Why the Body Needs Fat—and Why Too Little Is Harmful

Body fat is often seen as something to get rid of. Diet culture, fitness fads, and social media can all contribute to the perception that very lean physiques are the pinnacle of health.

But fat is not just excess weight to burn off. Adipose tissue stores energy, helps regulate temperature, and produces hormones and other signaling molecules that affect metabolism, appetite, reproduction, and immune function.

That doesn’t mean that more body fat is inherently healthier. Excess adiposity, especially visceral fat surrounding internal organs, is linked to an increased risk of diseases, including cardiovascular disease and type 2 diabetes.

The important point is that low body fat and low energy availability can also be a problem.

Fat as a store of energy

Fat is two things. There are two types of fat: the fat you eat and the fat you have.

Dietary fat is found in foods. During digestion, fats are split into fatty acids and other pieces that the body can use for energy and other biological functions.

Body fat is primarily stored in adipose tissue. Triglycerides are stored in fat cells when the energy intake is greater than the body's immediate needs.

Fat is more than a passive storehouse.

Now scientists call it an endocrine organ because fat tissue makes hormones and signaling molecules that communicate with other parts of the body. For example, leptin is produced mainly by adipose tissue and plays an important role in the control of appetite and energy balance.

When energy stores are very low, leptin levels may decrease. These hormonal changes are part of the body's response to low energy availability and can affect several physiological systems, including reproductive function.

Fat also provides insulation. On top of that, specialized brown and beige fat tissue can generate heat from stored energy.

Simply put, body fat is part of a larger biological system that helps the body regulate energy, temperature, and metabolism.

What happens when energy availability is too low?

The trouble with being very lean is not just the number on a body-fat scale.

Extreme thinness may result from long-term calorie deprivation, strenuous exercise without adequate nutrition, a disordered eating pattern, or another medical problem. In these conditions, the body may not have enough energy available to support all physiological processes normally.

Low energy availability can impair reproductive hormones, bone health, and sport performance. For women, such a condition could mean periods that are irregular or missed. In men, chronic underfelling may be associated with reductions in testosterone and other changes in reproductive and metabolic function.

These effects are especially relevant in athletes, where suboptimal nutrition relative to the training demands has been identified as a component of relative energy deficiency in sport, or RED-S.

“Starvation mode” is a phrase that can sometimes be misleading because it doesn’t mean the body literally “shuts down.” Instead, it responds to a lack of energy availability by changing the allocation of resources between different biological functions.

Severe or prolonged energy deficiency can lead to loss of muscle and bone and increase the risk of injury over time.

This is why striving for the lowest possible body-fat percentage is not necessarily an indicator of better health.

There’s no perfect body fat percentage.

One of the problems with online fitness advice is this idea that everyone should be aiming for the same body fat percentage.

There is no one percentage that is the healthiest body composition for every adult.

Body composition varies by sex, age, genetics, ethnicity, physical activity, and other factors. Methods of measurement also differ in accuracy.

Some clinical references provide broad body fat categories. For example, one Obesity Medicine Association reference classifies body fat of 30% or more in men and 35% or more in women as obesity when assessed by DXA, while other professional sources also use different reference ranges.

This variation shows why a single target should not be seen as a universal target.

Athletes may also have less body fat than the general population typically has, but very low levels are not necessarily healthier. Nutrition, hormonal health, bone health, recovery, and performance all matter for an athlete.

Location of fat storage matters.

Body fat percentage is only part of the story.

Subcutaneous fat is stored under the skin, while visceral fat is stored around internal organs and has a different metabolic profile. Higher levels of visceral adiposity are linked to adverse metabolic effects and an elevated cardiometabolic risk.

And that’s why two people of equal weight, or with equal body-fat percentage, can be completely unique in terms of health.

Body fat percentage alone does not provide important information on muscle mass, fitness, blood pressure, blood sugar, cholesterol, waist circumference, diet, physical activity, and other health factors.

The “obesity paradox”?

The original discussion of the so-called ‘obesity paradox’ in the article requires special caution.

Some observational studies have shown that individuals who are overweight or obese with certain cardiovascular conditions may sometimes have better short-term survival than normal-weight individuals. What has been called the “obesity paradox” is here.

But it’s a controversial phenomenon.

Possible explanations have been offered by researchers, such as differences in fitness, muscle mass, smoking, unintentional weight loss, disease severity, and the limitations of BMI as a measure of body composition. Some studies have shown that when researchers take these factors into account, the apparent survival advantage disappears or is much smaller.

So it would be a mistake to read the obesity paradox as evidence that excess body fat is protective against cardiovascular disease.

The well-established health risks of excess adiposity continue to be relevant.

“As little fat as possible” shouldn’t be the aim.

The science suggests a more complex picture than the notion that body fat is just something to get rid of.

Excess adiposity, especially visceral fat, can contribute to the development of metabolic and cardiovascular disease. But too little body fat, especially when caused by prolonged under-eating or excessive exercise, can also interfere with normal biological functions.

A healthier approach is to take a holistic view: adequate nutrition, physical activity, strength, cardiovascular fitness, sleep, metabolic health, and a body composition that is sustainable without extreme restriction.

Body fat is not simply “good” or “bad.”

It is a natural, biologically active part of the human body. The challenge is not to eliminate it but to understand its function and avoid the health risks associated with both excess adiposity and inadequate energy availability.