Should you train for your body type?

Where the three types came from

The words were coined by William Sheldon in The Varieties of Human Physique, published in 1940 and subtitled An Introduction to Constitutional Psychology¹. The project was not about training or nutrition. It was an attempt to link body shape to temperament, intelligence and character. That research program does not hold up, and modern psychology has long since abandoned it.

The training and diet advice came later. The fitness industry kept the three words, dropped the discredited claims about personality, and attached meal plans. The vocabulary survived because it is memorable, not because anyone went back and tested whether it predicts anything useful in the gym.

What the measurement actually says

There is a real anthropometric method in this space. The Heath-Carter system scores a body using skinfolds, bone breadths and limb girths, and it is still used in sports science to describe athlete populations. Two details in its own definition undercut the popular version.

First, it rates the present morphological conformation: your body as it is now². It is a snapshot, not a birth certificate. Second, the output is not a category. It is three numbers, one each for endomorphy, mesomorphy and ectomorphy, so a rating might be 3-5-2. Everybody has all three components. Nobody is an ectomorph the way they are left-handed.

Fast and slow metabolism

The strongest claim in most body-type articles is metabolic: ectomorphs burn everything, endomorphs store it. What actually predicts resting metabolic rate is mostly how much lean tissue you carry. In 150 adults, 63% of the between-person variation in basal metabolic rate was explained by fat-free mass, 6% by fat mass and 2% by age, while sex was not a significant factor³.

That is a meaningful finding for the body-type framing, because fat-free mass is not a type. It is a quantity, and it is one you change by training and eating. A bigger, more muscular person has a higher resting metabolic rate largely because there is more of them to run.

Does body shape predict how you respond to training?

Here the honest answer is more interesting than a flat no. One study trained 125 children aged 10 to 11 for eight weeks and scored them with the Heath-Carter method. Higher endomorphy reduced the odds of improving vertical jump, while higher mesomorphy and ectomorphy raised the odds of improving sprint performance, and higher ectomorphy raised the odds of aerobic gains⁴.

So body composition and proportions can relate to how much you improve at a specific task. That is worth knowing, and articles that claim somatotype is meaningless are overcorrecting. But look closely at what it does and does not support. These were prepubescent children over eight weeks. The components were measured, not self-selected from a list of descriptions. And the findings are about jumping, sprinting and aerobic fitness. Not one of them is a macronutrient split.

The result is also consistent with something simpler than body types: carrying more fat makes it harder to improve at moving your own bodyweight quickly. That is a statement about fat mass, which is modifiable, rather than about a category you are stuck in.

Personalizing by biology is harder than it looks

If sorting people by appearance worked, sorting them by actual biology should work better. It does not reliably. The DIETFITS trial randomized 609 adults to a healthy low-fat or healthy low-carbohydrate diet for 12 months and tested whether genotype pattern or insulin secretion predicted who did better on which diet. Neither did. Weight change was -5.3 kg on low fat and -6.0 kg on low carb, a difference that was not statistically significant, and the authors concluded that neither predisposing factor was helpful in identifying which diet was better for whom⁵.

Genotype and insulin response are measured in a laboratory. If those did not sort people usefully, a body shape you matched to a drawing is not going to.

What does vary between people

Individual variation in training response is real, and it is large. When 585 adults did the same 12-week arm training program, the change in muscle cross-sectional area ranged from -2% to +59%, and one-rep-max strength gains ranged from 0% to +250%⁶. Same program, same duration, wildly different outcomes.

That is the honest version of what body-type articles are reaching for. People really do respond differently. The problem is that this variation does not come pre-sorted into three bins you can identify in a mirror, which is why the practical answer is to measure your own response rather than predict it from your silhouette.

What to do instead

Train the goal, not the shape. The principles that drive results are the same across physiques: progressive overload for strength and muscle, enough weekly aerobic work for conditioning, a calorie intake that matches whether you are trying to gain or lose, and enough protein and sleep to recover from the work.

Then let your own data personalize it. Log your sessions, keep an eye on the trend in your lifts, your paces and your bodyweight over weeks rather than days, and change one variable at a time when progress stalls. That gives you information about you specifically, which is strictly more useful than a category that was invented to predict character from bone structure.

If you want a starting structure, our programs lay out the progression week by week, and the exercise library covers the movements themselves.