Each day, we learn more about how to lose belly fat efficiently through research and while many people believe in the positive effects of diets such as the keto diet and intermittent fasting, evidence suggest that in the long run, people will put the weight back on that they lost initially.
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When we try to lose weight, we work against our bodies, which likes to hang on to fat reserves as it expects bad times to come. "Human preferences for energy-dense sweet and high-fat foods may have evolved for reasons of survival", as this 2010 paper on Human Perceptions and Preferences for Fat-Rich Foods suggest. Our bodies are happy to reserve some fat because it knows that at some point, it will have to use it to keep us alive. So in theory, more fat reserves equals to an increased chance of survival.
One of the issues is that we live in a world where energy-dense food is abundant so there is no need for us to store fat. We can buy avocados and watermelons all year around and we also have access to a variety of supplements. As well as that, we also created an environment around us that preserves energy: we live in warm houses, move less and so on.
The Health Survey for England 2017 estimates that "28.7% of adults in England are obese and a further 35.6% are overweight but not obese". Despite all this, a good chunk of the population is still not obese. If we would crave all the food in the world all the time, surely everyone would become fat in no time? There must be other forces at play than just sheer will that regulates how much we weight.
One theory is that your body has a 'set point' or 'settling point': a bodyweight it likes to keep and return to most of the time. According to a research paper titled Set points, settling points and some alternative models: theoretical options to understand how genes and environments combine to regulate body adiposity, published in 2011, "the set point model...suggests that there is an active feedback mechanism linking adipose tissue (stored energy) to intake and expenditure via a set point, presumably encoded in the brain" while "the settling point model is based on the idea that there is passive feedback between the size of the body stores and aspects of expenditure."
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Both theories suggests that our bodies have a preferred weight it likes to keep whether we like it or not. This would explain why we pile the weight back on once we are finished with the latest 4-week diet-fad. The theory also implies that once we stopped forcing our bodies to do something it really doesn't like doing (i.e. getting rid of fat storages), it will return to point it thinks will serve your survival the best.
Bad news is, this system can not only be flawed but also abused by people. For example, our bodies can suffer from leptin-resistance, leptin being a hormone often referred to as the 'starvation hormone'. A study called Leptin resistance: underlying mechanisms and diagnosis suggests that "a decrease in tissue sensitivity to leptin leads to the development of obesity and metabolic disorders, such as insulin resistance and dyslipidemia".
Bad diet can also influence these theoretical set points, especially strict calorie restrictive diets. When we force our bodies to lose weight through starvation, it tends to reduce our energy expenditures as a response. After we return to our usual diet, the body will happily restock the fat it lost and does it even quicker since it already reduced its energy expenditure by lowering resting basal metabolic (BMR) rate, as explained in this research paper titled Reduced Metabolic Rate after Caloric Restriction—Can We Agree on How to Normalize the Data?
This could also explain why people put on more weight when they return to the normal food intake levels after low calorie diets. Since the body lowers its BMR, eating the same amount of food you used to eat will result in an even greater calorie surplus and therefore quicker fat reserve replenishment.
An article called Does Metabolism Matter in Weight Loss? and published by the Harvard Medical School mentions that "our bodies are also programmed to sense a lack of food as starvation. In response, our BMR slows down, which means fewer calories burned over time. That's one reason why losing weight is often difficult."
The more often and longer you go on extreme calorie restricted diets, the more likely you will damage your metabolic-system and sustain the 'losing weight-putting more back on cycle'.
Is it pointless to diet? Of course not. Normal BMI is associated with better health prospects in general, so it is worth losing some weight if at all possible. But how?
One solution might be to keep a somewhat calorie restricted diet all the while boosting metabolic rates. Metabolism can be boosted just by walking a bit more and including certain food items in your diet.
This might be more challenging to some than others – leptin-resistance can be at play in some cases – but in non-extreme cases, it is more than achievable to drop some weight through the combination of moderate exercising and a balanced diet. Resistance training is also said to help keeping the weight off, as noted in a paper aptly titled Resistance Training Conserves Fat‐free Mass and Resting Energy Expenditure Following Weight Loss.
An important thing to keep in mind is time, as in letting your body adjust to changes. Your body is extremely adaptable but also slightly pessimistic, so it can change quickly in response to negative stimuli such as a lack of food, but reacts slower to positive ones as it expects that lack of food to happen again in future...
A gentle decrease in calorie intake and moderate increase in exercise levels can help put the body on a more sustainable and healthy bodyweight trajectory in the long run.