All my lovely clients find losing weight rather easy – it is keeping the weight off that proves difficult. Last week some new research was published that help explains what might be happening.
We’ve known for a long time that when we go on a diet our body fights to keep the weight on. We think there is an evolutionary advantage to this. Back in pre-history people tended to die far earlier than they do today. Our ancestors didn’t have time to decline into middle aged spread. There were far fewer food temptations in pre-history. There were no 1000 calorie cakes, no pizza, no greasy hamburgers or sugary colas. In fact, there was no processed foods, no additives and no advertising agencies trying hard to get us hooked on the cheapest products at the highest prices. If we did happen to eat a little more one day, it would be in our interests to hang onto the extra calories so that we could withstand any famine that might be around the corner. Obesity was rare and the diseases of obesity, such as diabetes type 2, heart disease and cancer, because they coincided with an old age that many would never each, were also rare.
The calories we eat are the fuel that powers all our bodily functions. We all have a basic metabolic rate that we expend, even if we are asleep. On top of that we require more calories to power physical activity. If we are very active, the calories we need increase and if we are very inactive, our calorie needs go down. In addition, when we go on a diet our metabolism becomes sluggish because we are losing weight. Smaller bodies need less food. The whole process slows down.
There is a clear evolutionary benefit to this metabolic slow-down or adaptation. We observe that dieters have slower heart rates, they can feel colder or lethargic. They are using up less energy. A really stringent diet might interfere with periods. A famine isn’t safe for reproduction. In the most famous of famine experiments, that took place in Minnesota during WW2, young healthy male volunteers were fed a subsistence diet, designed to mimic the diet in prison camps in Europe at the time. They were given high levels of exercise at the same time, to mimic forced labour. They soon became emaciated and lethargic, some became depressed, food obsessed and even psychotic. 58% of their reduced calories could be explained by a reduction in physical activity. The rest was explained by a reduction in the energy required to maintain their lower weight (32%) and to digest their smaller rations (10%). When we diet, our body metabolism slows down and that makes dieting harder1.
It is due to experiments like the Minnesota experiment that we know how dangerous unmanaged diets can become. When I work with a client, I make sure that their diet is healthy and we work hard together to reduce the danger of over zealous and unbalanced eating. The conundrum is that even though dieting is hard, not dieting can be even more dangerous. The simplest way of improving the nation’s health would be to reduce obesity.
Unfortunately, there is no balancing evolutionary drive that increases activity when calories are more abundant. So, if you stop dieting and start eating as you did before, you’ll be in trouble. Last week (October 2019) a new study was published in the American Journal of Clinical Nutrition which looked at what happens when healthy young people are overfed by 40% for eight weeks. The study found that the volunteers all gained weight. They didn’t exercise more and their bodies didn’t adapt to the new calorie intake by turbocharging their metabolisms. They all simply got fatter!
When the researchers went back after six months of free eating and exercise, there was a marked variation in the response. Some people had lost all the excess weight, but many hadn’t. Those who had a naturally higher metabolic rate were able to get back to their old body size. Those with a naturally lower metabolic rate, had not lost the weight2.
Thinking about a diet, where the metabolism naturally adapts to a lower rate, we give ourselves a handicap if we then over feed ourselves. That is why maintenance is so much harder than dieting. It seems our bodies are naturally trying to conserve any energy that comes our way – as fat – just in case.
The solution is vigilance. Monitoring our eating and exercise after the diet is over is an important part of remaining svelte. Gradually our bodies will come back to balance. It gets easier with time. That’s why dieters need help and support during the all-important maintenance stage, just as much as during the dieting stage.
Further reading
1. Keys, A. The Biology of Human Starvation. University of Minnesota Press (1950).
2. Johannsen, D. L., Marlatt, K. L., Conley, K. E., Smith, S. R. & Ravussin, E. Metabolic adaptation is not observed after 8 weeks of overfeeding but energy expenditure variability is associated with weight recovery. Am. J. Clin. Nutr. (2019). doi:10.1093/ajcn/nqz108