While nutrition and fitness can play a role in weight management, new research shows that your body shape might be largely determined by your genes, metabolism or hormones. We take a closer look at understanding body shapes, including what you can and can’t control, so you can set health and fitness goals that are right for you…

When you’re trying to lose weight, it can feel like you’re surrounded by slim people who never need to say no to the crisps or slice of cake. The fact is, people are all shapes and sizes. Even siblings living in the same family, or friends with similar lifestyles, can find their weight is very different. But why do people have different body shapes?

Is body shape determined by genetics?

Studies of twins show a remarkable degree of similarity in their weight, even if they’ve been brought up in different families. And the weight of adopted children is often closer to that of their biological parents than their adoptive family. This strongly suggests that our weight does depend, in part, on our genes.

A tiny number of specific genes have been identified which can explain perhaps five per cent of severe, early onset obesity. But for the vast majority of people it’s not yet been possible to pin down the exact genetic differences that lead to differences in weight. It’s likely that for most of us, our weight is not due to one or two major genes but a mass of small variations in a multitude of genes.

One of these genes, which occurs in slightly different forms, is called FTO. People who inherit just one copy of the variant of this gene tend to weigh about one kilogram more than people who don’t carry it, while people with two copies of the gene weigh about three kilograms more.

‘Fat’ verses ‘lean’ genes

It’s a small difference, but it’s thought that there are many genes, each of which has tiny effects. Some people may inherit many of these ‘fat’ genes and others just one or two. Meanwhile, there may also be ‘lean’ genes that work in the opposite direction.

New research has suggested that some adults may have small quantities of brown fat. This is a cluster of a particular type of fat cell that tends to burn off excess calories rather than storing them. This could help them stay slim, but the effect is probably insufficient to explain large differences in weight between different people. This research greatly improves our understanding of body shapes.

understanding body shapes

Understanding metabolism and body shapes

Thirty or so years ago, it was thought that some overweight people must have a slow basal metabolic rate and burn off far fewer calories than people with naturally leaner body types. But accurate measurements of the calories we burn have shown that the differences in metabolic rate are much smaller than we imagined.

In fact, for people of the same sex and age, the calories burnt at rest are very predictable and depend almost entirely on your size. People with larger body shapes burn more, not less, energy. This is not quite as extraordinary as it might seem. The extra calories they burn are entirely due to their larger body size because every cell in the body needs fuel just to stay alive.

Can strict dieting slow your metabolism?

Recently, it’s been suggested that people who have dieted on and off for many years may have suppressed their metabolic rate below its normal level. This could be as a result of the body struggling to survive periods of strict dieting. Some, though not all, studies do show a very small reduction in metabolism compared to people of a similar weight who have never dieted.

Your metabolic rate will tend to decrease while you are dieting. This is especially true if you follow a very strict diet, say of less than 800 calories a day. Your body may move into starvation mode, but usually you metabolic rate will recover when your weight stabilises. However, regular exercise can help counter this.

How an underactive thyroid affects your metabolism and weight

Some medical conditions or certain medications can affect your weight and contribute to our understanding of body shapes. A common cause of unexpected weight gain is an underactive thyroid.

Thyroid hormones have a direct effect on your metabolic rate. If your thyroid is not secreting sufficient hormones, your metabolic rate will decrease. You may feel tired and sluggish and your weight may increase even if your diet remains the same. This is because you are not burning off the calories as fast as usual.

The good news is that this can be treated with tablets to mimic the effect of the natural thyroid hormones. Once you are stabilised on the correct dose of treatment, your metabolic rate will rapidly return to its normal level.

understanding body shapes

Body shapes and weight management

Our genes, metabolism and hormones can have an influence on our likelihood of becoming overweight. This can aid in our understanding of different body shapes. While no one body shape is ‘normal’ or ‘healthy’, you can increase or decrease your chances of gaining excess weight by the decisions you make each and every day. This is true, no matter your body shape.

If you are looking to reach a healthier weight for your body type, choosing a diet with less processed food and more fruit and vegetables will help. Regular exercise – at least 30 minutes on most days of the week – will also burn off a few extra calories that would otherwise be stored as fat, and will also improve your fitness.

Related: Intuitive exercise – how to listen to your body