Why is it so easy to put on weight and so hard to lose it? What does science say?

Two studies, presented recently at the International Congress on Obesity (ICO 2026) – the global congress organised by the World Obesity Federation in Mexico City – shed light on an aspect of the obesity pandemic that goes far beyond willpower. At the heart of it all is the way in which our bodies are literally ‘tricked’ by ultra-processed foods, and the human cost of this deception.

The first study, authored by Boyd Swinburn, an obesity expert at the University of Auckland (New Zealand), is based on the ‘Synergistic Systems Theory’. According to the expert, the obesity pandemic can be attributed to the perverse interaction between two complex systems, both of which function exactly as they were designed to. On the one hand, the ultra-processed food industry; on the other, the physiological mechanisms by which our bodies regulate energy balance.

The ultra-processed food system, explains Swinburn, is underpinned by a series of interlocking mechanisms that reinforce one another: product development, marketing, the distribution chain and political economy. Overall, the model identifies as many as 22 positive feedback mechanisms that continually drive this system to grow, at the expense of ‘real food’. The result is that everyone, including consumers, ends up becoming ‘semi-willing prisoners’ of a system from which they derive some immediate benefits (convenience, price, satisfaction of taste), whilst paying a high price in the long term.

At the physiological level, however, the energy balance system comes into play. Under normal conditions, our body precisely regulates calorie intake according to the energy requirements of our lean body mass. But ultra-processed foods – which are calorie-dense, quick to eat and designed to be irresistible – manage to circumvent this ‘balancing’ mechanism (between energy required and calories consumed), meaning we end up consuming more energy than we burn, without even realising it. This is what researchers call ‘passive overconsumption’. And this is where the most insidious trap springs: when weight increases, even significantly, the body reacts only slightly; when, on the other hand, you try to lose weight, the body reacts strongly, ‘slowing down’ the metabolism and increasing hunger. This is the so-called ‘ratchet effect’: an irreversible or one-way process, in which it is easy to keep gaining weight, but impossible to return to the previous level. In short, putting on weight is a piece of cake; getting the needle on the scales to go down, on the other hand, is extremely difficult, because it’s as if losing weight were a threat to be fought at all costs.

“If it were easy to resist the lure of ultra-processed foods, if they didn’t trick our bodies into making us put on weight slowly, and if losing weight were as easy as putting it on, we wouldn’t have an obesity pandemic,” said Swinburn, adding that action is needed on several fronts: by taxing sugary drinks, placing warning labels (the ‘traffic light’ system) on packaging, and banning junk food advertising aimed at children. Looking to the future, Swinburn points to GLP-1-based drugs as a tool capable of acting precisely on that ‘appetite dial’ that ultra-processed food keeps turned up to the maximum. ‘When these medicines become more widely available, their impact on the prevalence of obesity in the population could be significant.’