What Your Body Composition Metrics Actually Say About Your Health

When you step on a traditional scale, a few things happen: First, maybe a twinge of anxiety. Then, a bodyweight number readout. Digits flash for three seconds and that’s that. Back to zero, next in line please step up.

Whether you’re delighted, indifferent, or momentarily defeated by the number, we know it’s generally smart to keep tabs on your weight. A 2017 study of 294 female University of Pennsylvania freshman found that daily self-weighing could help mitigate unwanted weight gain, as has long been theorized.

However, there are loads of factors that play into our overall bodyweight. So it’s also important to understand the many specific measurements that contribute to the complete picture of your health.

Enter body composition. This refers to the percentages of fat, bone, water, and muscle in the body. Let’s break down these percentages, with average estimates for each.

Body fat percentage

What it is: Fat percentage consists of both essential body fat and storage body fat. According to smart scale makers Withings, normal ranges for fat mass are as follows:

“As a general rule, a lower percentage of body fat is indicative of better health,” says Dr. Eric Pham, a bariatric surgeon and weight loss expert with St. Joseph Hospital in Orange, California. “Generally speaking, an average adult male has 18-24 percent of total body weight as fat mass, while an average female has 25-31 percent. Athletes’ body fat percentages can be much lower, somewhere between 6 percent and 13 percent for men and 14-20 percent for women.”

Why it matters: Men and women with higher body fat percentage are at greater risk for cardiovascular disease, hypertension, diabetes, strokes, and some cancers, says Pham.


Bone mass

What it is: This is the total mass of skeletal bone in the body. According to Withings, normal bone mass for men is between 3 and 5 percent. This changes with age, as our bones reach peak mass in our late 20s, according to the National Institute of Health.

Why it matters: Low bone mass may increase the risk of fractures, says Pham. Things like exercise and a diet rich in whole foods will increase bone mass. According to Tufts research, increased bone density can help reduce risk of osteoporosis.

Total body water percentage

What it is: This is the percentage of the body’s mass that is fluid. The human body is 50-75 percent water, and how much you have at any given time hinges on age, sex, and hydration. Most men will have between 50-65 percent; women 45-60 percent.

Why it matters: Body impedance analysis (BIA—more on this in a bit) can determine the total percentage of water that’s found inside our cells and the percentage outside of the cells, called extracellular water, says Pham. “Generally, too much extracellular water may be indicative of heart, liver, malnutrition, or kidney disease.”


Muscle mass

What it is: The total mass of body skeletal muscle. The body has three types of muscles: skeletal, smooth, and cardiac. Skeletal muscle is under voluntary control (think biceps), smooth muscle contracts autonomously (or without any thought), and cardiac muscle makes up the main tissue of the heart’s walls. According to Withings, normal ranges for muscle mass are:

Why it matters: Higher muscle mass increases metabolic rate and prevents falls and illnesses, according to Pham. For example, a person who has a higher amount of their weight as muscle mass will burn more calories at rest compared to someone with a lower percentage of muscle mass. A higher level of muscle mass can also protect against developing diabetes, according research published in the Journal of Clinical Endocrinology and Metabolism.

“Fat mass causes hypertension and diabetes due to insulin resistance,” says Pham. “As we age, we lose muscle mass and are more prone to falls. Loss of muscle mass causes loss of balance and strength as commonly seen in the elderly. This is why exercise is critical.”

So how do I measure this?

We’re glad you asked. There are loads of scales on the market right now that you can purchase with the technology that gives you a body composition readout, which is called Bioelectrical Impedance Analysis (BIA). BIA sends a very low, safe electrical signal through your feet and into the body. This signal meets resistance when it hits fat tissue, and passes quickly through water in the body. The resistance, known as impedance, is what the device uses to determine your body composition percentages.

Before you get too hung up on your numbers, remember that this is just one part of the bigger picture. Make sure that you’re dedicating time to other aspects of a healthy routine and lifestyle, like what you’re putting into your body and the amount of sleep you’re getting (both factors that can affect these readouts, as well).

“BIA is best used in conjunction with a medically supervised weight loss program,” says Pham. “Use the first BIA measurement as a baseline measurement. Over time, as you might lose weight, BIA can be used to determine their body’s changes in fat and muscle mass.”

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