Body mass depends on the balance between energy intake and energy expenditure. When this balance shifts for long periods, the result can be weight loss, weight gain, or eventually obesity.

Energy Balance and Body Mass

Energy Intake and Expenditure

Energy from food is used for respiration and normal life processes. Body mass tends to stay stable when energy intake matches energy expenditure over time. Expenditure includes basal metabolic rate, physical activity, and maintaining body temperature. A long-term mismatch between intake and expenditure is called energy imbalance.

Short-term fluctuations happen, but a sustained imbalance changes body mass. If intake is lower than expenditure, stored fuels are used. If intake is higher, excess energy is stored.

Negative Energy Balance and Weight Loss

If a person remains in negative energy balance, the body needs more energy than the diet supplies. At first, it can use stored glycogen in the liver and muscles. As the deficit continues, triglycerides in adipose tissue are broken down to provide energy. In prolonged or severe energy deficit, proteins from muscle may also be used. Because stored materials are being broken down, body mass decreases.

Weight loss can be useful if a person has excessive fat stores, but large or prolonged losses can be harmful. Consequences of negative energy balance may include:

• reduced fat stores

• decreased muscle mass

• tiredness and weakness

• reduced ability to maintain body temperature

• lower capacity for normal physical activity

In children and adolescents, long-term energy deficit can also reduce normal growth and development. The rate of weight loss depends on how large the deficit is, how long it lasts, and the person’s starting body composition.

Positive Energy Balance and Weight Gain

If energy intake is greater than energy expenditure, the surplus cannot be used immediately and is stored. Some carbohydrate can be stored as glycogen, but glycogen stores are limited. Much of the excess energy is converted to and stored as fat in adipose tissue.

This labeled micrograph shows adipose tissue, where adipocytes are specialized for storing energy as triglyceride in large lipid droplets. The image helps connect ‘fat storage’ to a real tissue structure, making it easier to visualize how increasing triglyceride storage enlarges adipose tissue. This is the tissue-level basis for long-term increases in body fat during persistent positive energy balance. Source

Because fat is an efficient energy store, repeated surpluses over days, weeks, and months lead to increased body mass.

Weight gain does not always mean fat gain alone. Small short-term increases in body mass can also reflect extra water or glycogen. However, persistent positive energy balance usually increases fat stores. This is more likely when a person regularly eats energy-dense foods and has low levels of physical activity.

Even a small daily surplus can cause gradual but substantial weight gain if it continues over a long period. Persistent positive energy balance can eventually lead to obesity.

Development of Obesity

Obesity develops when excess energy intake over time causes a large increase in body fat. Adipose tissue enlarges as more triglyceride is stored. This usually happens gradually rather than suddenly. A person may not notice the change at first, especially if body mass increases by only a small amount each week.

Modern lifestyles can promote obesity because food is often constantly available, highly processed, and energy dense, while work and transport may involve little movement. If eating behavior is not matched by activity level, energy intake can repeatedly exceed energy expenditure.

The body has mechanisms that influence hunger and fullness, but these do not always prevent weight gain. Large portion sizes, frequent snacking, and low activity can all contribute to a long-term surplus. Once obesity develops, movement may become less comfortable, which can reduce physical activity further and make the imbalance worse.

Biological Effects of Obesity

Obesity is a consequence of long-term positive energy balance, but it can also produce further biological effects. Excess body fat can:

This figure summarizes major health risks that become more likely as excess body fat accumulates, including type 2 diabetes, hypertension, and cardiovascular disease. It reinforces that obesity is not just increased mass but a condition that disrupts normal physiology across multiple organ systems. Use it to support evaluative exam answers about biological consequences. Source

• place greater strain on joints

• reduce mobility and exercise tolerance

• increase the likelihood of high blood pressure

• raise the risk of cardiovascular disease

• increase the risk of insulin resistance and type 2 diabetes

These effects occur because excess adipose tissue changes normal body function. As body fat increases, organs and tissues may need to work harder to maintain normal conditions. Obesity is therefore not just an increase in mass; it is a condition that can significantly affect health.

Interpreting Changes in Body Mass

Changes in body mass should be understood over time, not from a single meal or one day of activity. A person may remain at a similar mass when intake and expenditure are close overall, even though both vary from day to day.

The key pattern is:

• negative energy balance leads to weight loss

• positive energy balance leads to weight gain

• long-term, substantial positive energy balance can lead to obesity

For exam answers, link the direction of the imbalance to what happens to energy stores. If energy is lacking, stored reserves are used. If energy is in excess, it is stored, mainly as fat.