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  • Brian Bender, PhD

What Counts as Fat?

The question isn’t as straightforward as it sounds.

There’s what society says. This depends on where you live as much as it does when you live.

What’s arguably more important, is what your doctor says. Or essentially, when does body fat become a concern for your health?

Diet and nutrition is best managed by a balance of key nutrients. Not too little. Not too much.

The same logic applies to body fat.

Too much body fat can lead to poor health outcomes. Obesity is linked to a slew of negative heart effects, increasing the risk of developing cardiovascular disease, type 2 diabetes, and some cancers.

But too little body fat can also lead to negative health issues. Fat can help regulate body temperature through thermogenesis. This means, when you are exposed to cold temperature, not only can a layer of body fat act like an insulating layer from the elements, but brown adipose tissue (brown fat) can also biochemically generate internal body heat. Body fat is also used as a vital source of energy when internal supply is low. And certainly, dietary fat, as a macronutrient, is an essential part of a balanced diet.

So what’s the happy medium?

Aim for a BMI between 18.5 and 25

The quick way to get an estimate is to measure your body mass index, or BMI. This relationship between your weight and height is used as a quick reference for what counts as fat. And disease risks tend to increase as BMI increases. Statistics that are presented by major health organizations and many journal publications often rely on BMI measurements to assess populations.

  • <18.5 is underweight

  • 18.5 - 24.9 is a normal weight

  • 25 - 29.9 is overweight

  • 30 - 34.9 is obese

  • 35 - 39.9 is severely obese

  • 40 is morbidly obese

BMI Isn’t Perfect

There are two primary reasons that a BMI score isn’t full-proof. First, your BMI doesn’t take into account the distribution of weight.

An obvious example of this is muscle. A body builder may have a BMI that would classify them as “overweight.” But the reason for weight classifications is to determine disease risk. And an individual with a high BMI due to high muscle mass doesn’t have the same disease risk as someone of the same weight with fewer muscles and a large waist.

But even after excluding Mr. and Mrs. Muscles, the story can still be misleading because not all fat is created equal.

The distribution of internal body fat appears to matter a great deal with respect to health and disease risk. Studies show that visceral fat is worse for disease risk factors like cardiovascular disease and diabetes than subcutaneous fat or brown fat.

This distribution of fat within the body can produce some counterintuitive health outcomes. For example, roughly 10% of obese individuals show no signs cardiometabolic risk factors; conversely, 10% of normal weight individuals (18 < BMI < 25) possess a body fat distribution similar to that of obese individuals. These so-called TOFIs (Thin on the outside, fat on the inside) are at high risk for developing metabolic diseases like diabetes because the location of fat is important.

In fact, this distribution of body fat may be the cause of the so-called “overweight paradox,” where some studies have shown a similar risk of cardiovascular disease among normal weight and overweight elderly individuals.

Waist to Hip Ratio

Measuring the ratio between your waist and hips is potentially a better predictor of health and disease risk as it relates to body fat. This is because the distribution of body fat matters a great deal.

Harvard Health suggests that disease risks rise greatly after a ratio of 0.95 for men and 0.85 for women.

The World Health Organization provides recommendations around this measurement and error associated with it. For example, the placement of the measurement on waist, the tension of the measuring tape, and the respiratory phase during measurement can all affect the accuracy.

Body Fat Distribution

Ultimately, the waist-to-hip ratio is aimed at better determining your body fat distribution. There are more accurate ways to determine this than the waist-to-hip ratio, but usually require more complex testing equipment.

Some of the more accurate techniques for measuring body fat percentage include underwater weighing, dual-energy x-ray absorptiometry, and whole-body air displacement plethysmography. The closest tools for home-use are bioelectric impedance analysis devices. However the accuracy of these is not yet good enough for individual, quantitative testing.

The most accurate techniques for truly determining differences in subcutaneous fat and the really bad visceral fat around internal organs is using CT or MRI. What better way to see how your insides are arranged, than to look inside? But again, these are not easy to come by.

And although our understanding of why this body fat distribution is worse for disease risk, precise numbers are still in flux and can vary relative to individuals. But the trend persists.

Body fat distribution is important for health.

Take Away

What counts as fat is a multifaceted question. Society may say one thing, the scale another, and an MRI machine yet another.

As 70% of America is now considered overweight or obese based on BMI, cardiovascular disease remains the number cause of death, and type 2 diabetes continues to rise, understanding what counts as a healthy weight is an important concern.

Despite the health risks associated with being overweight, concern over being fat can lead to eating disorders, stress (which can also impart negative health consequences), and and overall poorer quality of life. The psychological strain imposed by these issues is serious and another cause for concern that is outside the scope of this article.

And there will still be individuals with above-average weight or a traditionally negative body fat distribution that are active, have a healthy diet, and have no physiological markers of poor health. But these are exceptions, not the rule.

The correlations of weight to several disease-risk factors means that it’s never too late to start checking for yourself in a reasonable way. It’s easier to makes mistakes with waist-to-hip ratio measurements than BMI measurements. Calculating your BMI is the easiest, first step. But following up with a waist-to-hip ratio can help provide more information on abdominal fat distribution. This is a great first step towards personalizing your diet and nutritional needs.

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