1.9 Identify factors affecting a person’s energy requirements

This guide will help you answer 1.9 Identify factors affecting a person’s energy requirements.

Every person needs energy to function. This energy is measured in kilocalories (kcal) or kilojoules (kJ). Energy requirements mean the amount of energy a person must get from food and drink to support normal body functions, movement, growth, and repair.

The number of calories you need will vary based on several factors. These need to be identified and understood if you want to support your own health or advise others. Not everybody has the same requirements, and several elements play a part in this difference.

Age

Age is one of the largest factors. At different life stages, our bodies work differently and need different fuel.

Children and Adolescents

Young children and teenagers grow rapidly. Their bodies build new tissues, and their brains develop fast. Growth and physical activity demand more calories. For example, a teenager who is still growing may often need more calories than an adult. Boys often need more calories than girls during adolescence due to larger muscle mass and physical growth.

Adults

Once growth stops, the body’s energy needs stabilise. Adults need enough calories to support their basal metabolic rate (BMR)—the energy uses at rest—and physical activity. If an adult continues to eat like an adolescent but is less active, weight gain is likely.

Older Adults

As people age, muscle mass and physical activity usually decrease. Metabolism slows down. With reduced movement and muscle mass, older people need fewer calories. Eating too many can increase fat stores, while too few can lead to undernutrition.

Gender

Gender, sometimes called biological sex, plays a part in energy needs. On average, men need more calories than women. This is mainly because men tend to have more lean muscle. Muscle uses more energy than fat, even when resting.

Men’s bodies often create more testosterone, which helps maintain muscle mass. Women’s bodies, influenced by oestrogen and other hormones, store more fat. This difference in muscle to fat ratio means differing calorie use. For example, an active adult male will generally need more calories compared to an active adult female of the same age and size.

Body Size and Composition

Your overall size—height and weight—changes your energy requirements. A taller or heavier person uses more energy, even when sitting or lying down. Larger organs, more tissue, and a bigger body surface increase calorie use.

Body composition means the amount of muscle compared to fat. Muscle burns more energy at rest than fat. So, someone with a higher proportion of muscle needs more calories than someone of the same size with more body fat.

• More muscle = higher calorie use at rest
• More fat = lower calorie use at rest

This is why very fit people or athletes sometimes need more food, even if they are not currently active.

Physical Activity Level

The more you move, the more energy you use. Activity can range from housework, walking, and standing, to formal exercise or sports.

Sedentary Lifestyle

If you sit most of the day, at a desk or in front of a screen, you need less energy. Not using muscles much means fewer calories required.

Active Lifestyle

Manual workers, athletes, or people who have active jobs use up many more calories. Energy requirements go up with any extra movement:

• Walking to school or work
• Doing physical hobbies (gardening, cycling, swimming)
• Playing sports
• Doing housework

Exercise and Training

Frequent exercise builds muscle and burns calories. High-intensity or long-duration sports, such as running, boxing, or swimming, massively raise energy needs. Athletes often eat much more than non-athletes, even of the same age and size. This prevents fatigue and helps recovery.

Basal Metabolic Rate (BMR)

BMR is the energy your body burns at rest. This powers your heart, breathing, digestion, temperature control, and cell repair—even when you sleep.

Several elements affect BMR:

• Age (younger people tend to have higher BMR)
• Gender (men usually have higher BMR)
• Body size and muscle mass (more muscle, higher BMR)
• Genetics (everyone’s metabolism is slightly different)
• Hormones (thyroid hormones, for example, control metabolism speed)
• Illnesses (certain conditions like hyperthyroidism or hypothyroidism affect BMR)

A high BMR means more energy used at rest. A lower BMR means fewer calories are burned, so daily energy needs are lower.

Growth, Pregnancy, and Lactation

Periods of fast growth, such as infancy, childhood, and adolescence, need extra energy. The body is building new bones, tissues, muscle, and organs.

Pregnancy is another special time. A pregnant woman needs extra calories to support the baby’s growth, especially in the second and third trimesters. More energy is needed for building the placenta, increasing blood supply, and other physical changes.

Breastfeeding (lactation) increases energy needs further. Producing milk uses up a lot of calories. If these needs are not met, it can affect the mother’s health and her ability to feed the baby.

Illness, Injury, and Recovery

The body responds to illness, injury, or surgery with changes to energy use.

When repairing tissues, fighting infection, or healing wounds, the body often burns more calories.

Examples include:

• Recovery from burns—very high energy demand
• Infection or fever—increases BMR
• After surgery—body needs energy for healing

Long-term illness or conditions (for example, cancer or some digestive disorders) might need diet changes to maintain weight and strength.

Some illnesses or medicine side effects can reduce appetite. This can make it difficult for people to meet increased requirements, leading to malnutrition.

Environmental Factors

Temperature and environment play a part. In cold weather, the body uses more energy to keep warm. In very hot conditions, people may move less or lose appetite, but sweat loss can affect nutrient and water balance.

Other examples:

• High altitude—the body works harder to get oxygen, increasing energy needs
• Extreme climates may affect activity level, which in turn changes calorie needs

Hormonal Factors

Hormones act as chemical messengers in the body. Some change the amount of energy needed.

• Thyroid hormones can increase or decrease metabolism
• Insulin, from the pancreas, helps use glucose for energy
• Cortisol and adrenaline (stress hormones) increase energy use

Women’s energy needs can change with the menstrual cycle due to hormonal shifts, water retention, or changes in appetite.

Genetics

Genetic differences mean that some people burn energy more quickly than others. This is sometimes called having a “fast” or “slow” metabolism. Metabolism is the set of life-sustaining chemical reactions in the body.

Family traits may mean a person stores fat more easily, uses food differently, or gets hungry more often. People can inherit these traits from their parents.

Metabolic Conditions

Some conditions change the way the body uses, processes, or stores energy. Examples include:

• Hyperthyroidism—thyroid overworks, raising energy use
• Hypothyroidism—slows metabolism, so energy needs drop
• Diabetes—problems with insulin can affect how the body uses sugar

People with these conditions need advice from healthcare professionals. Diet and calorie needs vary.

Medications

Certain medicines can boost or reduce appetite. Some increase metabolism. For example, steroids or some mental health medicines can increase weight gain and appetite. Others, like those for hyperactivity, can reduce appetite and calorie intake.

Being aware of these side effects is important, particularly for people with long-term prescriptions. Adjustments in diet may be needed to avoid under- or over-nutrition.

Activity Level at Work or Home

Workplaces and household routines affect calorie needs. Jobs that involve heavy lifting, standing, carrying, or much movement require more energy.

In contrast, sitting jobs (office work, driving) use fewer calories. Being unemployed, retired, or at home may mean less daily movement.

Household tasks, children, pets, and hobbies can also add to total energy used, changing overall needs.

Dietary Habits

Eating patterns influence energy requirements both directly and indirectly.

• Skipping meals may slow metabolism.
• Overeating, especially high-calorie food, promotes weight gain.
• Irregular eating patterns can affect hunger cues and metabolism.

People who ‘graze’ or snack often may eat more energy each day than those who have structured mealtimes.

Emotional Factors

Emotions affect appetite and eating habits:

• Stress or anxiety may boost or lower appetite.
• Depression can reduce motivation to eat.
• Comfort eating, boredom, or emotional triggers can increase energy intake.

Eating more or less because of emotions can change calorie needs over time, especially if habits are ongoing.

Special Populations

Certain groups have distinct energy requirements:

• Athletes and sportspeople
• Pregnant and breastfeeding women
• Disabled people with limited movement
• People on special diets (for medical or ethical reasons)
• Babies and children
• Older adults

Each group has different patterns in muscle, fat, growth, hormone activity, or lifestyle factors.

Socioeconomic Status

Income and social status can limit access to nutritious food. People with less money may not get enough energy, or get it mostly from unhealthy sources (cheap, high-calorie processed foods). Both undernutrition and obesity can result, even within the same community.

Access to facilities for exercise or healthy eating options plays a role in energy requirements and habits.

Culture and Beliefs

Culture shapes food choices. Some cultural diets are lower or higher in energy. Religious fasting or holiday feasts affect daily calorie intake.

Norms and expectations about body size, women’s roles, and types of work all influence energy needs across different groups and communities.

Summary of Key Factors

Energy requirements are shaped by:

• Age
• Gender
• Body size and composition
• Physical activity level
• BMR (Basal Metabolic Rate)
• Periods of growth or pregnancy
• Illness, injury, or recovery
• Environment and climate
• Hormonal influences
• Genetics
• Medical conditions
• Medication use
• Work and household activity
• Dietary habits and meal patterns
• Emotional health
• Socioeconomic status
• Cultural practices

Each person is unique. Two people of similar age and size may still need different amounts of energy each day.

Supporting Healthy Choices

Knowing what affects energy needs can help people make informed decisions about food and activity. Eating the correct number of calories helps avoid weight gain or loss, supports growth in children, promotes healing, and allows for an active lifestyle.

Use practical knowledge, like food labels, meal planning, and activity tracking, to meet your own needs. Listen to hunger cues. Stay active when possible. Pay attention to health changes that may mean your energy needs have shifted.

Final Thoughts

Our bodies need energy for every process, from digesting food to thinking and breathing. Many factors decide how much energy you need each day. Some, such as age, size, and sex, can’t be changed. Others, like activity level and food choices, are more flexible.

By learning about these influences, you can help yourself and others make sensible, healthy choices. These adjustments can affect weight, wellbeing, and ability to fight illness or recover from injury. If you work with people in health, care, or nutrition, this knowledge helps you support others better.

Everyone’s needs change with health, age, hormones, environment, and life events. Be prepared to adjust food, drink, and activity to suit these changes. Keep learning and stay aware of your own signals and experiences.

Gaining insight into the factors affecting energy requirements allows people to eat and live in a way that suits their needs. Keep supporting yourself and those you care for, paying attention to the many ways our bodies use, store, and call for energy every day.