I have to say that I have enormous respect for Tucker Goodrich's self restraint in not commenting further on the single largest confounder in the latest Hall study:
Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake
which starts its abstract with:
"The carbohydrate–insulin model of obesity posits that high-carbohydrate diets lead to excess insulin secretion, thereby promoting fat accumulation and increasing energy intake."
If the simplest CIM model of obesity was correct then eating a diet which raised insulin, like this:
to a peak of 120microU/ml and kept it elevated for three hours, should result in marked sequestration of lipid in to adipocytes with subsequent weight gain.
The question is how this occurs.
Over the years I have had a couple of attempts to understand "carbosis", which this low fat group appears to be demonstrating. None of my attempts have been particularly satisfying from the metabolic point of view. Let's try again.
In the immediate aftermath of the low fat test meal glycolysis is very active. This is facilitated by the elevated insulin translocating GLUT4s to the cell surface, which will facilitate the ingress of glucose.
Insulin will also facilitate the ingress of FFAs via CD36 translocation to the cell membrane but, at the same time, insulin will simultaneously lower FFAs:
With plasma FFAs at around 0.1mM it doesn't matter very much how many CD36 receptors are present on the cell surface, fatty acid oxidation will be a limited source of both NADH and FADH2 supply to the electron transport chain.
In addition to facilitating glucose ingress in to cells, insulin also drives the activation of the pyruvate dehydrogenase complex. This increases the translocation of pyruvate via its proton gradient coupled transporter in to mitochondria and ensures its metabolism to acetyl-CoA.
Glycolysis to pyruvate generates two molecules of cytoplasmic NADH for each molecule of glucose utilised. Much of this NADH will be reconverted to NAD+ by the malate-aspartate shuttle, passing the electrons to generate NADH within the mitochondria. If the rate of generation of NADH exceeds the capacity of the malate-aspartate shuttle, cytoplasmic NADH levels will rise and, secondary to this, the level of lactate will rise to resupply NAD+, using lactate dehydrogenase.
We can get an idea of how much the cytoplasmic NADH levels rise from the output of lactate in to the bloodstream of the study subjects after the low fat test meal. Like this:
I find a post prandial lactate of just under 3mM quite impressive. I would suggest that the NADH:NAD+ ratio is high.
So the next question is, what might an elevated cytoplasmic NADH level do to the glycerophosphate shuttle?
If we assume that high cytoplasmic NADH activates the glycerophosphate shuttle we will have the transfer of electrons from cytoplasmic NADH to the intra-enzymic FAD of the mitochondrial component of the glycerophosphate shuttle, mitochondrial glycerol-3-phosphate dehydrogenase.
From here the input is from the outer surface of the inner mitochondrial membrane directly on to the CoQ couple. From the mitochondrial point of view, that cytoplasmic NADH never arrives (it should have entered using the malate-aspartate shuttle). Instead it is "seen" as an FADH2 input, with all of the implications that has related to the FADH2:NADH ratio intrinsic to the Protons thread.
Under the influence of insulin there is clearly a great deal of cytoplasmic NADH generated from glycolysis and it is this action of insulin which must be limited to avoid excessive caloric ingress.
With glucose at 120mg/dl (sorry for the quaint units):
and insulin at 120microU/ml there are lots of calories entering cells. With FFAs at or below 0.1mM they will not be a significant source of either NADH or FADH2.
Fatty acids are out of the equation. So the signal for cellular satiety, driven by ROS, is going to come from the rising FADH2:NADH ratio generated by the glycerophosphate shuttle converting NADH to FADH2.
Ultimately the sensing of a cellular "satiety" level of substrate ingress will be signalled the generation of high-physiological levels of superoxide and hydrogen peroxide, facilitated by the glycerophosphate shuttle. I won't mention negative feedback from complex III but it will contribute too.
What will not be involved to any significant degree is beta oxidation. FFAs are low at the time of peak calorie availability/storage. And of those FFAs there will be very, very little linoleic acid.
A very low fat diet side steps the problems caused by linoleic acid failing to allow satiety-facilitating levels of ROS to be generated. Linoleic acid is simply out of the equation, what little there is of it from the diet being squirrelled away in adipocytes during the period of peak calorie availability. It is simply not there to interfere when cellular satiety is being successfully signalled.
Couple that with the fact that the low fat meal plans provided a linoleic acid supply limited to 3% of calories, it is quite easy to see how a group of 20 slightly chunky young Americans (BMI 27ish, 32% body fat, not the Arnie look for the slightly high BMI!) might be suffering from chronic linoleic acid toxicity. Dropping to 3% energy from linoleic acid is going to be markedly less fattening than the level which might be found in any version of the SAD.
Low fat is synonymous with low linoleic acid. High carbohydrate/high insulin has its own satiety mechanism, more dependent on the glycerophosphate shuttle, which is impervious to small amounts of linoleic acid. And with a diet of 2000kcal supplying LA at 3.1g/1000kcal, then just over 6g/day is not a lot to worry about.
I'll stop here to keep it simple. Obviously the tendency to normalise weight in the low carb period is directly related to low insulin, facilitating lipolysis. Once insulin is low enough, ie once carbohydrate intake is low enough, then linoleic acid, even at a total of around 40g/d (as was eaten during the LC phase), becomes unimportant.
EDIT, missed the lack of fat loss on LC! I guess 40g/d of linoleic acid with low but not basal insulin is too much! The is an update on this here. END EDIT.
Until you get just a little bit of carb creep of course... As carbohydrate and associated insulin rises then that approximately 13% of calories as LA is going to facilitate weight regain with a vengeance.