The two primary factors that affect body composition are energy intake (what and how much you eat) and energy expenditure (how much energy your body burns). When these two forces are out of balance, your body either stores the surplus as fat or draws on its reserves for fuel. That simple equation, calories in versus calories out, ultimately determines your ratio of fat mass to lean mass over time.
But the real picture is more nuanced than a simple math problem. Each factor breaks down into several moving parts, and other influences like genetics, sleep, hormones, and aging shape how your body responds to that energy balance.
Energy Intake: More Than Just Calories
The food you eat supplies the raw energy your body uses or stores. When you consistently take in more energy than you burn, the excess gets deposited primarily as body fat. When you take in less, your body pulls from its energy reserves. This is the most fundamental driver of body composition changes.
What you eat matters just as much as how much. Protein plays a particularly important role in determining whether you hold onto muscle or lose it. The standard recommendation for healthy adults is about 0.8 to 0.9 grams of protein per kilogram of body weight per day, but research published in the Journal of Cachexia, Sarcopenia and Muscle suggests that a higher intake of 1.2 to 1.6 grams per kilogram is better for building or preserving lean mass. During periods of fat loss, getting enough protein is what tips the balance toward losing fat rather than muscle.
The quality of your overall diet also influences where your body tends to store fat. Chronic overconsumption eventually overwhelms your body’s ability to safely store fat under the skin. When that storage capacity hits its limit, fat spills over into more dangerous locations: around your organs and inside your liver. This visceral fat is far more metabolically harmful, driving insulin resistance and inflammation in ways that subcutaneous fat does not.
Energy Expenditure: The Three Components
Your body burns energy in three main ways. The largest share, roughly 60 to 70% of your daily total, goes to your basal metabolic rate: the energy needed to keep your organs running, your heart beating, and your cells functioning while you’re at rest. The second portion is the energy used to digest food, which accounts for about 10% of your daily burn. The third, and most variable, is physical activity.
Physical activity breaks down further into two categories: structured exercise and everything else. That “everything else” is called non-exercise activity thermogenesis, and it includes walking to the kitchen, fidgeting, standing, and doing household chores. For most people, especially those who don’t follow an exercise program, this low-grade daily movement is the single most variable component of total energy expenditure. Research in the Journal of Exercise Nutrition and Biochemistry found that people with obesity sat an average of two hours more per day than lean individuals. If they adopted the movement habits of their leaner counterparts, they could burn an additional 350 calories per day, equivalent to roughly 18 kilograms of body weight over a year. Small, accumulated movement matters enormously.
How Exercise Type Shapes Your Body
Not all exercise affects body composition the same way. A large meta-analysis comparing resistance training to aerobic exercise in middle-aged and older adults found clear differences. Aerobic training was better at reducing overall body mass, with participants losing an average of 1.23 kilograms more than those doing resistance training. But resistance training was significantly better at increasing lean body mass.
This distinction is critical. If your goal is simply to weigh less, cardio is efficient. If your goal is to change the ratio of muscle to fat on your body, resistance training is the stronger tool. Ideally, combining both gives you the benefits of fat loss and muscle preservation. The interplay between exercise and eating also complicates things: one of the biggest sources of individual variation in exercise results comes from how much people change their food intake in response to working out. Some people unconsciously eat more after starting an exercise program, which can offset the calories they burned.
Genetics Set the Baseline
Your genes don’t determine your body composition, but they do influence it more than most people realize. A study of identical twins, some raised together and some raised apart, found that about 60% of the variation in body fatness was attributable to genetics. Shared environmental factors like growing up in the same household accounted for 22%, and individual lifestyle choices made up the remaining 18%.
What this means in practical terms is that two people can eat the same diet, follow the same exercise program, and still end up with noticeably different body compositions. Genetics influence your metabolic rate, where you tend to store fat, how efficiently you build muscle, and even your appetite signals. You can’t change your genetic hand, but the 40% that comes from environmental and behavioral factors is entirely within your control.
Aging and Muscle Loss
Starting around middle age, your body begins losing muscle at a rate of roughly 1% per year if you don’t actively work to maintain it. By the eighth or ninth decade of life, this process can strip away up to half of your total muscle mass. The decline accelerates with time: longitudinal studies show that by age 75, men lose muscle at about 0.8 to 1% per year and women at 0.64 to 0.7% per year. Muscle strength drops even faster, at 3 to 4% per year in men and 2.5 to 3% in women.
This matters for body composition because muscle is metabolically active tissue. As you lose it, your resting energy expenditure drops, making it easier to gain fat even if your eating habits haven’t changed. Older adults also develop what researchers call “anabolic resistance,” where the body becomes less efficient at building new muscle protein in response to food and exercise. Just two weeks of reduced activity in older adults is enough to measurably decrease leg lean mass and trigger this resistance. Staying physically active, particularly with resistance training, and eating adequate protein become increasingly important with each passing decade.
Sleep Changes the Fat-to-Muscle Ratio
Sleep is an underappreciated factor in body composition. When you cut calories to lose weight, the amount of sleep you get influences whether you lose mostly fat or mostly muscle. Research comparing caloric restriction with and without sleep restriction found that people who slept less lost a higher proportion of their weight as lean mass rather than fat. Sleep deprivation also lowers leptin, the hormone that signals fullness, making it harder to stick with any eating plan.
The mechanism is straightforward: poor sleep disrupts the hormonal environment your body needs to repair muscle tissue and mobilize fat stores. It shifts your body toward preserving fat and breaking down muscle, which is the opposite of what most people want when they’re trying to improve their body composition.
How Fat Storage Location Matters
Body composition isn’t just about how much fat you carry. It’s also about where that fat sits. Fat stored deep in the abdomen, around your organs, is far more metabolically active than fat stored under the skin on your hips or thighs. Visceral fat releases fatty acids directly into the liver through the portal vein, which ramps up sugar production and lipid synthesis and drives insulin resistance.
Subcutaneous fat, by contrast, can actually serve a protective buffering role, safely storing excess energy away from your organs. Problems start when this subcutaneous tissue can no longer expand to meet demand. The fat cells become oversized, local oxygen supply drops, and an inflammatory cascade begins. Once that tipping point is reached, additional calories get redirected to visceral deposits and the liver. This is why two people at the same body fat percentage can have very different metabolic health profiles depending on where their fat is distributed.