Excess carbs coverted to fat?
Thanks to reader LynMarie for prompting me to exhume this post that has been in the "draft" hopper for months! I'll probably update with some comments at a later time, but wanted to get this article/link out there.
This is a repeated mantra in the LC community ... if only it were true to any significant extent.
We've heard it from Taubes, and Sisson, and -- the worst offender -- Nora Gedgaudas who goes so far as to claim that all fat in the body comes from glucose! I even emailed her once about this and she stuck by this claim -- "I didn't make it up"
The model of the human macronutrient energy economy that emerges from the study of McDevitt et al is consistent with previous work (2,3,8,9). In the hierarchy of fuels, dietary carbohydrate appears to have a higher priority for oxidation than does dietary fat; when both are present, carbohydrate is chosen. The 2 major macronutrient energy sources (carbohydrates and fats) are not, however, interconvertible energy currencies. Fat cannot be converted to carbohydrate in animals because animals lack the enzymes of the glyoxylate pathway, and carbohydrate is not converted to fat because of a functional block of uncertain cause.
What are the implications of this model? Some conclusions should not be drawn. First, these results do not mean that extra carbohydrate energy represents "free" energy in terms of body fatness. By sparing fat in the body's fuel mixture, surplus carbohydrate energy will make people fatter, even though it is not directly converted to fat. The absence of significant de novo lipogenesis is bad news for high-carbohydrate dieters for another reason, in that the high thermogenic cost of de novo lipogenesis cannot be invoked as an energy-dissipating feature of such diets. Second, the effects of carbohydrate-rich diets on macronutrient balances should not be confused with their potential effect on plasma lipids and atherogenesis. High-carbohydrate euenergetic or hyperenergetic diets consistently induce hypertriglyceridemia, the public health consequences of which remain controversial (10).
My personal feeling on carbs/insulin/fat accumulation is that insulin is not so much a storage hormone as it is an energy "traffic cop". Net storage is but the byproduct of excess consumption.
Comments
The basis for nutritional advice has been wrong for a very long time. In some ways, the current theory is just the other end of the same stick.
"She claimed that she ate between 3,500 to 5,000 calories a day while losing an incredible amount of weight on her high-carb/low-fat diet. I have no way of knowing if that claim is true or not, but I can say with a certain amount of confidence that for most women, that is way too many calories."
Sounds a lot like one of the examples in Atkins' original book where it was claimed he lost weight eating 5000 cals/day of meat and fat. Such claims are either not true or the person has a serious absorption issue. No doubt the hyper Powter burned off a few more cals than the average woman, but there are limits! I'm just imagining the physical bulk of high carb/low fat food she would have had to consume to reach that caloric level ... she stuffed herself to lose weight? LOL
I don't know that we have a whole lot of triglycerides on a very low fat diet unless its also super hypercaloric. I know the average 400g/day carbs figure gets bantied around quite a bit, but that's a lot of plain brown rice or kasha!! So I tend to think that most people who are eating that way to maintain their body weight probably don't have high trigs. Those that they do have are probably from that "recycling" of the fatty acid/triglyceride cycle but perhaps a few from DNL. Even 600g carbs = 2400 cal that would not be "excess" for many people. Now, those on a low fat reducing diet will still have higher trigs but if they are in caloric deficit, and even as GT points out carbs are necessarily reduced, again I think most of those trigs are coming from whatever dietary fat is not metabolized and/or the recycling of fatty acids.
If someone overeats on a very low fat diet then yes, DNL would be a more significant source of excess fatty acids and those trigs. This is not efficient for us as we only get about 75-85% of the excess energy to store rather than 90-95% for fats.
The main thing about "excesses" is that due to the heirarchy of energy utilization, if we're in calorie surplus, the first "excesses" are the fats, no matter how low, because we'll mostly burn those carbs and protein in excess of structural needs off first. This is the case in the SAD where some have added around 300 cal/day of mostly carb to our 1970's consumption. The "excess" may be carb, but to our bodies, the "excess" is 300 cal worth of the fat we consume that goes into net storage instead of being burned.
http://www.ncbi.nlm.nih.gov/pubmed/10744780
http://www.ncbi.nlm.nih.gov/pubmed/12499321
IMO this is a big problem with blood lipids. For the most part (NEFA aside b/c of the body of research indicating direct receptor mediated effects) lipids like LDL are a result, not a cause of things. That is of course a VERY generalized statement. But I suspect that, for example LDL is co-correlated with the real *cause* of CVD. In other words, A causes B and C so B correlates with C ... we need to target the A. Hope that makes sense!
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