Is it time to rethink calories?

How much real value is there in counting calories?

To a physical scientist it would seem quite simple. A calorie is a unit of energy. It is the amount of heat energy required to raise the temperature of 1 gram of water by 1 degree Celsius.

This was adopted with gusto by the early biologists in their understanding of energetics. 

According to the second law of thermodynamics, energy is neither gained nor lost, it just changes form. Hence, energy taken in, in the form of calories (in food or drink), needs to be balanced by energy expended (by physical activity, heat loss or metabolic rate) for body weight to remain stable.

The latter part of this equation is often ignored however, when applied to a biological organism. Heat loss (thermogenesis) and metabolic rate can differ between individuals. A simple calculation of energy balance is thus affected by individual differences. 

Still, most popular programs generally targeted at weight loss or maintenance were able to conveniently ignore this as these individual differences only account for up to 30% of total energy use: a little deviance here or there may be seen as not really worth worrying about.

Now, new findings on discrepancies in energy input have thrown the calculations out by even more, resulting in experts questioning whether a biological calorie really has much value in the design of an individual weight loss program any more.

There’s little difficulty in determining the energy level of foods (and drinks). As suggested above, that’s a physics problem. But it’s what happens to these, even early in the digestive cycle, that has changed modern thinking about just how much real value there is in counting calories.

Results from studies on gut microbiota are a case in point. For years we’ve known about the vast numbers of microbes that inhabit parts of the body, particularly the gut. We have even been aware of the various families, which make up what is called the microbiome in the gut. Two of these families, Bacteroidetes and Firmicutes, account for up to 70% of gut microbes in healthy individuals.

What has been less well known is that the ratio of these microbial ‘families’ can change, and as some are better at ‘harvesting’ energy from food in the gut, they can contribute to greater energy storage than might otherwise occur.

Studies with mice have shown that ‘germ free’ mice raised in sterile conditions with limited resident gut microbes take on the microbial structure of obese conventional mice, if living in the same cage. As a result they then also become obese.

Similarly, injecting gut microflora from obese into lean mice can make the latter fat, suggesting that the make up of the gut microbiome can change the fate of food in the diet. A physical calorie ingested is therefore not necessarily the same as a biological calorie stored – or used as energy.

Even stranger things can happen under circumstance of change in the gut microbiome. In ‘dysbiosis’, the immune system responds as if to an unrecognised antigen, the first sign of which is a form of low-grade inflammation. An inflamed gut can then become ‘leaky’, with the barrier created by epithelial tissue breaking down and allowing certain dietary fats and other nutrients into the bloodstream.

Whether a change in the gut microbiome can lead to obesity, or whether obesity leads to a change in the microbiome is not clear. What is known is that once these changes occur, it may take some time to change back. Is this the reason why the obese have much more difficulty keeping weight off after a diet?

Also, given all of the above, what are the effects of such microbial balances on our typical energy estimates based on a physics formula? It should be apparent that giving a food an energy value – say 200 calories for a piece of cake – has only passing relevance to the effects this might have on body weight in any particular individual. 

It should also be clear that a calorie in vitro, can have a different outcome to the same calorie in vivo, and that the formulae used by the popular magazine diets and just about every weight loss book, must be taken with a grain of (low calorie) salt. 

Perhaps it’s time to re-write the traditional weight loss formulae. Given the ongoing march of obesity in the community it seems the old model hasn’t worked. It might also help us to see obesity as a ‘systems’ rather than a ‘linear’ problem, which is much more complicated and needs to be managed accordingly. 

Professor Garry Egger

Director, Centre for Health Promotion and Research, Sydney; Professor of lifestyle medicine and applied health promotion, Southern Cross University, Lismore, NSW.