Body composition describes what the body is made of. There are several ways to categorize the components of the human body, as shown below.
Using the molecular model, we can quantify four components: body water, protein, minerals, and body fat.
Body composition varies by sex, age, and individual traits and is closely related to personal condition. We can assess balanced development through body composition analysis.
The body consists of four components: water, protein, fat, and minerals. Their typical proportions are: body water 55%, protein 20%, body fat 20%, minerals 5%. Balance among these components is fundamental to health.
1. Water balance
Body water is divided into intracellular fluid (ICF) and extracellular fluid (ECF). Normally the ICF:ECF ratio is 2:1. These fluids make up 50–60% of body weight, supply nutrients and oxygen to cells, and carry CO2 and waste for processing. The distribution remains stable: two-thirds inside cells and one-third outside.
When metabolism is impaired, edema or dehydration may occur and this balance is lost. ECF changes are often more evident. In edema, both ICF and ECF may increase, but ECF rises more, increasing its share of total body water. In dehydration, ECF decreases more than ICF because ICF is more vital for survival.
Various diseases disturb this distribution and increase ECF, leading to edema. In elderly and chronic disease patients with muscle atrophy, ECF may be relatively elevated even if edema is not apparent, resulting in malnutrition-related edema.
2. Protein balance
Protein is an adhesive component formed by many chemicals in cyclic structures. Muscles contain abundant protein; bone and fat also contain some. Protein deficiency means inadequate musculature in the limbs and organs. Since muscles power movement and organ function, deficiency leads to weakness; in many cancer and chronic disease patients, malnutrition causes organ failure.
Many overweight individuals have low muscle mass despite higher body weight. Some thin people are weak, others strong—strength relates closely to body composition. Healthy muscle is about 73% water and 27% protein. Muscle mass matters more than fat mass. When energy intake is inadequate, protein is broken down for energy; chronic disease patients should take special care.
3. Fat balance
Excess nutrients are stored as body fat under the skin and around visceral organs. Carbohydrates, protein, and fat are the three energy sources. Each gram of carbohydrate or protein yields 4 kcal, while fat yields 9 kcal and stores energy densely. Carbs and protein retain more water, making them less storage-efficient.
Obesity is not simply being heavy. A heavy wrestler may not be obese, while some lighter young women may have excessive fat. Properly, obesity is explained by the ratio of fat to muscle. Fat stores/releases energy; muscle consumes energy to move. When this balance is broken and fat predominates, true obesity results.
Typical percent body fat: women 23±5%, men 15±5%. For boys under 17, about 20% at age 7, decreasing by ~0.5% per year to 15% by age 17. If percent fat is below standard, either high-activity muscular builds (healthy) or malnutrition (unhealthy) may be present.
About 50% of fat is in the limbs, ~5% intramuscular, and ~45% in the trunk. Abdominal fat ratio (fat distribution) is assessed by WHR (waist-to-hip ratio): normal values are 0.75–0.85 for males and 0.70–0.80 for females.
In many young women and adolescents, weight may be normal or low but body fat percentage is high—low-muscle obesity. Excess fat relative to muscle circulates and deposits on vessel walls, leading to atherosclerosis, thicker, narrower vessels, hypertension, and potential stroke from thrombus detachment.
4. Mineral balance
Minerals support the body’s structure and shield vital brain organs. Protein-calcium complexes form strong bone. If calcium is lost in urine, bone density decreases, creating empty spaces and osteoporosis. Mineral levels correlate with muscle mass; body composition imbalances from excess fat and low muscle contribute to poor bone mass and density. Active individuals with robust muscles and normal fat usually maintain bone mass and are less prone to osteoporosis.
5. Weight loss and body composition changes
Diet, exercise, and body composition are closely related. To lose weight successfully, understand the principle: energy intake must be less than expenditure. Changes in substrates and body composition during energy use occur in four stages.
Stage 1: Blood glucose supplies energy. When insufficient, hepatic glycogen is broken down to glucose. Water excretion increases during glycogenolysis, so weight drops rapidly.
Stage 2: After fat-derived glucose is used, protein is broken down into glucose. Muscle is lost, so exercise must be increased to synthesize muscle. Protein contains water, so water excretion also increases.
Stage 3: Protein breakdown decreases and fat becomes the main energy source. Fat yields twice the energy per unit compared with carbs/protein, so weight loss slows. The brain uses only glucose; after ~2 days of fasting, glucose and glycogen are depleted. The brain then uses ketone bodies, peaking after ~10 days. In starvation, diabetes, low-carb/high-fat diets, ketones may exceed demand, acidifying blood and threatening life.
6. FitMao body composition analyzer
The popular FitMao analyzer uses bioelectrical impedance analysis (BIA). Impedance is resistance to current. Fat and skin conduct poorly versus muscle and blood, so impedance is higher with more fat for the same weight. Some high-end analyzers measure segmental impedance (right/left arms, trunk, right/left legs). All BIA devices measure total body water because water inversely relates to impedance. Healthy muscle contains about 73.3% water, so water content indicates muscle mass; muscle correlates with mineral content, enabling mineral estimation. Body fat is calculated as body weight minus lean body mass (LBM: water + protein + minerals).
FitMao analyzers are primarily for fitness scenarios. For medical contexts, stricter measurement methods are required. The data can indicate poor condition or chronic issues but is not for diagnosing specific diseases.
Body composition analysis has two major purposes:
1. Preventive function. For example, if fat accumulation is found, you may be advised to exercise to lose weight. If you have body edema, you may be advised to undergo further examination of your heart and kidney function. If your inorganic matter content is low, it is best to check for osteoporosis.
2. Function for monitoring changes in body composition. It can be used to measure changes in body composition before and after sports exercise, during nutritional therapy, especially changes in fat and muscle. Or to measure changes in body water.
7. The value of muscle
Muscle cannot be stored indefinitely like fat. Unlike fat, which should not exceed thresholds, higher muscle mass is not harmful and raises basal metabolic rate and benefits bone. The best outcome is balance: between fat and muscle, across body segments, and in fluids. Balance arises from health, proper exercise, and diet. Among components, fat and muscle levels are the basics for assessing balance. If muscle exceeds fat on the obesity chart, that’s good; if equal, fat is effectively higher, suggesting potential issues like obesity. But do not judge by fat and muscle alone; consider health status, abdominal fat, and fluid balance as well.
8. What “underdevelopment” means
People often think of bodybuilders’ triceps as “muscular,” and thinness as “underdeveloped.” In reality, appearance can be misleading. Regular exercisers with developed muscles feel firm; those with low weight often have low muscle and fat and appear underdeveloped. Some who don’t look underdeveloped actually are, due to low muscle masked by fat—common in sedentary young women and children. Underdevelopment also describes muscle degeneration in the elderly.