Fat balance and body weight stability
There are two mechanisms by which a new steady-state of body weight and body composition achieved following a positive or negative perturbation in fat balance. First, changes in behaviour can lead to adjustments in either intake or oxidation of fat (e.g. altering total energy or fat intake and altering physical activity). Second, in the absence of sufficient behaviour changes, fat oxidation will be altered following alterations in the body fat mass. As an example of behavioural adjustments, the negative fat balance produced by reducing energy intake could be eliminated totally by a compensatory reduction in physical activity. As an example of metabolic adjustments, overconsumption of total energy and fat will produce positive energy balance. If behavioural adjustments are absent or insufficient, increases in the body fat mass will result. Increased body fat mass is associated with increased levels of circulating free fatty acids which elevate total fat oxidation. Thus, a stable body weight will be reached at the point where the body fat mass has increased sufficiently so that fat oxidation equals fat intake.
Metabolic differences between carbohydrate and fat
Based on known differences in macronutrient metabolism, we can begin to predict how the composition of the diet, and specifically the carbohydrate to fat ratio of the diet, might impact upon body weight regulation. It must be realized that the pathways by which nutrients are metabolized (particularly carbohydrate) vary with the overall state of energy balance and this must be considered when predicting the impact of diet composition. For example, conversion of carbohydrate to fat would occur during situations of excess carbohydrate intake and not under situations of normal or below normal intake.
Changing diet composition with no energy intake change
Altering diet composition without a change in total energy intake should have relatively modest effects on body weight and body fat content. There are at least two ways that such a change in diet composition could affect body weight. First, the thermic effect of carbohydrate is greater than the thermic effect of fat. Changing to a lower fat diet (assuming total energy and protein intake remain constant) means changing to a higher carbohydrate diet, which will increase total energy expenditure. The magnitude of increase in energy expenditure depends on the magnitude of change of the carbohydrate/fat ratio, but is probably relatively small and of questionable importance in body weight regulation for reducing dietary fat from 35-40% to 20-25% of total energy intake. Second, altering the carbohydrate/fat ratio of the diet requires that substrate oxidation rates be readjusted to the new macronutrient intakes. If total energy expenditure is not changed, these changes occur relatively rapidly, with carbohydrate and protein balance being reachieved more quickly than fat balance (108,109). Negative fat balance and some loss of body fat will occur until fat balance is reachieved. It is difficult to predict the rapidity with which fat balance will be reachieved following a reduction in fat (and an accompanying increase in carbohydrate intake).
Effects of diet composition during positive energy balance
It is during periods of positive energy balance that differences in carbohydrate and fat have the greatest impact upon body weight regulation. This is because of differences in the efficiency of metabolic pathways involved in disposing of excess carbohydrate vs. fat. One study (107) demonstrated that while the majority of excess energy is stored regardless of its composition, a greater proportion of excess energy is stored when the excess is from fat as compared to when the excess is from carbohydrate. This is a clear example of a situation where fat intake leads to more body energy storage than the same amount of energy from carbohydrate.
Total energy expenditure increases more with carbohydrate overfeeding than with fat overfeeding. This is because carbohydrate oxidation increases to a greater extent than fat oxidation decreases during carbohydrate overfeeding. The difference between carbohydrate and fat in the proportion of excess energy stored is greatest during the first week of overfeeding. This suggests that the more sustained the overfeeding, the less the difference between carbohydrate and fat overfeeding. If obesity develops due to brief, periodic episodes of overeating, differences between fat and carbohydrate are likely to be more important than if obesity develops from sustained positive energy balance.
Carbohydrate type and body weight regulation
The effects of different types of carbohydrates on body weight regulation have been reviewed recently (110). While there are clear differences in metabolism of carbohydrates and fat that could affect body weight regulation, there do not appear to be such metabolic differences between types of carbohydrate. The majority of comparisons have been made between simple sugars and complex carbohydrates. There is little scientific support for the commonly held perception that consumption of high amounts of simple sugar contributes to obesity. There is no evidence that simple sugars are used with a different efficiency than complex carbohydrates (other than dietary fibre or resistant oligosaccharides). While there is substantial data suggesting that high levels of dietary fat intake are associated with high levels of obesity, at present there is no reason to believe that high intake of simple sugar is associated with high levels of obesity.
Does carbohydrate make you fat?
The idea that increased insulin concentrations subsequent to carbohydrate intake lead to conversion of significant amounts of carbohydrate to fat is misleading. First, it takes an extreme excess of carbohydrate to produce de novo lipogenesis, and even under these conditions, very little net fat is produced from carbohydrate. Second, the idea that persons with insulin resistance are particularly prone to become obese when eating high carbohydrate diets is unsubstantiated by scientific evidence. In fact, low-fat, high-carbohydrate diets are commonly recommended to prevent further weight gain for these individuals who are at risk to develop non-insulin dependent diabetes and coronary heart disease. Finally, substantial data suggest that voluntary energy intake is higher in many people when the diet is high in fat content and low in carbohydrate content. Excess consumption of energy in any form leads to accumulation of body fat. There is no serious scientific evidence to suggest, however, that diets high in carbohydrate promote weight gain when consumed in amounts which do not exceed energy requirements.
Prevention of obesity
Because excess dietary fat is stored more efficiently than excess dietary carbohydrate, eating a low fat diet may be helpful in obesity prevention. If one assumes that everyone overeats occasionally, less of the excess energy will be stored as adipose tissue if a low fat diet is consumed than a high fat diet. It remains prudent to recommend a high carbohydrate diet for body weight maintenance. Diets high in fat are likely to promote excess energy consumption and excess dietary fat is stored as adipose tissue with extremely high efficiency. Eating a high carbohydrate diet reduces the likelihood of overeating and, if overeating occurs, results in slightly less of the excess energy being stored as adipose tissue.
Some things are right, some aren't. I disagree with this:
"Diets high in fat are likely to promote excess energy consumption and excess dietary fat is stored as adipose tissue with extremely high efficiency. Eating a high carbohydrate diet reduces the likelihood of overeating and, if overeating occurs, results in slightly less of the excess energy being stored as adipose tissue."
Oh, just looked, the article was written by the Department of AGRICULTURE and Consumer Protection. Clearly biased.
"Storage" here is being used equivocally. Assuming two metabolically healthy individuals, carbohydrate will go to the muscles to be stored as glycogen, whereas fat will be more likely to go to adipose tissue. However, consuming carbohydrate inhibits lipolysis and consuming fat doesn't. Therefore, if we eat carbohydrates, we will be burning carbohydrates. If we eat more fat, we will be burning more fat. Just because the fat is "stored" as adipose tissue does not mean that it will stay there or even accumulate, because if we are primarily burning fat, it will be constantly broken down into free fatty acids and used in place of carbohydrate. Neither low/medium glycemic index carbohydrate nor fat will make one fat in the absence of insulin and leptin resistance.
Last edited by Stabby; 07-20-2010 at 03:03 PM.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!
Well, without a meticulous review of the numerous sources they cite, it would be hard to make any concrete conclusions. Studies can be quoted out of context to convey a message that is not what the study had intended. Again, one would have to go through each study to make sure that the info was used in good faith. What I will comment on is what I think about the article, in and of itself.
1) It only talks about diet and obesity, not necessarily health. Obesity is typically defined in terms of BMI or a similar method. Using that approach, you can be not-obese on a diet of twinkies and mountain dew, but that doesn't mean it's the best diet for you. People on the brink of starvation are not obese either. It doesn't mean that a diet of "no diet" is the best, even though it staves off obesity.
2) The article is published by the Food and Agriculture Organization of the United Nations' Agriculture and Consumer Protection Department. Take that for what you will.
3) This is not a study. It is, instead, an interpretation of MANY studies. This, also, leaves a lot of room for speculation.
That's my opinion, FWIW.