bites you.

Peter

Addendum. There is a similar known problem with feeding whole ground rabbit carcasses to cats (of which I was aware, but had missed the selective taurine deficiency in white poultry meat):

Rabbit Carcasses for Use in Feline Diets: Amino Acid Concentrations in Fresh and Frozen Carcasses With and Without Gastrointestinal Tracts

Fructose (10) A can of cola

At some stage we have to transition from "fructose is good, it increases glycogen storage in the liver" through "if fructose is storing glycogen in the liver you can be damned sure it's storing lipid" through to "it causes hepatic insulin resistance" and the follow-ons from there.

This next study is pretty well supported by an established base of in-vivo and end-of-vivo studies which I think I can safely pass by and look at the induction in changes to insulin signalling, which might be interesting

Fructose Selectively Modulates c-jun N-Terminal Kinase Activity and Insulin Signaling in Rat Primary Hepatocytes

We're looking at this:

Aside: c-jun N-terminal kinase is what we expect to kill cells severely injured by being cultured in "fasting" levels of unadulterated palmitate plus glucose at 25mmol/l for 24 hours. You know the studies. Assume intolerable ROS. End aside.

The basic message from the paper is this:

The interesting parts of the paper allow us to ask hepatocytes very, very carefully, about the level of fructose exposure which inhibits insulin signalling. We can accept JNK activation as a crucial messaging step in converting fructose exposure to insulin resistance. The concept that this might be ROS driven is my own rather than anything in the paper per se.

This is the bar chart:

This model used steady state fructose exposure over four hours and is quite convincing that there is a "switch" somewhere between 0.4mmol/l and 0.6mmol/l. Our previous steady state study was that of dogs using fructose at 2.22 micromol/kg/min and this achieved a portal vein fructose of 0.1mmol/l:

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog

We can combine the data from both studies and suggest that, in summary, exposing hepatocytes to fructose a 0.1mmol/l is insulin signal augmenting and exposing them to fructose at 0.6mmol/l induces insulin resistance.

It is also perfectly reasonable to assume that the level of ROS which indicate that is a necessary time to induce insulin resistance are converted to a signal to be carried by the JNK pathway, exactly the one which carries the mitochondrial ROS indicator that it is necessary to induce insulin resistance.

Whether the pre-emptive signal generated by palmitic acid even at low delta psi is the same one as is generated by simple caloric overload (ie failure to resist insulin in time, ie linoleic acid) remains to be seen (by me at least, so far).

We can summarise that nibbling an apple might augment storage of a bowl of porridge as hepatic glycogen but downing two cans of fructose sweetened soft drink might do other things, not least of which is to induce hepatic insulin resistance.

Looking at things fundamentally, this is a story told by ROS signalling. All the signals downstream are certainly interesting and complex but tell us little about the underlying essential process, the information derived from which they are carrying and refining.

Peter

Fructose (09) The Surwit hepatocyte

I hope everyone has forgotten this diagram

which I simplified to this:

Well, now it's time to butcher it further, to an even simpler diagram:

And now I can get rid of the background faint image and shift things around a little to make some more space. I've also converted all "carbohydrate" pathways to blue.

which leaves room to add in mitochondrial, saturated fat derived ROS, the physiological antagonist to the ROS signal which we name as the "insulin" response, though insulin is but a partial contributor to the genuine ROS signal. We can show the blockade like this:

All very simple. Now lets look at the Surwit diet, 59% fat calories, mostly coconut oil, very low PUFA (~2% LA) and modest fructose (~6% of calories).

Aside. Oooh, look, the Surwit diet provides something very close to the 5% of calories as fructose which was used to augment glycogen formation in dogs. Neat. End aside.

I was going to get in to a deep morass at this point about why MCTs fail to generate an ROS signal in proportion to their chain length and degree of saturation. The aside became progressively larger and, not unexpectedly, more theoretical. For the sake of the discussion of the Surwit diet we just have to accept that coconut oil contains fatty acids which are dealt with differently to longer chain fatty acids and which generate a limited ROS signal.

So let's go back to the Surwit diet, 59% fat, mostly coconut, 13% sucrose, some maltodextrin and some casein.

First, at 6% fructose this will produce an insulin-augmenting level of ROS generation, solid blue arrow below. Next is the glucose from the sucrose and maltodextrin, generating a fairly low level of ROS, mostly via the insulin receptor, shown as a thin arrow because this diet is almost a low carbohydrate diet. Both the above generate ROS which signal "insulin" activation "downstream".

Next let's add in octanoate. For whatever reason this is a poor generator of ROS under physiological levels of exposure. It will do nothing to inhibit the "insulin" effector actions of the carbohydrate generated ROS:

I suppose you could even make a case that the relatively minor production of ROS from octanoate might actually produce an activation signal for the "insulin" effect. A possible explanation for this paper. Then you're really in trouble.

Or at least your liver is.

During the earlier posts on this thread about the actions of fructose I've cited papers which suggest that fructose derived ROS augment the formation of glycogen within hepatocytes. Very clever people are very welcome to look at which substrates change in which direction activating which enzyme pathways to generate this glycogen. It's complex.

To me it's much simpler. Augmented low level ROS -> "do what insulin does". One effect being glycogen accumulation.

In a Surwit type diet the directly supplied fatty acids are heavily slanted towards medium chain fatty acids. Mammals do not use MCTs for bulk caloric storage, the preference is toward a mix of palmitate +/- oleate. The liver is quite capable of converting caprylate to palmitate +/- oleate. In fact MCTs are segregated away from chylomicrons by the enterocytes in the gut and are diverted, as free fatty acids, to the portal vein and so directly to the liver for this to happen.

Given augmented hepatocyte insulin signalling combined with augmented access to free fatty acids, what is the likely effect of augmented "insulin cascade activating" ROS levels?

Could that be the accumulation of lipid in the cells subject to this combination of circumstances?

We call this fatty liver.

That's the first step of several.

Peter

Fructose (08) Acipimox and FFAs

Just a tidy up post. I got bored with acipimox. This is why inhibiting lipolysis using acipimox doesn't make you fat.

Different acute and chronic effects of acipimox treatment on glucose and lipid metabolism in patients with type 2 diabetes

I've taken this from the top left panel of Fig 1 and removed the "day three" line so we just have the pre acipimox treatment solid line and the 28 day dashed line. I've added in red arrows to mark the acipimox dose times. Across the bottom are clock times for 24 hours:

It should come as no surprise that the AUC for FFAs on day 28 is virtually the same as that for pre treatment.

Whether this is a simple drug withdrawal effect or an effect mediated via increased size of already distended adipocytes ramping up basal lipolysis in the aftermath of being stretched by acipimox is hard to say, but the bottom line is it won't make you fat.

The beauty of linoleic acid as an obesogen is that it's there all the time from diet or released from lipid stores. All it needs is a decent level of carbohydrate induced insulin signalling for it to accentuate and you're away.

Under hypoinsulinaemia linoleic acid becomes (almost) powerless to augment insulin signalling because there's not much insulin there. Hence the efficacy of the low carbohydrate diet.

Should get back to fructose next.

Peter

Fructose (07) Acipimox tangent II

Acipimox is a drug which keeps on giving. Before I get back to fructose thoughts there's this post and maybe another given over to acipimox.

This is the current paper of interest:

Overnight Lowering of Free Fatty Acids With Acipimox Improves Insulin Resistance and Glucose Tolerance in Obese Diabetic and Nondiabetic Subjects

It has good points and bad points. The worst is the fact that the figures are of such low resolution as to be illegible in places, especially numerical scales to graphs.

On the plus side the tables are fine and the data extraordinarily confirmatory to my biases. Today I'm just looking at the "normal" slim people.

Here is the effect of acipimox on HOMA-IR from an on-line calculator using numbers from the slim, non-diabetic group:

Despite these subjects not supposedly having impaired glucose tolerance the HOMA-IR score derived from their mean fasting values is >2.0 suggesting some degree of insulin resistance. It's not easy to be healthy even as a slim 40 year old in Brazil.

This is easily corrected by acipimox giving an HOMA-IR score of 1.13, well under the cut off for IR.

It does this by locking fatty acids in to adipocyte triglyceride droplets and keeping them there. This, fundamentally, is what acipimox does. It locks lipids in to adipocytes.

From the ROS point of view there is then minimal fatty acid oxidation, minimal superoxide produced by reverse electron transfer through complex I, minimal inhibition of the insulin cascade at the level of insulin receptor substrate so maximal insulin signalling. The end result, in the basal state, shows as increased glucose oxidation and decreased fat oxidation:

These changes are absolutely secondary to the suppression of lipolysis by acipimox at the level of the adipocytes.

I hope all of this is starting to sound familiar.

I can't get this table out of my head

taken from here:

Insulin sensitivity is increased and fat oxidation after a high-fat meal is reduced in normal-weight healthy men with strong familial predisposition to overweight

Acipimox is reproducing both the low HOMA-IR score and reduced lipid oxidation seen in slim people who are destined to become obese. Ignore the comment about genetics at the end of the abstract. People with obese parents are going to become obese themselves via the action of linoleic acid locking triglycerides in to adipocytes.

Before obesity develops they have "excellent" insulin sensitivity because they are metabolically hypocaloric due to concurrently starting becoming obese via lipid *loss/sequestration* in to adipocytes. They will have to eat more [carbohydrate] to make up their metabolic needs by however much lipid is sequestered in to their adipocytes. They will only become insulin resistant once those adipocytes become large enough that lipolysis cannot be adequately suppressed by insulin.

Acipimox recapitulates linoleic acid's insulin sensitising and obesogenic effects, but only for 6 hours at a time.

Linoleic acid accumulates in your adipose stores (and deep fat fryer) and is available continuously for years making you hypocaloric [ie hungry], especially when you avoid saturated fat. As cardiologists have advised for decades.

Of course, if you took acipimox every 6 hours for the rest of your life you would get fat, wouldn't you? I only discovered acipimox in pre-Protons days when I worked from the simplistic, partially correct idea that insulin inhibits lipolysis to make you fat. Pubmed "lipolysis" "inhibitor" "obesity" and acipimox pops out.

BTW It doesn't make you fat. Basal lipolysis was another gift of acipimox.

Peter

Fructose (06) Acipimox tangent

I've glibly stated that I consider insulin resistance to be a simple but imperfect adaptation to the inability of insulin to suppress basal lipolysis, that is the rate of FFA release from isolated adipocytes in an ex vivo situation.

One of the best suppressors of basal lipolysis is the nicotinic acid derivative acipimox.

There's a review of our knowledge in 2007 here:

Nicotinic Acid Receptor Subtypes and Their Ligands

which has a nice diagram like this, slightly edited:

The process is quite simple. Acipimox, a nicotinic acid derivative (NA), interacts with a cell surface receptor (here HM74A but it has many, many names) which is coupled to a signaling G protein (G) which inhibits adenyl cyclase (AC), lowering cellular cyclic AMP so deactivating a cAMP dependent protein kinase (PKA) which does something magical to all lipases to shut down lipolysis.

Despite the fact that acipimox can reverse diabetes overnight it has been a clinical complete flop and is nowadays only used to improved a few lab numbers of people eating a diet of complete crap.

I woke up this morning thinking about why "Magic happens" in the above diagram. Not how, but why.

Niacin is a drug acting on the ketone body receptor of adipocytes. The endogenous activator is shown as beta-hydroxy butyrate:

The obvious explanation for ketone bodies inhibiting lipolysis is that this is a negative feedback loop. The original diagram also had FFAs inhibiting lipolysis too.

There are upper limits to the body's tolerance of elevated FFAs and ketosis is a marker that decreasing the FFA supply to the liver might be a good idea.

The most obvious inhibitor of lipolysis via hormone sensitive lipase is insulin. Under ketosis (ignoring MCT exposure) insulin is at absolutely rock bottom levels and if you want something to limit lipolysis under hypoinsulinaemia you had better develop a system which does so independent of insulin's action.

Ketones do that (pax hyperglucagonaemia and ketoacidosis, there are limits).

So if a cell has elevated basal lipolysis (which cannot be shut down by insulin) choosing an alternative mechanism, insulin independent, for suppressing lipolysis is effective stratagem.

Acipimox activates this process in the absence of the normal physiological ligand, ie ketones.

ATGL is key to mediating lipolysis triggered by increased adipocyte lipid droplet size where as elevated insulin exposure cannot do this. You can shut down this elevated basal lipolysis with a "ketone mimetic".

Which reverses type two diabetes. Until the drug wears off.

Peter

Oh, another thought: Is this how exogenous ketones improve insulin sensitivity? Never mind all those complicated intracellular switches (I told you I was lazy). The "problem" is that adipocytes are leaking FFAs. Acipimox fixes this problem. Acipimox is a ketone mimetic. Do exogenous ketones "mimic" acipimox? Amusing thought for the day!

Fructose (05) Four out of five diabetics

This follow on study looking at fructose in people with type 2 diabetes was a bit of a disappointment. 

TLDR: Fructose works pretty well in four out of five people with diabetes just as it does in "normal" people with a relatively poor OGTT result.

Acute Fructose Administration Improves Oral Glucose Tolerance in Adults With Type 2 Diabetes

This is what the title of the study is describing:

It's clear that adding 7.5g of fructose to a 75g OGTT load improved both glucose and insulin curves. There is nothing exciting about this. What I thought might be interesting was that, in a study of five subjects, two of them were outliers of sorts.

I'd hoped these outliers might give more insight in to the state of ROS signalling within hepatocytes of people with severe diabetes. Not really, much of the data you need is not in the paper. For completeness here are the anomalous results:

Subject 2 failed to drop their systemic glucose in response to added fructose, like this

Not unsurprisingly they also failed to drop their insulin level, clearly insulin level follows exposure of the pancreas to systemic glucose, which didn't change, so neither did insulin.

Can we guess what might have been happening in hepatocytes to nullify the "beneficial" effects of fructose? Let's assume that fructose is being absorbed, entering hepatocytes and generating ROS via a NOX enzyme in proportion to the rate of fructose ingress. So the question becomes

"Under what circumstances does a modest increase in ROS fail to activate the insulin cascade?"

We have no idea what the fasting insulin level was, nor the fasting glucose, so this is a little difficult. We are told that subject 2 had an HbA1c of 9.0 or 10.1, one of the highest in the study, which implies the worst average glucose excursion over the last three months or so. We could choose extreme insulin resistance, failing beta cell function or a combination of both to explain this. The fact that they produced a minimal increase in AUC for insulin when presented with 75g of glucose with or without fructose suggests that beta cell function was limited and underlying hyperglycaemia might both be present. I favour this explanation.

Both hyperinsulinaemia -> NOX4 activation via G protein coupled signalling and hyperglycaemia -> NOX2 activation via calmodulin kinase signalling have the potential to spill ROS over from activating via phosphatase inhibition to being inhibitory via insulin receptor substrate inhibition. Adding a small increase in ROS from fructose to a maximally stimulated system may affect both aspects, producing no net change. That's my guess.

The second unusual result was from subject 5. They produced a perfectly reasonable drop in systemic glucose in response to fructose addition but had an unusual increased insulin response despite reduced systemic glucose.

I struggle to explain this "paradoxical" rise in insulin despite a successful fructose induced fall in pancreatic exposure to systemic glucose. You could argue that the increase in insulin came first and this lowered the systemic glucose level but this would make quite an exception and need either a pancreatic response to a very small systemic fructose rise or a pancreatic effect derived from gut hormone signals, none of which were measured and none of which are needed for an explanation of the fructose effect in all other subjects.

If I had to guess I would suggest the fasting insulin level was very high in this person. We know from this 2011 study that OGTTs are reproducible on repeat testing (in "normal" people) over 3 days unless you have to be hyperinsulinaemic in order to be "normal":

Reproducibility of multiple repeated oral glucose tolerance tests

"However, our cases of individuals who exhibited hyperinsulinaemia in order to maintain glucose homeostasis, suggest that repeated OGTT’s may not produce a reliable estimation of insulin sensitivity for people with pre-diabetes and diabetes."

Subject 5 was also an individual with very high HbA1c (9.0 or 10.1) suggesting poor control and compatible with high insulin levels. So the insulin anomaly may just be random finding in one person secondary to chronic hyperinsulinaemia... We'll never know.

Overall the fructose effect on hepatic glucose output seems to be genuinely maintained in patients with DMT2 unless they are approaching seriously poor levels of control.

Should individuals with more "mild" diabetes add a little fructose to each bowl of porridge they consume in real life?

Rhetorical question.

Peter

Fructose (04) Normal Adults

Now it's time to think about this paper

Acute Fructose Administration Decreases the Glycemic Response to an Oral Glucose Tolerance Test in Normal Adults

These are the results from an OGTT on a set of healthy human volunteers who took 75g of glucose with or without 7.5g of fructose. From the top graph you can see the results are more than a little inconclusive. If you instead plot change from baseline you get the slightly more convincing pair of curves below and if you compare the areas under the curves for the second plot you get p less than 0.05.

So fructose addition"almost" or "just" works. I think it was this set of findings which made Cherrington's group go for the canine study with a continuous duodenal infusion of glucose with or without extremely low dose fructose inclusion. So much more control and stability, over hours, than a 75g OGTT in a human where an awful lot of changes occur in the first 30 minutes.

Never the less some very interesting findings did come out of the human study.

As the authors noted, and published, some "normal" people have better OGTT results than others. And they also noted that, if you have a relatively poor OGTT result, you respond well to fructose. If you have a good OGTT result you don't respond to supplemental fructose at all. There is a correlation. It looks like this:

Okay, along the x axis we have a measure of how "good" an individual's OGTT result was. Low values are best, higher values are poorest. The 

y axis indicates how much improvement in OGTT occurred with adding 10% fructose. There is a clear cut cluster at the left hand end. These individual people have the best normal OGTT results and do not respond to fructose supplementation, at all:

At the right hand area of the plot we have people who are trending towards impaired glucose tolerance by having a poor OGTT result and these clearly have a nice response to supplementary fructose:

We can then take those five people at the bottom left of the correlation plot and make a plot of their OGTT results with and without fructose and it comes out like this (using change from baseline glucose, not the absolute values, of course):

I think this is clear cut, if your OGTT only spikes your BG by 4.0mmol/l there is no benefit from adding fructose.

Next these are the six individuals for whom the straight glucose OGTT spiked their blood glucose by 5.0mmol/l:

Very clearly these six people benefit from added fructose.

If you have poor glucose tolerance you benefit from adding fructose. You become almost normal!

How come?

You could, if you wanted, take a diagram like this

which I found in this comprehensive review:

Fructose metabolism in humans – what isotopic tracer studies tell us

(as a link kindly supplied by Jaromir in previous comments.) and try to work out exactly what nudging 75g of glucose metabolism by the co-adminstration of 7.5g of fructose might do to OGTT results in a real live human being. Sadly this is utterly beyond my abilities so I have to fall back on my own trusty crutch, the very simple ideas of ROS and what is meant by impaired glucose tolerance.

The last post was a summary of what I think the role of ROS from differing sources have on insulin signalling. In particular this concept matters:

Insulin acts (among many other places) far, far away from the liver, on adipocytes to suppress lipolysis when there is a copious supply of glucose/insulin available. My definition of impaired glucose tolerance is the inability of adipocytes to limit their release of FFAs to the circulation under the influence of this insulin. In the above doodle insulin shuts down FFAs to 50micromol/l. That's normal. Now let's repeat this illustration with a residual supply of FFAs at around 200micromol/l under an OGTT.

Insulin still docks with the insulin receptor, NOX4 still produces activating ROS but mitochondrial FFA oxidation is producing some degree of partial blockade of the insulin cascade at the insulin receptor substrate level:

The solution to this problem is simple, hyperinsulinaemia. This will not reduce the FFA levels from high basal lipolysis but will allow a "forced" increase in activating ROS and so increase phosphatase inhibition and so increase the insulin cascade to overcome the partial "obstruction" at the IRS level, like this:

Let's be absolutely clear. The hyperinsulinaemia is a sticking plaster placed on to the excess basal lipolysis secondary to adipocyte distension (linoleic acid derived). It is NOT a cure, it's a bodge.

The above is what I consider the metabolic explanation of people with poor glucose tolerance under an ordinary OGTT. The message is that the hyperinsulinaemia is just a tool to increase NOX4 derived ROS to a high enough level to allow the cell to overcome the inhibitory effect of excess FFA oxidation which should not be there.

The extra ROS are what makes this happens and the cost is hyperinsulinaemia.

Now lets add in some fructose.

The fructose (all in green) enters the cell rapidly and talks to its own NOX enzyme generating its own extracellular superoxide.  This dismutates and re enters the cell at the location where ROS are used to activate insulin signalling by disabling inhibitory phosphatases. So fructose derived ROS accentuate the insulin cascade to restore insulin signalling, much as hyperinsulinaemia would do, but without the hyperinsulinaemia.

Again, the signal is the ROS.

Again, it's not a cure, it's a sticking plaster. We are simply using an alternative source of supplementary ROS to activate the insulin cascade. I've changed this last feature to green as well, just to signify it's being helped by fructose derived ROS. I don't mean to suggest this is fructose catabolism, just fructose-ROS facilitated pAKT activation et sequitur.

Activating the insulin cascade in hepatocytes without any action anywhere else will lower glucose penetration to the systemic circulation and normalise the OGTT result.

Recall there is a specific problem causing the poor OGTT result and we have not fixed this problem. Things look better but the abnormal basal lipolysis derived FFAs are still there.

In people with an excellent OGTT result the insulin cascade pretty well fully activates and adding a few extra ROS seems to make little difference in this model. Of course we know that there are changes in the liver of very normal individuals secondary to low dose fructose because we already know the results of the subsequent canine studies.

However two of the normal people had a worsening of their OGTT area under the curve, one by quite a lot, clearly visible circled in red here on the correlation graph:

These two individuals had an excellent OGTT without fructose so we can assume they are perfectly capable of suppressing adipose lipolysis to levels which don't interfere with insulin signalling. You have to ask yourself if it is possible that the ROS generated by fructose and its NOX were sufficiently numerous that they diffused as far inwards from the cell surface as to start to act on the insulin receptor substrate in a mild version of the marked inhibitory effect of FFA oxidation derived ROS. I've put this speculation in as a dotted green line and reduced the overall insulin cascade curving arrow to show some limitation of downstream signalling:

When we come to think about grossly toxic doses of fructose this generation of impaired insulin signalling is likely to be come a primary feature. Perhaps we have a hint here. Just guessing. 

We are not given enough individual subject level data to see what happened to any parameter other than glucose in this study. We only get the whole group aggregate data for most parameters.

I think the concept outlined makes sense and seems quite simple. Fructose feels like it is starting to yield some of its secrets.

Time to look at fructose and type 2 diabetes next.

Peter

Fructose (03) NOX vs RET

This post is not about any specific study. It just gives the background to understand the post after, which is already written but I need to sit on it for 24h and correct the worst of the typos/logic errors. Okay...

Before we can go on to understand the role of fructose as an insulin sensitiser/mimetic I think a little more explanation of NOX enzymes might be in order. So this is a very simple view of NOX and ROS vs reverse electron transfer through complex I and ROS.

So let's begin here again, don't forget this is the cell surface, mitochondria will reappear further down the page:

I want to start with the red square which we can pull out thus:

The basic story is that insulin "talks" to NOX4, activates it and the resulting ROS deactivate a number of (inhibitory) phosphatases, which allows the insulin signalling cascade to take off. That little red arrow needs some elaboration and is an oversimplification.

I have mentioned before that the basic NOX core looks extremely primordial (thought it appears it is actually an eukaryotic invention) and it is better represented like this, taken from here:

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

The NADPH binds to a protein containing an FAD moiety which accepts a pair of electrons to form FADH2, temporarily. This seems to be a "bolt-on" NADPH oxidase which can supply high energy electrons to any process you care to bolt the oxidase on to. Here it feeds electrons to a "wire" to the outside of the cell. The six transmembrane helices form a tunnel containing two haem iron groups which are the "wire". One electron at a time travels from FADH2 "down hill" towards the exterior of the cell. But to fully traverse the cell membrane it is travelling against the cell membrane voltage and can only make it if oxygen is docked on to the outer end of the "wire" as terminal acceptor, like this:

Obviously superoxide, as a charged particle, is not going to re-enter the cell. Some NOX members have another "bolted-on" sub-unit which means the NOX produces H2O2 rather than superoxide, which has a much better chance of entering the cell. There is also extracellular SOD3 which can dismutate superoxide to H2O2 to re-enter the cell.

So we can now look at the activation of insulin signalling again but with a modified NOX doodle:

There might be quite a lot of arrows but the set-up isn't really that complicated. The core message is that the ROS which activate the insulin cascade are of extracellular origin. We will see later that they can influence more distant intracellular sites but I want to stay with simple activating physiology of the insulin cascade here for today. As far as insulin signal activation the triggering ROS are of extracellular origin. Hence the location of the "ROS" labels in the initial image from which the above doodle is derived.

Next we have to think of fasting. Here's a view of ROS signalling under fasting conditions. FFAs can be between 1000 and 3000micromol/l in healthy people under an extended fast. To survive under these conditions it is helpful to reduce both the absolute level of insulin and the level of signalling in the insulin cascade from whatever insulin is present. This is how it works:

The oxidation of FFAs will generate ROS by reverse electron transfer through complex I irrespective of mitochondrial membrane potential (as long as this is within physiological limits) and will be a primary mechanism for resisting glucose usage wherever FFAs can substitute to allow glucose sparing for tissues where glucose is an essential requirement. Of course long chain saturated fats do this best but all mitochondrially targeted fats do it to some degree.

Before I pause to make some more doodles on more interesting subjects we just have to add in how FFA supply is regulated. At the start of an OGTT glucose is rapidly absorbed and penetrates to the systemic circulation and stimulates insulin secretion (pax GLP-1 and related overlays to this system) from the pancreas. In healthy people this insulin might drop FFAs as low as 50micromol/l:

This has nothing to do with an individual cell, it's a distant effect of insulin acting on adipocytes to suppress lipolysis and so suppress FFA supply to the whole body. Which will markedly reduce mitochondrially generated ROS. Any residual mitochondrial FFA derived ROS might even be at insulin facilitating levels:

Ultimately the above doodles describe models, which are simple extremes and only distantly relevant to the integrated performance of the complexities of glucose and fatty acid derived energy production and ROS control. But they form a reasonable framework to go on to explore the findings in Cherrington's human OGTT studies.

TLDR

Insulin -> NOX -> cell surface low ROS -> activating

FFAs -> RET -> mitochondrial high ROS -> inhibiting

So these are next, normal adults first:

Acute Fructose Administration Decreases the Glycemic Response to an Oral Glucose Tolerance Test in Normal Adults

Acute fructose administration improves oral glucose tolerance in adults with type 2 diabetes

Peter

Fructose (02) Obesogen

I guess we could do worse than to being this post here;

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog 2002

It's a follow on study after an initial dog study from 1998 which demonstrated that, much like glucose, in a tightly controlled somatostatin/insulin/glucagon model, low dose fructose markedly suppresses hepatic glucose output and markedly increases glycogen formation in healthy dogs. On a fixed insulin background, low dose fructose behaves as an insulin mimetic.

This follow on study in 2002 used hormonally intact dogs instrumented for portal vein, hepatic vein, femoral vein and femoral artery access. In addition they were cannulated to allow intra-duodenal infusion of glucose or glucose + fructose. This gets rid of all of the pesky delay in gastric emptying involved in an OGTT and allows time to assess effects under steady state conditions rather than the brief and dynamic changes which occur under an OGTT. Between the first and second canine studies the group had performed a couple of sets of human OGTT studies +/- fructose with somewhat challenging results, which I suspect is what led to this latest canine study, to really control as many variables as practical.

Aside: There is a horrible typo in the plasma glucose graphs throughout the study. For glucose they clearly mean milli moles/l, not micro moles/l. Arghhh. I think the fructose at 100micromoles/l is likely to be correct. I can't face trying to confirm whether their infusion rates are correct re micro vs milli. Ugh. I think their data are fine per se, unfortunate re typos. End aside.

The intra-duodenal glucose infusion was pitched to generate a modest, stable hyperglycaemia at ~10mmol/l. In the intervention section this glucose infusion remained unchanged at 44.4μmol/kg/min but an additional infusion of fructose was added at 2.2μmol/kg/min, ie around 5% fructose in addition to the original glucose. This is what happened to the systemic plasma glucose levels:

If anyone had developed a drug to produce this effect in diabetic patients it would be impressive. But that's not all. Look at what insulin levels were needed to achieve that drop in blood glucose:

Adding in 2.2μmol/kg/min of fructose to an hyperglycaemic glucose infusion profoundly lowers the insulin levels needed to maintain a modestly reduced hyperglycaemia value. In a hormonally intact dog preparation.

That's amazing if correct. Fructose appears to be profoundly insulin "sensitising" or mimicking.

Almost nothing is being "done" with the fructose. It enters the liver, drops though fructolysis and is excreted largely as lactate, well in excess of that produced by the glucose alone. I hope we're all aware that fructolysis bypasses those regulatory steps which make glycolysis a tightly regulated process. Here's the lactate change:

Next we can quickly doodle in the fructolysis pathway taken from here

Fructose Metabolism in Cancer

which has this as a generic cell capable of metabolising fructose. In the liver the various GLUTs will be different but the same principle applies:

We can add in fructolysis with its unregulated, high flow pathway to lactate as typically seen in our current discussion of hepatocytes. Like this:

Equally we can put in the tightly regulated glycolysis route to either ox phos and/or lactate, usually a bit of both. But regulated by pH, citrate, ATP or oxygen availability.

We can put both pathways in together and throw in the insulin receptor too:

We are now in a position to add in the core features to an understanding of what is going on. Like this:

and then we can fade the background to let us concentrate on the generation of ROS:

Here we have the situation under intra-duodenal glucose infusion alone. ROS are being generated by glucose ingress per se using NOX2 and also by insulin docking with the insulin receptor to activate NOX4. Both insulin level and the entrance of glucose in to cells without insulin are tightly controlled.

Neither glucose nor insulin are the signal to activate the insulin signalling cascade. That is purely the prerogative of ROS, at physiologically appropriate levels. The end result is a glucose value of around 10mmol/l and an insulin value at around 250pmol/l.

This is what happens if we add 2.2μmol/kg/min of fructose to the glucose infusion. We achieve a portal vein fructose concentration of 100μmol/l which will generate significant ROS, in direct proportion to the rate of entry of fructose to the cell, which will be high:

The fructose generated ROS will increase the ROS signal. This is the activator of the insulin cascade. The ROS signal will result in the phosphorylation of AKT, the activation of glycogen synthesis and will decrease the penetration of glucose past the liver and in to the systemic circulation. Systemic glucose will fall, less pancreatic secretion of insulin will be needed.

We get what we see in the study.

My personal definition of an obesogenic drug is one which activates "insulin" signalling in the absence of insulin. Fructose generates "unexpected" ROS when entering liver cells at a concentration of 100μmol/l. I think it is perfectly likely that, should one achieve an adipocyte exposure of 100μmol/l of fructose, that a similar effect might occur.

In Laughlin's 2014 review

Normal Roles for Dietary Fructose in Carbohydrate Metabolism

there is the suggestion that peak systemic fructose concentrations healthy men after a high sugar exposure might be about 0.5mmol/l, ie 500μmol/l and a fasting level might be a fraction of a μmol/l.

There are a number of studies showing fasting fructose levels as high as 2.0mmol/l (ie 100 times Laughlin's accepted level) with post fructose loading reaching over 17.0mmol/l. That makes life easy for everyone. If you want to show fructose is harmless or beneficial you chose the low studies. If you want toxicities you choose the high studies.

But are low doses of fructose beneficial at all? Is insulin cascade activation good or bad? Are the benefits of reduced absolute insulin exposure offset by the ROS mediated activation of insulin cascade?

Whichever fructose measurement camp you fall in to, a normal human being can easily maintain systemic fructose levels > 100μmol/l in the aftermath of even a modest fructose load, which in the dog studies I consider generates enough ROS to be markedly facilitating of what we could call an insulin sensitising or mimetic exposure. Perhaps we should apply this to adipocytes.

Does fructose make you insulin sensitive? Of course it does. Does activating insulin signalling make you fat? Of course it does.

Except, of course, when it does the opposite.

Peter