Last Friday my blog focused on a study which found that fat intake (total fat, as well as levels of, saturated, unsaturated and polyunsaturated fat) appeared to have no impact on change in body weight over a period of several years. Partly in an attempt to explain how this can be so, I outlined the biochemical basis of the accumulation of fat in the fat. Several biochemical truths point to an excess of carbohydrate as a major driver of obesity. This throws up the possibility that a diet high in fat but low in carbohydrate might affect weight in a way that is independent of calories.
The idea that a particular diet might have weight loss benefits that cannot be predicted utterly by the calories contained in that diet is often referred to as ‘metabolic advantage’. I’ve found it to be a vexed topic, with commentators on it belonging to one of two camps. On the one hand we have those that believe ‘a calorie is a calorie’, and that the form that calories come in have not bearing on their impact on weight. On the other hand, we have those who maintain that the form calories come in can influence body weight in a way that is independent of the number of those calories, or who are least open to the idea that his can be true. Just to be clear, I am in the latter camp.
One of the commenters (Nigeoopoo) regarding my post last Friday asked, quite reasonably, what comes of fat calories eaten, say, in the absence of carbohydrate. My suggestion was that they might be metabolised into energy. Here’s my thinking on this:
As I described in my post, carbohydrate appears to play a key role in the ‘fixing’ of fat in the fat cells as something known as triglyceride (formed primarily from fatty acids). It follows that lower levels of carbohydrate in the diet will result in a shift in the conversion of triglyceride into fatty acids. The importance of this is that triglyceride cannot be burned as a fuel in the cells, but fatty acids can. Low-carb diets also can stimulate the production of ketones, at least some of which will be manufacturered from fat. So, in theory at least, cutting back on carbs ‘frees up’ fat to allow it to be metabolised (converted into energy).
Nigeepoo pointed out, quite correctly I think, that fat does not have much impact on what is known as the thermogenic effect of food. This, in short, is the rise in metabolic rate that occurs when food (fuel) is consumed.
The thing is, though, could the liberation of fatty acids (increasing fuel supply) stimulate calorie burning in other ways?
It’s well known that if when blood sugar levels drop, so do energy levels. Restoring blood sugar levels to normal generally causes energy levels to normalise too. In other words, altering fuel levels in the bloodstream can affect the amount of energy people feel they have. Interestingly, there is evidence that when calorie intake is restricted, individuals move less [1,2]. Could it be that making fuel (including fatty acids) more readily available in the body will enhance metabolism through increased activity? In theory at least, this is one way in which adjusting the make-up of the diet might have a ‘metabolic advantage’.
From what I can see, the search for conclusive proof that metabolic advantage exists in humans has been somewhat elusive. For example, studies in which quite carefully monitored individuals have been fed different diets of the same calorie content have failed to elicit a metabolic advantage. These studies are problematic, in that it is virtually impossible to control what human beings eat, even when installed, on a hospital ward. Also, such studies are expensive and labour intensive, and therefore tend to be of short duration. They may simply not go on long enough for any genuine metabolic advantage to be realised.
One way found these issues is to study animals. The diets of mice, for instance, can be utterly controlled and very accurately measured. Plus, researchers can conduct studies for as long as the mice remain alive.
One interesting animal study that appears to provide some evidence for metabolic advantage was published in the American Journal of Physiology, Endocrinology and Metabolism in 2007 . In this study, mice were red one of four diets:
1. Regular mouse food (chow)
2. Regular mouse food but calories restricted by 1/3 compared to diet 1.
3. A high-fat, high-sugar diet containing the same number of calories as diet 1.
4. A high fat, low-carb (ketogenic) diet containing the same number of calories as diet 1.
The mice had their body weights monitored for 9 weeks.
Here are the results of this experiment.
The mice eating the regular chow and the high-fat, high-sugar diet gained weight.
The mice eating the calorie-restricted diet lost weight.
The mice eating the high-fat, low-carb (ketogenic) diet lost weight. This, despite the fact that they ate the same number of calories as mice on regular chow and high-fat, high-sugar diets who gained weight.
The weight loss seen in these mice was about the same as that seen in the mice who had been given 1/3 less calories to eat.
Seems to me that this is proof, in mice anyway, that the macronutrient make-up of the diet (irrespective of calories) can influence the impact it has on body weight.
The researchers went further in this study by taking mice that had been made ‘obese’ in the first phase of the study, and putting some of them of the high-fat, low-carb (ketogenic) diet. Others were left of the high-fat, high-sugar diet. Mice put on the high-fat, low-carb diet lost their excess weight over a period of 5 weeks. The other mice, not surprisingly, did not.
Again, both groups of mice consumed the same number of calories. So, here again, we have evidence of ‘metabolic advantage’.
One interesting aspect of this study is that it monitored gene expression, which can provide information in terms of what is going on in the body of these mice that could explain how a high-fat, low-carb diet has metabolic advantage.
Evidence was found of increased metabolism of fat, as well as reduction in biochemical pathways that cause the manufacture of fat.
Of course, the high-fat, low-carb diet would be expected to do this, because it should, by rights, lead to a reduction in insulin levels (insulin generally stimulates the manufacture of fat, while reducing it’s breakdown).
Analysis revealed that insulin levels declined dramatically on the high-fat, low-carb diet, and ended up at only 10 per cent of the level seen in the mice eating the calorie-restricted diet.
But what of the theory that a high-fat, low-carb diet might increase energy production? This study actually measured energy production by assessing heat production from the mice. The results showed that the high-fat, low-carb diet led to a significant enhancement in total energy production. This appeared to explain where the ‘missing’ calories went.
While this study was done in mice, it nonetheless provides at least some evidence for the contentious phenomenon known as metabolic advantage.
There is, as it happens, some evidence in humans too. In one review, researchers looked at the impact of diets of different composition on several body measurements including weight loss, fat loss and body fat percentage . 87 individual trials were included in the review. Lower-carb (generally higher-protein) diets, compared to higher-carb ones, were found to bring about enhanced results in all the parameters assessed.
Specifically, in trials lasting 12 weeks or more, lower carb diets led to:
- an additional 6.5 kg in weight loss
- an additional 5.6 kg in fat loss
- an additional 3.5 per cent reduction body fat
But look, even if like Nigeepoo you just don’t believe that the metabolic advantage exists, there is another reason for favouring lower-carb diets. In one of this comments, Nigeepoo alludes to the fact that such diets tend to sate the appetite more effectively than higher-carb ones. And he’s right. Generally speaking, when individuals migrate from a higher- to lower-car diet they end up eating less. Sometimes a lot less. Without undue hunger.
Whether the metabolic advantage is a real thing and clinically relevant or not is probably not as important as the fact the lower-carb diets generally are much more effective at sating the appetite than low-fat ones. And, even more important than this is the fact that, generally speaking, low-carb diets trump low-low fat ones in terms of weight loss and other important markers of health including blood fat and blood sugar levels.
1. Redman LM, et al. Metabolic and behavioral compensations in response to caloric restriction: implications for the maintenance of weight loss. PLoS One. 2009;4(2):e4377
2. Weyer C, et al. Energy metabolism after 2 y of energy restriction: the biosphere 2 experiment. Am Clin Nutr. 2000;72(4):946-53
3. Kennedy AR, et al. A high-fat, ketogenic induces a unique metabolic state in mice. Am J Physiol Endocrinol Metab 2007;292:E1724-E1739
4. Krieger JW, et al. Effects of variation in protein and carbohydrate intake on body mass and composition during energy restriction: a meta-regression. Am J Clin Nutr. 2006;83(2):260-74