View Full Version : Dietary Fat, Carbohydrate Balance and Weight Maintenance: Effects of Exercise.
04-07-2009, 07:03 AM
Most individuals maintain a stable bodyweight during long periods of their life, indicating that energy intake and energy expenditure, although quite variable from day to day, tend to remain adjusted to each other in the average. Obviously, such a steady state is likely to be sustained only if the fuel mix oxidized by the body is equivalent, in its average composition, to the nutrient content of the diet consumed, so that protein balance, carbohydrate balance, and fat balance are all achieved.
Because deviations from these balances undoubtedly occur from day to day, the maintenance of a long-term steady state of weight maintenance further implies that depletion or accumulation of body constituents elicits three possible corrective responses: to alter the composition of the substrate mix used for energy generation, to influence food intake, and/or to alter the rate of energy expenditure.
This discussion considers the consequences that the constraint of having to adjust the composition of the fuel mix oxidized to the nutrient distribution in the diet may have in the regulation of body weight. This aspect of the problem is not usually dealt with when the regulation of the overall energy balance is studied because, often, only the sums of the energy contents of the nutrients and of the substrates oxidized are taken into consideration.
04-07-2009, 11:35 PM
Strange paper. I appreciate the attempt to view fat gain as a consequence of metabolic disregulation, and like the analogy of the little tank/big tank.
The overall model posits that the body is much more sensitive to changes in glycogen than in fat, and that changes in glycogen drive overall food intake. Generally this, mechanism works to keep weight stable over time, but a metabolic mistake that drives fat accumulation may run for a long time before self-correcting, because the body is much less sensitive to changes in fat than to glycogen storage. The paper posits a mechanism whereby the modern diet keeps glycogen storage constantly full due to high availability of high carbohydrate foods, driving fat storage from the high fat content of the western diet. (The phenomenon of insulin resistance is mentioned in passing as an alternative driverof fat accumulation, but the overall model explains why both might happen without self-correction).
That model would explain glycogen depletion favors fat mobilization and oxidation, and would explain how both a high fat low carbohydrate diet would favor fat loss (so long as the carbohydrate intake was low enough to prevent full glycogen replenishment).
It would also give a model for explaining why HIIT promotes greater fat loss than steady state aerobics (again, through glycogen depletion).
The strange part of paper is the dismissal out of hand of a clear pathway from high carbohydrate ingestion to lipogenesis (based on one paper). There's a theoretical argument in favor of low fat diets for weight loss/weight maintenance, but no reflection of the real-world fact that low fat diets tend to do worse than low carb diets in clinical trials.
Then there some just silly stuff about how weight lifting tends to promote weight gain and long distance aerobic exercise tends to promote weight loss, based on a theoretical model calculated from the ratio of food quotient to respiratory quotient, leading the author to conclude that short duration work is counterproductive to weight loss (at this point the author, who previously was careful to talk about body composition and adipose accumulation/loss, now talks more vaguely and much less usefully about weight gain/loss).
The overall sense is of an author who is on firm footing when discussion empirical findings in metabolic studies on mice, but falls substantially when trying to apply those findings to the real world...
04-08-2009, 04:10 AM
I tried to read the whole thing, but I couldn't make it through.
But Arien's analysis seems to be right from the stuff I read.
04-08-2009, 07:30 AM
First thing I looked for was when this paper was written, 1987. OK, so we are looking at a paper that is over 20 years old. Keep that in mind...the age of MAKE EVERYTHING FAT FREE!
04-09-2009, 06:01 AM
I suspect the paper didn't discuss the role of de novo lipogenesis because DNL only really becomes a factor in cases of significant and prolonged overfeeding of CHO. In terms of day to day regulation your body deals with any excess CHO by increasing it's rate of oxidation proportionally and it's this preferential oxidization of CHO that causes dietary fat to be stored rather than be oxidised. (Link (http://www.ncbi.nlm.nih.gov/pubmed/10365981?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsP anel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=2&log$=relatedreviews&logdbfrom=pubmed))
It might seem logical to avoid this problem by cutting carbs and increasing your fat intake but as the paper and this passage from Clinical Sports Nutrition 3rd ed. (Ch. 5.3; p.115) suggests it is likely that your dietary fat intake ultimately determines adiposity not your CHO intake (with the exception of significant overfeeding of course).
Under normal physiological condition CHO, protein and alcohol are not easily converted to bodyfat (Swinburn & Ravussin 1993; Prentice 1998). Thus, increases in the intake of non-fat nutrients stimulate their oxidation rates proportionally. Conversely an increase in dietary fat intake does not immediately stimulate fat oxidation, thus increasing the probability that excess dietary fat will be stored as adipose tissue (Abbott et al. 1988; Westerterp 1993; Prentice 1998; schuts 2004a).
This may well explain why the average Japanese salaryman stays relatively slim eating their traditional high carb low fat diet with rice or noodles at every meal while his western contemporaries gain weight on their traditional mixed diet.
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