Weight loss is just about calories in and calories out, right? We’ve all heard this a lot, and after all it’s simple physics. You take calories into your body, and you use them in the course of daily living and exercise. If there’s a surplus you store it, if there’s a deficit you lose weight. Simple. And trivially, it’s even true. But it’s actually a lot more complicated that, to the point where such a simplistic maxim can be cruelly misleading.
Calories in
Surely this bit’s easy. There are calorie labels on everything. A calorie is a specific, unchanging, amount of energy. The energy in any food can be precisely calculated, so calories in must be cut and dried? Obviously not or we’d not be discussing it. We can leave aside the inevitable errors made because ingredient composition and portion sizes vary, even in standard meals. Although these variations can be quite high, they are obvious, and a deviation most people are happy to accept. What is more important, and more subtle, is the format of the food.
Many people have noticed that eating a certain number of calories from some foods impacts weight loss differently from the same number of calories from different foods. One reason for this is the bioavailability of what’s in the food, given how it has been prepared. Tree nuts, for example have significantly fewer calories when consumed whole as opposed to when ground. This is because the food matrix is disrupted, increasing the bioavailablity of the energy in the nuts. The same is true of smoothies, compared with their unblitzed ingredients.
There is a similar story for ultra-processed foods. Processing food has a long history. Many methods maintain, or even improve, the nutritional value of the food. The levels of processing vary from simple mashing, canning and freezing, through to smoking, curing and fermenting.
Ultra-processed foods, in contrast, have ingredients (often in the form of various chemicals) added during commercial processing. Skeptics have often scoffed at the idea that if you haven’t heard of it, or can’t pronounce it, then you ought to be wary. Everything is, after all, a chemical, and that the chemical terms for well-known foods may be unpronounceable is also a truism. But foods aren’t known by their chemical names.
One way to think about this is to list the ingredients you may use to bake a cake, then read the ingredients on a commercially produced cake, and ask yourself what the additions are. Their purpose is simple: to improve the taste, colour, mouthfeel, longevity or stability of the cake. But their addition makes the food ultra-processed, and we now know that this ultra-processing can have negative consequences in terms of diet quality, gut flora and calorie density.
So although the calories on the label may be identical to, or even less than, a similar meal you have made yourself, they will have different effects on your short and long term health and weight. Alternatively, you may discover, after the fact, that you just ate a great deal more, in terms of excess energy, than it had appeared.
Calories out
Our calorie expenditure is just as tricksy as our intake. Many of us have watched the calories ticking up on the treadmill, or have read just how far we have to walk to burn off 100 calories. As you may suspect, this is worse than guesswork, and is more akin to wishful thinking, calculated as if we were machines, rather than the complex biological beings we are.
There are two main ways this complexity manifests. Firstly, in our exercise response. When we start a new form of exercise, we may indeed burn a fair amount of energy, but as we become more practiced, we have biological adaptations which kick in. Our muscles become more efficient, requiring fewer calories for the same effort.
Secondly, if we try to lose weight by limiting our intake, we have biological adaptations for that too. Before our current over-abundance of food, in some parts of the world, we were subject to repeated famine. Those most able to survive these famines are those who passed on their genes. We have been gifted, in this way, with a complex famine response which has a number of effects. We become hungry, often with a desire to binge on energy dense foods, we become cold and lose our libido, we fidget less. All with the result that our energy requirements go down, and the same intake no longer leads to weight loss
Other factors
People’s appetites – not something under a great deal of control – come in for close, often unkind, scrutiny from other people. Resisting food is pretty easy if you feel satiated, but it can feel impossibly difficult if you’re hungry. Some people are simply hungrier than others. People who wouldn’t dream of accusing a heroin addict or an alcoholic of lack of will power often happily level the same accusation at the hungry. The hormones which regulate our appetite are currently a major area of research, but we do know that they interact with, and contribute to, our psychological attitude towards food, and in fact the first appetite hormones available in drug form are just appearing on the market.
Our gut microbiome is another major area of research at the moment. We are discovering that it has a significant effect on many aspects of health, and it is encouraging that we can affect this by what we eat. Keeping it well-fed and diverse looks likely to have a big effect on our weight and general health.
And the much vaunted willpower? In brief it’s unreliable, ineffective and we are inordinately good at weaselling our way round it. Decision fatigue, although more complex that we originally thought, is real. What we all know from experience is that it takes one brief craving too much, combined with a brief loss of focus, to render hours of willpower irrelevant.
So, in summary, calories in/calories out is a physics-based truism which makes for a simple slogan, and a nice stick to beat unsuccessful dieters with, but reality is far more complicated than that.
Does that mean that healthy weight loss is impossible? Not necessarily, and there are times when someone may have a specific reason for needing to lose weight. Calorie restriction, however, very rarely leads to long term weight loss, and overall health is more important than weight. It is easy, with our current UK diet, to both become unhealthy and to gain weight, so cultivating a good relationship with food is sensible. A good first step might be to keep ultra-processed food to a minimum, feed your gut bacteria, don’t starve, and exercise. And the next time some smug know-it-all tells you that energy balance is a simple matter of physics, you’ll know that they are simply wrong,
