It was in late 2007 that I first blogged about the concept of adipocyte insulin resistance and of course it is back in my mind while I work through ideas on metabolic flexibility and insulin resistance in general. It is a very simple concept that the fatter adipocytes become (using whatever delivery system you like, ASP if you must) the harder it becomes to push more fat in to them. And certainly the harder it becomes to keep it there once it is installed. So this idea of adipocyte insulin resistance limiting fat gain is very intuitive and probably correct. How big adipocytes can get is probably determined by how strong your pancreas is combined with how responsive your adipocytes are to insulin as they swell. A pancreas of steel and relatively insulin-resistance resistant (no typo) adipocytes combine to get you to the over 200kg mark. This came up in comments on the last post. Is this true?
A rather nice paper was published back in the 1960s showing this very clearly. I have seen it cited as purporting to show that elevated fasting insulin is a consequence of obesity, rather than a cause. This is a fascinating and rather counter intuitive concept, so you just have to go have a look see at the paper. Luckily it's free access.
It does show, very convincingly, that adipocyte size correlates with adipocyte insulin resistance on the adipocyte cellular level. I rather like that.
It also demonstrates quite clearly that forced, brutal adipocyte size reduction by a couple of months on a 600kcal/d diet improves adipocyte insulin sensitivity as adipocyte size shrinks.
There are two core concepts which need to be taken away from this paper.
The first is that as adipocytes swell they become progressively less able to respond to insulin. This obviously translates in to insulin resistance of adipocytes ultimately limiting fat gain within the limits of the pancreas to secrete or hypersecrete insulin. That is if you accept that insulin is in any way involved in fat storage.
Now. What does this mean for the carbohydrate hypothesis of fat gain?
It is the RESISTANCE of adipocytes to insulin which limits fat gain.
And the corollary is??? Sensitivity to insulin drives fat gain. You can't have one conclusion without the other.
Anyone telling you that adipocyte insulin resistance limits fat gain and yet insulin per se has nothing to do with fat gain... Well, you decide. I have.
Although the group measured many, many things the only information we get about fasting insulin levels and post challenge insulin levels are these five paired graphs:
There is nothing in the text or tables giving any numeric data about insulin levels in obese individuals and no details at all from the normal groups. I don't mind this too much as the study was really aimed at adipocyte size and adipocyte glucose metabolism in response to exogenous insulin. This was the main drive of the paper. Note that they didn't look at adipocyte beta oxidation, no one had any idea this might be compromised back in the 1960s, so we get no idea about the ability of adipocytes to carry out this essential function.
Look, fasting insulin in five obese people is not generally elevated, it's reported as being only slightly elevated in two out of the five obese patients. This obviously implies that elevated fasting insulin does not predict weight gain. There we go. Time to pack up and go home.
Ah yes, but which fasting insulin are we looking at? Remember that group of starved obese folk we chatted about previously who had three different fasting insulin levels? One level on their normal (obesogenic) diet, one on a calorie and carbohydrate limited diet and another on the full starvation non-diet (ie complete carbohydrate restriction): 45 or 38 or 15-20 microIU/ml.
In obese people (but not in people who have normal metabolic flexibility) you can simply dial fasting insulin by carbohydrate intake. The question we cannot answer from Hirsch's study is what the fasting (and the 24h AUC) insulin values were for the five obese participants while they were free living on their normal obesogenic (high carbohydrate, you can bet) diet and slowly gaining weight? Remember we only need an average of 5g/d adipose tissue accumulation for long term obesity.
We are given an insulin value during phase I on a weight stability diet with a carbohydrate intake fixed at 45% of not-quite-enough-for-comfort calories. This is not what a given individual would normally choose to eat. In real life these people would not be on a weight stable diet. They certainly would not have been limiting their carbohydrate to 45% of calories. So we have no idea what their fasting insulin level would have been before stabilisation on phase I, but is certainly going to have been higher than the graphs show.
After massive weight loss during phase II of the study (on 600kcal/d for several months, probably only bearable because carbohydrate was limited to around 50-55g/d and the doors were locked [jk!]) we go in to phase III and get our second set of curves. Here we are now maintaining weight stability at a markedly reduced body weight with a smaller portion size of a still 45% carbohydrate diet, so total carbohydrate intake will be a bit lower. Hence the slightly reduced fasting insulin... But of course none of this represents the life which led to the enrolment in the study.
Subjects will be hungry.
While ever they stay hungry and limit carbohydrate to 45% of their never-quite-enough calorie intake, their insulin levels will stay low and they will, hungrily, stay slim.
Four of the five patients managed this for quite some time. Kudos to them and their willpower. You have to wonder about the fifth patient. Lost to follow up? Not lost to follow up but fatter than pre study? Just got fed up with people sticking needles in their butt?
How effective for long term weight control is chronic caloric restriction? Answers on a postage stamp to...
Are these people fixed? Their adipocytes certainly have scope to respond better to insulin and will inhibit lipolysis more effectively than during obesity. This limits FFA leakage due to insulin resistance which decreases FFA delivery to muscles and so allows muscles to take up glucose better, so both glucose and insulin curves improve. But are they really, really fixed? Will they will simply regain their lost weight, unless they enjoy being hungry all the time? Especially if they increase their total carbohydrate intake? And why are they hungry? Another post in this series there.
Addendum: Running through the methods section of Petersen's paper it is actually worth noting that fasting insulin and simple derivatives of fasting insulin plus glucose, such as the HOMA score, are rather blunt instruments for picking up insulin resistance. The more complex insulin sensitivity index is better but even this failed to pick out two out of twelve apparently insulin sensitive participants who turned out to be insulin resistant on the hyperinsulinaemic clamp, the current gold standard for picking out insulin resistant subjects. So, while insulin resistance is core, simple fasting insulin has to be accepted as a blunt instrument. Clamps, unfortunately, are not simple to perform. End addendum.
Of course you cannot dial fasting insulin by carbohydrate intake in normal individuals. So all you have to do is include enough normal people in your longditudinal studies and there will be no significant correlation between fasting insulin and subsequent weight gain. What would you expect?
Anyhoo, back to adipocyte insulin resistance. Stretching adipocytes appears to have effects on their sensitivity to insulin. As adipocytes stretch this translates in to progressive pathology as the adipocytes are running out of their ability to function normally. As they get fatter they leak more FFAs at a given level of insulin. This is important. Very important.
Before we go on to the next post: Is there any other form of adipocyte insulin resistance, other than that due to fat distension?
I rather like physiological insulin resistance. It keeps me alive. Simple carbohydrate restriction or a couple of days of frank starvation produces whole body insulin resistance to spare glucose for brain use. You know what I mean. Take a young fit healthy human and starve him for three days and he will immediately become intensely insulin resistant on a whole body basis. If not he would become intensely dead. Are adipocytes part of this physiological insulin resistance response, in the same way as muscle cells are?
We get a partial answer to this when Hirsch cites Tucker's study and suggests that the reason she found no difference between the adipocytes of obese and slim rats was because both were maximally insulin resistant after a 20 hour fast, even those from skinny rats...
"However, these studies were performed upon tissue from animals fasted for 20 hr, a manipulation known to decrease the insulin response of adipose tissue in vitro."
Ad hoc number 3523, but highly plausible. Every body knows this... Physiological insulin resistance mimics pathological insulin resistance. The mechanism through FFAs is likely to be the same.
This would again be logical as you do not want rats in starvation hanging on to their adipocyte energy stores or to be allowing precious glucose in to adipocytes (however little glucose adipocytes use) and so allowing it to be "wasted" when needed by the brain.
Is there a third factor affecting adipocyte insulin sensitivity?
Well, of course adipocytes have mitochondria. Are they breakable? Probably.
If you break them I would assume that they behave much like those in muscle tissue and they do the best they can with pyruvate while leaving the FFA derivatives in the cytosol, ie adipocytes should become insulin resistant if they have broken mitochondria. But this insulin resistance is not stretch related and it's not physiological. It's a mitochondrial break and could happen at any stage of distension of adipocytes. So mitochondrial failure should lead to adipocytes leaking FFAs when glucose and insulin are elevated. Possibly at minimal distension size, ie while you are still slim.
This would worsen whatever state of insulin resistance the muscles were in from their own mitochondrial problems. If the pancreas is not up to overcoming the supplementary FFA-induced insulin resistance (due to its own mitochondrial problems as suggested by Petersen et al) then hyperglycaemia will result and you get that label of T2DM... Possibly while still slim.
The plateau in your weight here might be mistakenly attributed to the satiating effects of insulin on your brain finally kicking in, somewhat belatedly, after 50 years or so of hunger.
If you have an unbroken pancreas of steel you can still argue with the broken adipocyte mitochondria and you can still get even fatter. Ditto if you have T2DM due to insulin resistance and some joker gives you a bottle of injectable insulin plus some syringes. Especially if they also tell you to eat a ton of bagels and cover the hyperglycaemia with a ton of exogenous insulin. And chide you for overeating.
Summary: Adipocytes become fatter under the influence of insulin. Resistance to insulin by adipocytes limits fat storage and hence eventually limits weight gain. It also elevates FFA supply. Important.