1.5 Identify 3 main sources of energy from food

This guide will help you answer 1.5 Identify 3 main sources of energy from food.

Understanding where we get energy from in our diets helps us to make better choices for health and wellbeing. Food contains a range of nutrients, but only some of these provide energy that our bodies can use. Energy in this context refers to the ability to perform physical activity, power our internal functions, and keep us alive and alert.

Three groups of nutrients are recognised as the main sources of energy in the human diet. These are:

• Carbohydrates
• Fats
• Proteins

Each of these has a different role in the body and provides differing amounts of energy. In this guide, we will cover each source, and show where they come from in our daily diets.

Carbohydrates as a Main Source of Energy

Carbohydrates are the body’s favoured and most immediate source of energy. They are often thought of as sugars and starches. When eaten, carbohydrates are broken down into glucose. Glucose moves into the bloodstream and is carried around the body, where cells use it to produce energy.

Carbohydrates provide 4 kilocalories (kcal) per gram. The term kilocalorie is another name for what we often call a “calorie” on food labels. This is a measure of how much energy is produced when a food is digested and used by the body.

Main Types of Carbohydrates

• Sugars: Found in fruit, honey, milk, and foods with added sugar like sweets and soft drinks.
• Starches: Present in foods such as bread, rice, pasta, potatoes, and cereals.
• Fibre: Part of the plant that the body cannot digest. Fibre does not provide energy, but helps the gut to function and keeps us feeling full.

Where Carbohydrates Are Found

• Rice, pasta, bread, potatoes, breakfast cereals
• Fruits like apples, bananas, pears
• Vegetables such as carrots, peas, corn
• Sugar-rich foods like biscuits, cakes and sweets

Why Carbohydrates Matter

Carbohydrates are important because they provide quick energy. For someone who is active or has a physically demanding day, carbohydrates help to keep energy levels up. Wholegrain and fibre-rich sources give energy more slowly, helping us to feel satisfied and reducing the risk of sharp drops or spikes in blood sugar.

Fats as a Main Source of Energy

Fats are a dense source of energy in the diet. They provide 9 kilocalories per gram, which is more than double the amount from carbohydrates or protein. The body stores any extra energy from food as fat, to be used later when energy is needed but food is not available.

Fats are broken down into fatty acids and glycerol. These can be used straight away for energy, or stored for use during periods of fasting or longer gaps between meals.

Types of Fats

• Saturated fats: Usually solid at room temperature. Sourced mainly from animal products like butter, cheese, cream, and fatty meats.
• Unsaturated fats: Usually liquid at room temperature. Come from plant-based oils like olive oil, sunflower oil, avocado, and fatty fish.
• Trans fats: Manufactured fats found in some processed foods, such as some margarines, biscuits, and cakes.

Where Fats Are Found

• Butter, margarine, oils (olive, sunflower, rapeseed)
• Cheese, cream, full-fat yoghurt
• Meat, especially fatty cuts and processed meats (sausages, burgers)
• Oily fish such as salmon, mackerel, sardines
• Nuts, seeds, avocados
• Ready meals, cakes, biscuits, crisps

How the Body Uses Fats for Energy

When carbohydrates are low, the body turns to fat for energy. Fat is particularly important for endurance exercise or long periods between meals. The body stores fat under the skin and around organs for use in emergencies. This store can support the body for days or even weeks, if food is in short supply.

Proteins as a Source of Energy

Protein is best known for its role in growth, repair and maintenance of body tissues. It helps build muscles, repair wounds, and supports our immune system. However, protein can also be used for energy if carbohydrate and fat intake is low or during intense exercise.

Each gram of protein provides 4 kilocalories of energy, the same as carbohydrates. If the body cannot get enough energy from carbohydrates and fats, it will start to use protein from the diet or even break down muscle tissue to get energy.

Types of Proteins

• Complete protein: Contains all the amino acids the body needs. Mostly from animal sources, such as meat, fish, eggs, dairy.
• Incomplete protein: Lacks one or more essential amino acids. Often from plant sources, like beans, lentils, nuts and seeds.

Where Protein Is Found

• Lean meat, poultry, fish
• Eggs, milk, cheese, yoghurt
• Beans, lentils, chickpeas
• Soya products (like tofu, soya milk)
• Nuts and seeds
• Quorn and other meat alternatives

How the Body Uses Protein for Energy

The body prefers to keep protein for growth and repair, but will convert it for energy if there are not enough carbohydrates or fats. This process happens during starvation, extreme dieting, or in untreated diabetes. Using protein for energy is less efficient, and can reduce muscle mass if continued for long periods.

Comparing the Energy Provided by Each Source

Different nutrients provide different amounts of energy. Here is a quick summary:

• Carbohydrates: 4 kcal per gram
• Protein: 4 kcal per gram
• Fat: 9 kcal per gram

Fat provides the most energy per gram. This is why fatty foods contain a lot of calories in small amounts. Carbohydrate and protein have equal energy per gram, but carbohydrates are usually the main energy source in a balanced diet.

Recommended Intakes for Each Energy Source

Health guidelines offer advice on how much of each nutrient should provide energy in the diet.

Carbohydrates

• Make up about half of our daily calorie intake
• Most energy should come from starchy and wholegrain foods
• Limit intake of free sugars (sugar added to food and drinks)

Fats

• Should make up less than a third of total energy
• Saturated fat should be less than 10% of total energy
• Choose unsaturated fats where possible

Protein

• Needs vary by age and situation
• For adults, about 0.75 grams per kilogram of body weight each day
• Should make up about 10-15% of daily energy intake

How Energy is Released from Food in the Body

The body has a few steps it follows, no matter the food source:

1. Eating: Food is chewed and mixed with saliva to start breaking it down.
2. Digestion: Stomach acid and enzymes break food into smaller parts.
3. Absorption: Nutrients move into the bloodstream through the gut wall.
4. Metabolism: Cells use these small molecules to make energy, using oxygen in a process called respiration.

Energy is released for movement, keeping warm, running body systems, and even thinking.

Factors That Influence Energy Needs

Everyone needs energy, but the amount needed changes from one person to another. Factors that affect energy requirements include:

• Age (children and teenagers need more as they grow)
• Sex (males usually need more than females)
• Activity level (active people need more energy)
• Health status (illness or injury can raise needs)
• Pregnancy and breastfeeding (increases requirements)

Eating more energy than needed can lead to weight gain, as excess energy is stored as fat. Eating less energy than the body burns leads to weight loss.

Practical Examples from Daily Life

Let’s see what typical meals contain in terms of energy:

Breakfast

• Porridge oats (carbohydrate, small amount of protein)
• Low-fat milk (carbohydrate, protein, small amount of fat)
• Sliced banana (carbohydrate, mainly sugar)

Lunch

• Wholegrain sandwich (carbohydrate and some protein)
• Chicken breast (lean protein)
• Salad with olive oil dressing (fat from oil)

Snack

• Handful of nuts (mostly fat, some protein, low carbohydrate)
• Low-fat yoghurt (protein, some carbohydrate)

Evening meal

• Grilled salmon (protein, fat)
• Steamed potatoes (carbohydrate)
• Green vegetables (carbohydrate, fibre)

These examples show that most meals provide a mix of energy sources. The healthiest diets include all three sources, in sensible amounts and varied forms.

Technical Terms

• Kilocalorie (kcal): The unit used to measure energy in food.
• Metabolism: The sum of all the chemical reactions in the body that turn food into energy.
• Amino acids: The building blocks of protein.
• Fatty acids: The components of fats broken down by digestion.
• Starch: A carbohydrate made of long chains of glucose, found in foods like bread and rice.
• Free sugars: Sugars added to foods and drinks, and sugars naturally found in things like honey and fruit juices.

Energy From Food and Health

Energy from food is needed for:

• Physical activity—walking, running, lifting, movement
• Basic body functions—breathing, heart beating, keeping warm
• Growth and repair—building new cells and tissues

Problems can arise if the balance of energy sources is wrong:

• Too much fat is linked to heart disease and obesity
• Too many sugars contribute to tooth decay and diabetes
• Too little protein can reduce muscle mass and immunity

Eating a variety of foods from all three energy sources helps support health.

Final Thoughts

All the energy we need to live, move, and think comes from three main sources in our diet: carbohydrates, fats, and proteins. Carbohydrates provide quick and easy energy for most daily needs. Fats are energy dense and support the body during longer gaps between meals or during exercise. Protein, while vital for building and repairing tissues, can also give energy, especially if the body is low on carbohydrate and fat.

Choosing balanced portions of each helps support health, maintain weight, and prevent disease. It’s wise to favour wholegrain carbohydrates, limit saturated and trans fats, and snack on foods that give more lasting energy. Each person’s needs are unique and may change over time.

To support good health, meals should include a mix of these energy sources, mostly from starchy foods and vegetables, healthy fats, and moderate protein from both animal and plant sources. A varied and balanced diet, mindful of portion sizes, supports wellbeing and helps avoid health problems linked to energy imbalance.

Learning about the three main energy sources gives you more control over your diet and can help you support others with positive changes. This foundation makes it easier to plan nourishing meals and meet everyday needs for yourself and those you care for.