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.


LynMarie Daye said…
Just to give the low-carb proponents a break, let's talk about Susan Powter of "Stop the Insanity!" fame. She was also not up on DNL like she should have been. She preached something to the effect that the only way to put fat on your body was to ingest it as your body can not make fat. There's BIG difference between not being able to make fat and not readily being able to make fat. 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. Practically all the fat being eaten with the carbs would be stored in fat tissue and not oxidized and DNL would be up-regulated as well, probably after an adaptation period. when it comes to weight loss diets, the devil is definitely in the details.
CarbSane said…
Flashback! That woman was whacked ;)

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
Todd said…
I'm very confused. Can you help me understand how this is possible? If DNL is such an minor pathway, how does the body create fat to store in the absence of dietary fat on a very low fat diet? And where do all the triglycerides come from?
CarbSane said…
Hi Todd, it's getting to the point that especially with common names I'm losing track of who has posted before, but in case this is your first comment let me extend a welcome to my blog!!

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.
Todd said…
That was my first comment on your blog. Thanks for the welcome. If the issues all relate to excess calories, how do you interpret this eucaloric study? Or the next isoenergetic study? A lot of these short term studies seem to be all over the map. how can someone be so confident that DNL plays such a minor role?

CarbSane said…
Aahh, I see the problem now! Distinguishing the role of DNL in body weight vs. blood lipids. In rodents, it is my understanding that DNL does play a significant role in body weight - e.g. it is on an order of magnitude where one can consider these critters pretty efficient at converting carbs to fat for long term storage. THIS is what Jequier & Hellerstein have demonstrated is not a major pathway in humans. Off the top of my head, we're talking perhaps a few grams from well over a hundred grams, and evidence that these synthesized lipids are subsequently "burnt" off. Circulating triglyceride levels are mg/dL and we have ~5L blood, so a fasting level of 100 mg/dL * 50 = 5000 mg = 5g triglyceride ... we're talking a few grams differential. In the very low fat eucaloric diet there's no reason to believe these are detrimental.

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!