Why is it so hard to lose weight? Genes, environment and what you can influence

Is weight gain down to your genes?

Partly, yes. Studies of twins, families and adopted children estimate that inheritance explains 40–70% of the differences in body weight between people [1]. That is a large proportion. It means that, in the same environment, some people find it easier to accumulate body fat than others.

This inheritance does not depend on a single gene. There are hundreds of variants, each with a tiny effect. Together, all those identified in large genetic studies explain around 6% of the variation in body mass index [1].

Many of these variants act in the brain, in the circuits that regulate hunger, fullness and the pleasure of eating [1]. So rather than having a “slow metabolism”, people with a greater predisposition often feel hungrier, want second helpings more often and feel less full.

Only in rare cases does a single gene cause severe obesity from childhood [1]. An endocrinologist investigates this with specific tests, and it is very different from what happens to most people.

Is there a “thrifty gene”?

It is the best-known explanation, and you have probably heard of it. Geneticist James Neel proposed it in 1962. His idea was that our ancestors lived through alternating periods of plenty and famine, selecting genes that made it easier to store fat. In an environment where food is always within reach, those same genes could become a problem.

The idea is appealing, but the thrifty gene hypothesis is now disputed. A review of the evolution of body fat points out important flaws and considers the evidence that famines shaped it to be weak [3]. There are other proposals. One suggests selection happened more recently; another is that the body regulates weight within a lower and an upper limit [3]. None is settled.

What is clear is the practical part. We live surrounded by food that is energy-dense, inexpensive and appealing, and we move much less than we did a few decades ago. Genes have barely changed during that time, while the environment has changed considerably. It is the combination of the two that determines whether a predisposition has an effect [1].

Why does weight come back after losing it?

Because the body cannot tell the difference between weight loss you chose and a period of scarcity. When it loses body fat, it activates mechanisms to regain it. This is called metabolic adaptation, and it has two sides.

The first is energy expenditure. When you weigh less, you use less energy, and your body also becomes slightly more energy-efficient. For every kilo lost, daily energy expenditure falls by around 20–30 kcal [2].

The second is hunger: it matters more and gets less attention. For every kilo lost, appetite rises by around 100 kcal a day above its previous level [2]. After losing ten kilos, your body asks for considerably more food than the energy it no longer uses. This is a biological signal, and you can plan for it.

That is why the pattern in weight-loss programmes is so similar: rapid loss at first, a plateau within the first year, and then gradual regain [2].

Does your metabolism stay slow after losing weight?

It does slow down somewhat, but it is not “broken”. The drop in energy expenditure is real and continues while you maintain the weight you have lost [2]. But it is small compared with the increase in hunger. The hardest part is living for years with a stronger appetite that makes it difficult to maintain your progress.

Maintaining weight loss is estimated to require a sustained effort equivalent to around 300–500 kcal a day, to counter both the drop in energy expenditure and the rise in appetite [2]. That is why maintenance is a different task from losing weight. And it is why very restrictive plans that produce rapid losses do not prepare you for this second phase.

What mistakes do we see in appointments?

There are three ideas we hear almost every week that make it harder to maintain progress.

• Comparing yourself with someone who eats the same and weighs less. With the same food, some people find it harder than others [1], and measuring yourself against their result only adds frustration.
• Seeing hunger as a lack of willpower. Increased appetite after losing weight is a response from your body [2]. It is easier to manage when you expect it from the outset.
• Treating a plateau as failure. It is a normal part of weight-loss programmes [2], and it is easier to get through when you know it may happen.

What you can influence

You cannot choose your genes, but you can influence their effects. For the genetic variant most studied in obesity, in the FTO gene, physical activity or a healthy diet reduces its effect by 30–40% [1]. Having a predisposition means it may be harder for you than for someone else, but there is still a lot you can do.

Once you understand how your body responds, the most helpful steps are easy to grasp. A moderate rate of weight loss avoids the large energy deficit that drives hunger up from the first week. Filling meals with protein, vegetables and pulses help manage the increase in appetite.

Moving more is one of the habits associated with maintaining weight loss in the long term [2], and strength training helps preserve muscle while you lose body fat. Plan for maintenance from the start, as a stage with its own plan and follow-up [2]. If you weigh yourself, do it no more than once a week to avoid becoming preoccupied with it. Waist and hip measurements are more useful than weight for tracking changes; a belt you wear often can also help. And before demanding more willpower from yourself, it helps to change the environment in which you eat.

Knowing that your body defends its body fat changes the question. It stops being “what am I doing wrong?” and becomes “how can I organise things so I can sustain this?”

The Canadian clinical practice guideline on obesity in adults sums this up as a change in perspective. Obesity is a complex chronic disease, and the goal is to improve health, not just lower the number on the scales [4].

If you want to see how to approach fat loss without regaining it, our guide to weight-loss diets takes you through it step by step. What nutrition can and cannot do when you have obesity has its own guide. And if you have been doing everything right for a while but the scales are not moving, read why am I not losing weight? Common mistakes and how to address them.