Why do we regain weight?

It’s a simple question which has stumped obesity researchers for decades: why do most people who diet to lose weight, regain it? A drop in metabolism gets the most blame, but the hidden culprit could simply be an increase in appetite.

Losing weight is easy; keeping the weight off is the hard part. No need to cite scientific studies to support that claim with decades of failed dieting attempts endemic in the population. All the popular diet trends can lead to weight loss, but only a tiny minority of people maintain their weight loss long-term.

One of the key explanations for weight regain is a drop in resting metabolism. A slower metabolism makes it more likely that even with less food eaten, it can still exceed a person’s needs. There is merit to this idea, but it probably does not explain the whole picture.

Looking at metabolism and energy expenditure is only half of the equation. The other half is having an accurate fix on just how much food a person eats, even when restricting calories. Measuring how much someone eats is hard to study in controlled research. A person’s food intake changes substantially from day-to-day, and we don’t have many accurate methods to track this in a free-living situation.

Weight regain explained

A year-long clinical trial with a new drug for treating diabetes gave researchers an opportunity to see how food intake can change with weight loss. The drug used in the trial was canagliflozin. The drug works by causing more glucose to be lost in the urine. Losing excess glucose in the urine also means a loss of a potential energy source. The favourable side-effect is weight loss.

A total of 153 people with type 2 diabetes took canagliflozin for 1 year. For a comparison group, 89 people took a placebo pill. Both groups lost weight, but it was around 3 kilograms in the drug treatment group and just 1 kilogram in the placebo group.

A puzzling question was why the drug-treatment group hadn’t lost more weight. From laboratory tests, the amount of energy lost in the urine from excreted glucose was about 1500 kJ (360 Calories) per day. Even with this degree of lost kilojoules from the body, weight loss plateaued.

What explains the weight loss plateau?  The research team burrowed into mathematical models of how body weight and energy intake are related. And the answer they came up with? Appetite.

People in the drug treatment group were hungrier. The hunger drove them enough to eat around 420 kilojoules (100 Calories) extra each day for every kilogram of weight they lost.

Even though people in the study didn’t know how many kilojoules the drug was limiting each day by excretion, their bodies fought against the weight loss, prompting them to eat more to make up the deficit.

The influence of appetite is three times stronger than the slowing of metabolism from weight loss. Add the two together and it makes it almost inevitable that lost weight will be regained.

A clear downside to the study was that it was only done in people with type 2 diabetes using a specialist drug where weight loss was more a side-effect. How appetite changes may play out in healthy people is unclear.

What it all means

The research helps to explain that weight regain is not a simple matter of poor self-control. You can ignore hunger cues for a time, but they persist while the body remains in a transient state of weight loss. Taking the focus off ‘the best way to lose weight’ and more onto helping and support a person maintain weight loss, or even just keep their current weight stable and instead focus on their health, is where effective therapies for weight loss need to be going.