Body types and diet: why the same diet works differently for different people

Two friends, same height, same weight, start the same diet on the same day. After two months one has lost 5 kg and the other has hardly lost anything. Is it laziness, a "slow metabolism" or a different "body type"? Here is what the research really says, in simple words.

By Muhammad IshfaqUpdated 29 October 20269 min read

The three "body types": where the idea comes from

You have probably seen pictures online showing three body shapes:

These names were made by an American psychologist, William Sheldon, in the 1940s. He claimed that body shape was linked with a person's personality and character. That part of his work was later rejected by scientists, and his own assistant accused him of faking data.

Later, two researchers (Heath and Carter) turned the idea into a proper measuring method using height, weight, bone width, muscle size and skin folds. Sports scientists still use this method today to describe the build of athletes, for example to compare sprinters with long-distance runners.

But describing a body is not the same as predicting how it will respond to food. We could not find good studies (randomised trials) showing that an "endomorph diet" or an "ectomorph diet" works better than a normal healthy diet for that person. Most people are also a mix of the three, not one pure type.

Reason 1: same weight, different body inside

A weighing scale only tells you your total weight. It cannot tell how much of it is muscle, fat, bone or water. Look at these two people:

An example: same height, same weight, same BMI, but Bilal carries about 10 kg more fat and 10 kg less lean mass (muscle, organs, bone and water) than Ali.

On paper they look the same. Inside, they are very different, and this matters in three ways:

• Calories burned at rest. In a study of 150 adults in Scotland, the amount of lean mass explained about 63% of the difference in resting metabolism between people. Fat mass explained only about 6%.
• Handling sugar from food. Muscle is the main place where the body stores sugar from a meal. Less muscle means sugar from rice, roti and sweets has fewer places to go, so blood sugar and insulin stay high for longer.
• Where the fat sits. Fat deep inside the belly is more harmful than fat under the skin. See belly fat and waist size.

Does muscle burn a lot more calories than fat?

Yes, but less than many gym posts claim. Researchers estimate that 1 kg of muscle burns about 13 calories a day at rest, and 1 kg of fat burns about 4.5. So if 10 kg of fat were swapped for 10 kg of muscle, the body would burn about 85 extra calories a day at rest (130 minus 45). That is roughly one small banana. It helps, but the bigger benefits of muscle are better blood sugar control, strength and more movement in daily life.

Reason 2: genes

Studies of twins show that genes explain somewhere between 40% and 70% of the differences in BMI between people. Genes affect appetite, how full you feel, where your body stores fat and how much you move without thinking about it.

The clearest proof comes from a famous experiment from Canada in 1990. Twelve pairs of identical twins were overfed by 1,000 calories a day, 6 days a week, for 84 days. Every man ate the same extra amount, yet weight gain ranged from 4.3 kg to 13.3 kg. The twins in each pair gained in a similar way to each other, but the differences between pairs were about three times larger. In simple words: your genes strongly shape how your body responds to extra food.

Genes are not destiny, though. A large review of 8 trials with over 9,500 adults found that people carrying the best-known "fat gene" (FTO) lost just as much weight on a diet and exercise plan as people without it.

Reason 3: how much you move without exercising

Walking to the shop, standing while talking, fidgeting, doing housework and taking the stairs all burn calories. Scientists call this NEAT (non-exercise activity).

In a 1999 study, 16 adults ate 1,000 extra calories a day for 8 weeks. Some gained very little fat, others gained much more. The biggest reason was NEAT: the people who automatically moved more, without planning to, stored the least fat. Two people on the same diet can burn very different amounts simply because one sits all day and the other is always on their feet.

Reason 4: the same meal, a different blood sugar

In a large study from Israel, 800 people wore blood sugar sensors for a week. The same meal raised blood sugar a lot in some people and only a little in others. The differences were linked with each person's gut bacteria, body measurements and blood tests.

A later study from the UK (PREDICT) with over 1,000 adults, many of them twins, found the same thing: big differences in how people respond to the same food. Genes explained only a small part of it (less than 10% of the difference in blood sugar). The make-up of the meal, the gut bacteria and other daily habits also played a part.

Reason 5: gut bacteria

Trillions of bacteria live in our gut. They help digest fibre and affect how much energy we take out of food. In one experiment, scientists took gut bacteria from human twins where one twin was heavy and the other was lean, and put them into mice. The mice that got bacteria from the heavy twin gained more fat. When those mice ate a healthy, high-fibre diet, bacteria linked with leanness could move in and the extra fat gain was prevented.

This research is still young, and most of it is in animals. But it shows that what you eat over months shapes your gut bacteria, and your gut bacteria can shape how you respond to food. Fibre from daal, chana, vegetables, fruit and whole wheat atta feeds the "good" bacteria.

Reason 6: hormones

• Thyroid. An underactive thyroid slows the body down and can cause weight gain, tiredness and feeling cold. A simple blood test (TSH) can check it.
• Insulin resistance. When the body stops responding well to insulin, it stores fat more easily, mostly around the belly. Read about insulin resistance.
• PCOS in women is often linked with insulin resistance, which can make weight loss slower.
• Menopause. A study that followed women through menopause found that belly fat went up and calories burned went down during these years.
• Stress. Long-term stress and the hormone cortisol are linked with more hunger and more belly fat.
• Some medicines, such as steroids, insulin and some medicines for mood or seizures, can cause weight gain. Never stop a medicine on your own; ask your doctor.

Reason 7: sleep

In a small but careful study, overweight adults followed the same low-calorie diet for 2 weeks twice: once sleeping 8.5 hours a night and once sleeping 5.5 hours. They lost about the same weight on the scale both times. But with short sleep, they lost 55% less fat and more muscle, and felt hungrier. So two people on the same diet can lose the same kilos, but one loses fat and the other loses muscle, just because of sleep.

Reason 8: age, sex and your dieting history

• Men usually carry more muscle than women of the same weight, so they burn a little more at rest.
• With age, most people slowly lose muscle unless they do strength work.
• After losing weight, the body often burns a bit less than expected for its new size, and hunger goes up. Scientists call this adaptive thermogenesis. It is one reason why weight loss slows down after the first few months, and why someone who has dieted many times may find it harder than a first-timer.

Why this matters even more for South Asians

Research from India and the UK shows that South Asian people, on average, have more body fat and less muscle at the same BMI than Europeans. Indian doctor Chittaranjan Yajnik and British doctor John Yudkin showed this with a famous photo of themselves: both had the same BMI, but the Indian doctor had much more body fat. They called it the "Y-Y paradox". Researchers also use the term "thin-fat Indian": thin on the outside, but carrying hidden fat inside.

A study of nearly 1.5 million people in England found that for South Asians, the risk of type 2 diabetes at a BMI of 23.9 was the same as the risk for white Europeans at a BMI of 30. This is why Asian BMI cut-offs are lower. See BMI for South Asians.

Add a diet high in white rice, maida, sugar and fried snacks, plus long hours of sitting, and it is easy to see why the same "standard" diet plan can give a desi person weaker results than the plan promised.

Does any one diet win?

A large study from Stanford University (DIETFITS) put 609 adults on either a healthy low-fat or a healthy low-carb diet for one year. On average, both groups lost about the same weight. But inside each group, results ranged from losing over 25 kg to gaining about 7 to 9 kg. The researchers also tested whether a gene pattern or how much insulin people made could predict which diet suited them. It could not.

The lesson: there is no magic diet for a "type". The diet that works is the one that gives you a small calorie gap, enough protein and fibre, and that you can follow for months.

What to do: find your own response

1. Measure, do not guess. Check your body fat and waist, not only your weight. If your waist is high, you likely have more fat and less muscle than your weight suggests.
2. Start with a sensible plan. Use the calorie calculator to find your daily need and eat about 300 to 500 calories less.
3. Give it 3 to 4 weeks. Weigh yourself at the same time 2 to 3 times a week and look at the weekly average, not single days. Measure your waist every 2 weeks.
4. Adjust based on your result. No change after 4 weeks? Cut about 150 to 200 calories more, or add more daily steps. Losing more than about 1 kg a week? Eat a little more.
5. Build muscle. Do strength exercises 2 to 3 times a week and eat enough protein. See protein in desi food.
6. Move more in daily life. A 10 to 15 minute walk after meals helps blood sugar. See walking for weight loss.
7. Protect your sleep. Aim for 7 to 8 hours.
8. Get tested if progress is very slow. Ask your doctor about thyroid (TSH), blood sugar (HbA1c) and, for women with irregular periods, PCOS.

Common questions

Am I an ectomorph, mesomorph or endomorph?

Most people are a mix of all three. The labels describe your shape, but they do not tell you which diet to follow. Your body fat percentage and waist size tell you much more.

Is there a special diet for endomorphs?

We could not find good studies showing that a special "endomorph diet" works better than a normal healthy diet. People with more body fat often do well with fewer refined carbs and more protein and fibre, but this is true for most people, not only one body type.

Can I change my body type?

You cannot change your bone frame or height, but you can change what your body is made of. Strength training and enough protein build muscle, and a small calorie gap reduces fat. Over months, this can change your shape a lot.

Why does my friend lose weight faster on the same diet?

Your friend may have more muscle, move more during the day, sleep better, have different genes or hormones, or simply be eating a little less than you think. Compare your own progress over weeks, not with someone else.

Is "slow metabolism" real?

Resting metabolism does differ between people, mostly because of muscle and body size. A medical problem such as an underactive thyroid can also slow it. But for most people, the difference is smaller than they think, and daily movement and food intake matter more.

Sources

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