Jul 24 2009

Calories Revisited

I am a proponent of the notion that the dominant factor in determining weight gain or loss is the formula – calories in vs calorie out. In fact, this formula is inescapable – change in weight must be equal to the total calories consumed minus the total calories burned.

Whenever I write about this topic, however, angry critics (food is always highly emotional) charge that this formula is not true because it is too simplistic. But I think they misunderstand the formula. For example, some argue that this formula does not necessarily explain the differences among people. This is partly correct, but irrelevant. It does largely explain the difference – overweight and obesity does in fact correlate with higher caloric intake, and all weight loss diets work by reducing calories.

But, people are different – they have different digestive systems and different metabolisms. They have different amounts of brown fat – the kind of fat that burns calories to generate heat. Therefore, some people burn more calories while sitting and watching tv than other people, and they are more likely to be thin.

However, at present there is no safe and effective way to change people’s metabolisms. Without going into detail, I do not think that stimulants are a safe way to achieve this. And drugs to block food absorption have been pulled from the market due to side effects.

Therefore, while metabolism may explain why person A is thin and person B is overweight, if person B wants to become thin they need to either decrease their caloric intake and/or increase their caloric expenditure.

And keep in mind that the evidence suggests that most people who are overweight are so not because they are victims of their genetics, but because of their lifestyle. This is actually a good thing – because we can change our lifestyle.  (This study shows that the morbidly obese are sedentary 99% of the time.  Data shows that per capita food consumption is up 16% in the US since 1970. – just to give a couple examples. )

One of the simplest and most effective ways of altering behavior to consume fewer calories is keeping a food diary. Essentially, anything that makes us pay attention to how much food we are eating is likely to decrease how much food we are eating – which is probably why most diets work in the short term. It also makes sense that we should pay attention to the caloric density of the food we eat – an apple may be about the same volume as a slice of cheesecake, but their caloric content is not the same. We need some way to estimate the amount of calories that we consume, so that we do not accidentally over eat.

A recent article in the NewScientist points out that one barrier to accurate calorie estimating is that the number of calories reported on various food items may be misleading. Food calories are calculated using the Atwater system, which assumes that our body burning food for energy is literally equivalent to incinerating the food and measuring the heat produced. I remember seeing this on a “Mister Science” type show when I was a kid – burning a pile of jelly donuts and saying that the energy released is the same as if we ate the jelly donuts. Cool.

But there’s a catch – processing, digesting and absorbing nutrients from food costs calories, and some of the calories in food are consumed by the bacteria in our guts or excreted as waste. Therefore what we really want to know about the food we eat is not how many calories are in the food but how many calories we will net from eating it. If a piece of food has 100 calories, but it costs 20 calories to extract that energy, and 10 calories are lost to bacteria and stool, then we are only netting 70 calories from that piece of food.

If all food were 70% efficient then this would not be a big deal – in fact lowering the number of calories reported on food labels may contribute to more overeating.  But the real issue is that different foods have significantly different efficiencies. Soft foods are easier to consume than hard foods. Cooked foods are easier to digest than raw foods. Cooking food actually gave a huge boost to human evolution because our ancestors could extract many more calories and nutrients from their food, and expand the number of foods they could eat.

Cooking is interesting because we are literally using energy from fire to replace energy we would have to expend in digestion, and we can get more energy from the food.

Differences in food calorie efficiency can range from 5-25%. Some argue that food labeling should reflect this – not just the total Atwater calories. Therefore we would be better able to estimate our caloric intake. At present people may falsely assume that soft and easily digested food is equivalent to mechanically hard and fibrous food of the same calories.

The differences between total and effective calories may also explain why highly processed food seems to be associated with being overweight. Processed food is not necessarily bad for you – it is just highly efficient. Finely milled wheat is more efficient than whole grain. Chopped or processed meat is more efficient than whole meat.

Efficiency was a good thing when we were scraping out a living in the wild. It is also a good thing in those parts of the developing world where hunger and starvation are a problem. But in the affluent West – we are victims of our own efficiency. If you want to lose weight, you want to be caloricly inefficient.

This one realization about effective calories vs Atwater calories is not going to solve our obesity problem. But each little bit helps. I maintain that the weight loss industry has been largely counterproductive – promising easy solutions and distracting people from the real issue. For any individual hoping to maintain a slimmer and more healthy weight, keep it simple. Find a fun exercise you can do on a regular basis. And keep some basic track of your effective calorie intake and keep it under control.

Calories in – calories out.

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