Body composition is one of the five health-related components of physical fitness, alongside cardiorespiratory endurance, muscular strength, muscular endurance, and flexibility. It refers to the proportion of fat versus everything else in your body: muscle, bone, water, and organs. Unlike the other four components, which measure what your body can do, body composition describes what your body is made of, and that ratio has a major influence on your health, your metabolism, and your physical performance.
Fat Mass vs. Lean Mass
Body composition splits your total weight into two broad categories. Fat mass is all the stored fat in your body, both the essential fat your organs need to function and the extra energy reserves stored under your skin and around your organs. Lean mass (sometimes called fat-free mass) includes everything else: skeletal muscle, bone, tendons, ligaments, water, and internal organs.
Healthy body fat percentages generally fall between 12% and 20% for men and 20% and 30% for women. Those ranges reflect the biological differences in how men and women store fat, particularly the essential fat women carry for reproductive function. Falling well below or above those ranges signals potential health problems, from hormonal disruption at very low levels to metabolic disease at high levels.
Why It Matters More Than Body Weight
Two people can weigh the same and have completely different health profiles. One might carry a high proportion of muscle with relatively little fat, while the other might have low muscle mass hidden under excess fat. Body composition captures this difference in a way that stepping on a scale never can.
BMI, the most commonly used screening tool, calculates a ratio of height to weight. It can misclassify muscular people as overweight or obese, and it misses a condition called normal-weight obesity, where someone has a healthy BMI but an elevated body fat percentage that raises their risk for metabolic syndrome, type 2 diabetes, and cardiovascular disease. A 2024 study in the Annals of Family Medicine found that body fat percentage was a significantly better predictor of 15-year mortality risk in young adults than BMI. When the researchers compared how each measure sorted people into healthy or unhealthy categories, BMI and body fat percentage only agreed about 60% of the time. BMI showed no statistically significant relationship with all-cause mortality after adjusting for other variables, while body fat percentage remained a strong, independent predictor of both all-cause and heart disease mortality.
Not All Fat Is Equal
Where your body stores fat matters almost as much as how much you carry. Visceral fat, the fat packed around your internal organs deep in the abdomen, behaves very differently from subcutaneous fat, the layer you can pinch just under your skin.
Visceral fat is the main driver of insulin resistance and is strongly linked to type 2 diabetes, cardiovascular disease, and chronic inflammation. It releases inflammatory compounds at higher rates and disrupts the hormones that regulate blood sugar and appetite. Subcutaneous fat, by contrast, is more metabolically neutral and in some contexts may actually improve insulin sensitivity. The distinction explains why two people with identical body fat percentages can face very different health risks depending on how that fat is distributed. A person who carries more fat around the midsection typically has more visceral fat and a higher metabolic risk than someone whose fat is distributed more evenly across the hips and limbs.
Sarcopenic Obesity: The Hidden Risk
One of the most dangerous body composition patterns is sarcopenic obesity, a condition where excess fat coexists with low muscle mass and poor muscle function. It becomes more common with age as people naturally lose muscle while gaining fat, but it can also develop in younger adults who are sedentary. The combination is worse than either problem alone. Obesity and low muscle mass interact in a feedback loop: excess fat promotes inflammation that accelerates muscle loss, and declining muscle mass lowers your metabolic rate, making it easier to accumulate more fat. The result is a synergistically higher risk for metabolic disease, physical frailty, and mortality compared to what you would expect from adding the risks of obesity and sarcopenia together.
How Exercise Changes Body Composition
Both aerobic exercise and resistance training improve body composition, but they work through different mechanisms. A study comparing the two in overweight and obese adults found that aerobic training was more effective at reducing total body weight and fat mass, while resistance training was more effective at building lean mass. Participants who did aerobic exercise lost more fat. Participants who did resistance training gained muscle but didn’t lose significant fat. Interestingly, the resistance training group still saw their body fat percentage drop, not because they lost fat, but because they added enough muscle to shift the ratio.
The combination of both produced the best overall body composition results. People who did aerobic and resistance training together lost fat at rates similar to the aerobic-only group while also gaining lean mass at rates similar to the resistance-only group. If your primary goal is losing fat, aerobic exercise is the most time-efficient approach. If your goal is building or preserving muscle, resistance training is essential. For the best shift in overall body composition, doing both is the clear winner.
Common Ways to Measure It
Several methods exist for estimating body composition, each with different tradeoffs between accuracy, cost, and accessibility.
- Skinfold calipers measure the thickness of fat folds at specific sites on the body. They’re inexpensive and widely available, but accuracy depends heavily on the skill of the person taking measurements.
- Bioelectrical impedance analysis (BIA) sends a small electrical current through your body and estimates fat and lean mass based on how quickly the current travels. Many consumer scales use this technology. It’s convenient but can be thrown off by hydration levels, recent meals, and exercise.
- Hydrostatic weighing involves being submerged in water. Because fat and lean tissue have different densities, your underwater weight can be used to calculate body fat percentage. It’s accurate but requires specialized equipment.
- Dual-energy X-ray absorptiometry (DXA) uses low-dose X-rays to distinguish between fat, muscle, and bone. It provides the most detailed breakdown of body composition, including regional data showing exactly where fat and lean mass are distributed. It’s considered one of the most reliable methods but requires a clinic visit.
For tracking changes over time, consistency matters more than the specific method. Picking one approach and using it under the same conditions (same time of day, same hydration status) will give you a more useful picture of trends than switching between methods.
How Body Composition Connects to Other Fitness Components
Body composition doesn’t exist in isolation from the other four fitness components. Aerobic exercise builds cardiorespiratory endurance and burns calories, directly contributing to a healthier fat-to-lean ratio. Resistance training builds muscular strength and endurance while increasing the amount of metabolically active tissue in your body, which raises your resting energy expenditure. Even flexibility work, while not a major driver of body composition on its own, supports the mobility you need to train effectively in the other areas.
Improving body composition is rarely about targeting it directly. It’s the downstream result of consistently working on the other components of fitness, combined with nutrition that supports the balance of energy your body needs. Tracking body composition over time gives you a more complete picture of your fitness than weight alone ever could, capturing the muscle you’re building alongside the fat you’re losing.