The Case Against Extreme Fat Loss: Why Our Bodies Need Fat

From releasing hormones to regulating our temperature, fat has myriad roles in the body. Chasing extreme fat-loss goals, however, is unhealthy, and experts are growing concerned that the trend for being super skinny is once again resurfacing, in some cases fuelled by the popularity of weight-loss drugs.

Fat has been somewhat demonised for years. Diet culture tells us to burn it, food companies market low-fat products at us, and weightlifting forums share tips on how to reach as little body fat as possible. A moment spent scrolling through social media soon turns up advice on how to get shredded, a sinewy physique defined by extremely well-defined muscles. Yet while too much fat poses health concerns and can lead to obesity, too little is not good for us either.

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?

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 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, causing the cells to expand in size.

Rather than just sitting there passively, body fat is now understood to be 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 is 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. White fat stores excess energy but also acts as a layer of insulation, while brown and beige fat contain specialised cells that burn energy to produce heat, helping the body maintain its temperature.

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 is not a healthy situation, he says, 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. 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. The mental health condition anorexia nervosa, for example, can lead to long-term organ damage or death.

It is surprisingly tricky to measure how much body fat you have yourself, and there is no universal percentage to aim for. Individual needs vary. Nevertheless, as a broad guide, healthy adult men tend to have around 10 to 20% body fat, while healthy adult women tend to have around 20 to 29%. Those living with obesity tend to have body fat above 30%. In terms of how much is optimal, it is 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. In her research, Tamara Hew-Butler, a sports performance researcher at Wayne State University in Detroit, and colleagues scanned over 700 athletes using a Dexa scanner, which measures the body's composition including internal fat. The lowest body fat we ever found in an athlete was 10% for a male and 16% for a female, she says. Some participants were surprised at this result, as the athletes in the study trained regularly and believed their body fat was as low as 1 to 2%.

It also matters where you store fat. Visceral fat, also known as belly fat, is especially harmful if you have too much of it. Unlike fat that lies under the skin, visceral fat wraps around vital internal organs and releases inflammatory chemicals that raise your blood pressure and interfere with insulin, making it harder for the body to control blood sugar and raising the risk of type 2 diabetes.

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, though these require specialist equipment and can be expensive. One popular home test is the skinfold test, which 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, and visceral fat cannot be measured directly this way. Skinfold tests are not particularly accurate and may give misleadingly low figures.

Another at-home option is a set of smart scales, which work by sending a low electric current through the body to estimate fat mass. Readings are influenced by how much fluid we have in our bodies, however, and so they are not considered accurate for measuring fat levels. They also cannot tell you where your fat is stored, or distinguish between visceral fat and other types.

Attempting to measure your fat levels in any detail is not generally necessary for maintaining a healthy body. But two measures can help you stick to a healthy weight and keep track of where fat is being stored. A tape measure can give a quick insight. The waist-to-height ratio measures how much central abdominal fat a person has relative to their height. Once you have measured your waist circumference, an online calculator can find your ratio. A healthy range is around 0.4 to 0.49. A person's waist-to-hip ratio can also give a good indication. To get this figure, measure around your belly button and divide it by the measurement of the widest part of your hips, or use an online calculator. A measurement above 0.90 for men and 0.85 for women is considered high, as excess fat around the waist is an indicator of poor metabolic health. Body mass index is not an accurate predictor of body fat, as it cannot distinguish between the body's fat and muscle content. People with a lot of muscle and very little fat may have a high BMI.

You may want to keep a closer eye on your body composition as you age. Our fat mass tends to increase while our muscle and bone levels decline. As this can cause adverse health effects, it is important to eat well, exercise and take part in strength training. However, we should learn to value the many important things fat does for us and not obsess over extreme fat-related goals. It is important to have the right amount, rather than striving to have as little as possible. Or, as Oral puts it, we believe fat is both powerful and beautiful.