What happens when you have too little body fat

From releasing hormones to regulating our temperature, fat has myriad roles in the body. Here’s why chasing extreme fat-loss goals is so unhealthy.

Fat has been somewhat demonised for years. Diet culture tells us to burn it, some food companies market low-fat products to us, and weightlifting forums share tips on how to reach as little body fat as possible.

You only need to spend a moment scrolling through social media, and you’ll soon find advice on how to “get shredded”. This sinewy physique is defined by extremely well-defined muscles.

This year, experts are concerned that the trend to be super skinny is once again resurfacing, in some cases fuelled by the popularity of weight-loss drugs.

Yet while too much fat poses health concerns and can lead to obesity, too little is not good for us either. The goal of ridding ourselves of fat can be damaging biologically as well as socially, numerous experts have told me.

“We have to love our fat,” Elif Oral, an endocrinologist and professor of medicine at the University of Michigan, tells me. “It plays such an important biological function.”

So how much do we really need? And why is too little body fat bad for us?

Why body fat matters

Broadly speaking, there are two different types of fat: dietary fat and body fat. Dietary fat is the fat you eat in your food, while body fat is nature’s way of storing energy you don’t need.

When you eat a meal, the dietary fats, carbohydrates, and proteins in the food are broken down into simple building blocks your body can use for fuel.

Carbohydrates become glucose, proteins become amino acids and fats become fatty acids. If you consistently eat more calories than you consume, fat cells – known as adipocytes – store these excess nutrients as lipids. This causes the fat cells to expand in size.

However rather than just sitting there passively, we now know that body fat is an active organ which regulates the body’s functions.

Just like in the story of Goldilocks and the three bears, our body works best when things are “just right”. Too little, just like too much, is dangerous for our health.

We can think of our body fat as a specialised storage vault, taking up nutrients when energy is abundant and releasing them only when needed, says Mikael Ryden, professor of clinical and experimental fat tissue research at the Karolinska Institute in Sweden. It’s a system that helped our ancestors survive periods of food scarcity.

Another important role is communication. Fat cells produce hormones that tell the rest of the body how much energy we have and how full we are. The best studied of these is leptin, explains Oral.

“Leptin tells the brain there is sufficient energy storage, and then the brain can almost relax,” she says, which leaves the body able to focus on important biological processes such as reproduction and fighting infection.

Without leptin, the brain starts to worry about the amount of energy available, meaning essential functions start to be deprioritised.

Body fat is also important for staying warm. While white fat stores excess energy but also acts as a layer of insulation. Meanwhile, brown and beige fat contain specialised cells that burn energy to produce heat. This helps the body maintain its temperature.

What happens when we have too little?

The health consequences of having too much fat are well known. They include an increased risk of cardiovascular disease, type 2 diabetes, certain cancers and osteoarthritis. However, having too little is not good either.

When body fat drops too low due to severe calorie restriction, it can have profound effects on our health and physiology, explains Jessica Maung, a graduate student researcher of metabolic diseases at the University of Michigan.

“Having too little adipose tissue leads to disrupted organ and bodily function,” she says.

When the body has too little fat, it responds by trying to save energy, essentially putting itself into starvation mode.
Systems start to shut down, including our immune function. With no reserves left, the body starts to break down muscle instead. Bones are no longer built and repaired.

This results in frailty and increased risk of fractures, says Fredrik Karpe, professor of metabolic medicine at the University of Oxford.

“That’s not a healthy situation,” says Karpe, especially because it leaves the body with no reserves to cope with a serious infection or other medical emergency.

Too little body fat can affect our hormones and reproductive health too. In men who become extremely lean because they are chronically under-fuelling themselves, testosterone can fall, as can energy levels and physical performance.

Some female athletes with very little body fat experience irregular periods or find that they stop altogether.

Several studies have found that carrying extra weight in middle and older age is associated with better survival odds during a critical illness.

For instance, while people living with obesity are more likely to suffer from cardiovascular disease, they are less likely to die from it than leaner individuals also suffering from the condition – a phenomenon known as the obesity paradox.

A cushion of fat reserves appears to help the body withstand serious cardiac events, strokes and other acute illnesses, according to Oral.

In extreme cases, having too little fat can lead to dire health consequences or be fatal. For example, the mental health condition anorexia nervosa can lead to long-term organ damage or death.

How much do we really need?

It’s surprisingly tricky to measure how much body fat you have yourself – but more on that later. There is also no universal percentage to aim for. Individual needs vary.

Nevertheless, as a broad guide, healthy adult men tend to have around 10–20% body fat. Healthy adult women tend to have around 20–29%. Those living with obesity tend to have body fat above 30%.

In terms of how much is optimal, “it’s very difficult to say exactly a range,” says Ryden. “This might differ between people, sex and ethnicities.” Among athletes, the typical amount of body fat tends to be lower.

How to measure our fat

There are a few ways to measure the amount of fat in your body.

The most detailed assessments come from DEXA full-body scans, which can differentiate between body fat, lean muscle mass, and bone mass. However, these require specialist equipment and can be expensive.

One popular home test is the skinfold test. This involves measuring the thickness of the fat layer just below the skin by pinching it using a specially designed pair of callipers.

However, this instrument can only measure the thickness of fat folds at specific body parts. Visceral fat can’t be measured this way directly.

But skinfold tests are not particularly accurate and may give misleadingly low figures.