Energy Needs and Nutritional Analysis
Where Does Energy Come From — and How Much Do You Need?
Every time you move, breathe, digest food or maintain your body temperature, you use energy. That energy comes from the macronutrients in food: carbohydrates and protein each supply 4 kcal (kilocalories) per gram; fat supplies 9 kcal per gram; alcohol supplies 7 kcal per gram (not on the spec but worth knowing for context). The total energy a person needs each day depends on two factors: their basal metabolic rate (BMR) and their physical activity level.
Basal Metabolic Rate (BMR)
BMR is the energy your body burns at rest — doing nothing but keeping the heart beating, the lungs breathing and the cells functioning. It accounts for roughly 60–75% of total daily energy use. Several factors affect BMR:
- Age — BMR declines with age as muscle mass decreases and metabolism slows. A 16-year-old has a higher BMR per kilogram of body weight than a 60-year-old.
- Gender — males typically have higher BMR than females of the same weight because they generally have more muscle mass. Muscle tissue burns more energy at rest than fat tissue.
- Body composition — a person with more lean muscle mass has a higher BMR. A marathon runner has a higher BMR than a sedentary person of the same weight.
- Hormones — thyroid hormones regulate metabolic rate; an overactive thyroid raises BMR, an underactive thyroid lowers it.
- Pregnancy — BMR increases during pregnancy because the body is supporting fetal development and preparing for breastfeeding.
BMR is the energy the body needs at complete rest to maintain vital functions. It is affected by age (declines with age), gender (males typically higher), body composition (more muscle = higher BMR), hormones and life stage (pregnancy increases it). BMR accounts for approximately 60–75% of total daily energy expenditure.
Physical Activity Level (PAL)
Physical activity level (PAL) is a multiplier that reflects how much energy a person uses through movement on top of their BMR. A sedentary adult (desk job, little exercise) has a PAL of roughly 1.4; a moderately active adult (some walking, 3–4 sessions of exercise per week) has a PAL of around 1.6–1.7; a very active person (manual job or elite athlete in daily training) can have a PAL above 1.9.
Total daily energy requirement = BMR × PAL. For example, a 15-year-old girl with a BMR of 1,400 kcal/day who plays netball four times a week and walks to school (PAL ≈ 1.7) needs approximately 2,380 kcal per day. If she eats less than this consistently, she loses weight; if she eats more, she gains weight.
Energy Balance
Energy balance means intake equals expenditure. Positive energy balance (more in than out) leads to weight gain over time, as excess energy is stored as body fat. Negative energy balance (more out than in) leads to weight loss. For a person trying to manage their weight, increasing PAL (more physical activity) or reducing energy-dense food intake are both effective strategies.
A common misconception is that eating fat directly causes weight gain while eating carbohydrate does not. In reality, weight gain comes from positive energy balance — consuming more total kilocalories than you expend. Fat contributes more kcal per gram (9 kcal/g vs 4 kcal/g for carbohydrate or protein), so high-fat foods are energy-dense and easy to overconsume. But a diet high in refined carbohydrates and sugar can equally lead to positive energy balance and weight gain. The focus should be on total energy intake relative to PAL, not on demonising any single macronutrient.
Recommended Percentages of Energy from Nutrients
Current guidelines specify that the balance of macronutrients in the diet should be:
- Protein: 15% of total energy intake.
- Fat: 35% or less of total energy intake (of which no more than 11% from saturated fats).
- Carbohydrate: 50% of total energy intake — of which 45% from starches, lactose in milk and fruit sugars, and a maximum of 5% from free sugars (added sugars, honey, syrups and fruit juices).
These are percentage targets for the whole diet over time, not for every individual meal. In practice, a typical UK diet has too many free sugars and saturated fats and not enough dietary fibre. A dinner of grilled salmon (protein and unsaturated fat), a large portion of brown rice (complex carbohydrate) and steamed broccoli (fibre and micronutrients) moves the balance in the right direction.
Target energy split: protein 15%, fat 35% or less, carbohydrate 50% (of which maximum 5% from free sugars, 45% from starches, lactose and fruit sugars). These are the exact figures the written paper may ask for directly.
Carrying Out Nutritional Analysis
Nutritional analysis means calculating how much energy and how many nutrients a recipe or meal provides per serving. In a school kitchen or professional setting this is done using nutritional analysis software (e.g. Nutritionix, Cronometer or bespoke food-technology software). In an exam, it is done using food tables (printed tables of nutritional values per 100 g for each ingredient).
The method is: identify each ingredient and its weight in the recipe; look up its nutritional values per 100 g in the food table; scale to the actual weight used (e.g. if the table gives values per 100 g and you use 250 g, multiply by 2.5); add up all the ingredient contributions to get the total for the whole recipe; then divide by the number of portions to get the value per serving. This can be done for energy (kcal), protein (g), fat (g), carbohydrate (g), fibre (g), and individual vitamins or minerals.
Once you have the analysis, you can compare it to the Dietary Reference Values (DRVs) or Reference Intakes (RIs) for the target person and identify nutrients that are too high or too low. If the recipe is too high in saturated fat, you might substitute full-fat cream for a lower-fat yoghurt, or grill the meat instead of frying. If it is low in iron, you could add spinach, kidney beans or fortified cereal to the recipe.
The written paper may present a recipe analysis and ask you to suggest modifications. The key is to apply the energy percentages and RIs as benchmarks. Common modifications: (1) Too much saturated fat — swap butter for an unsaturated spread or olive oil; use skimmed or semi-skimmed milk instead of full-fat; remove the skin from chicken before cooking. (2) Too much salt — use herbs and spices to season instead of added salt; choose low-salt stock cubes; reduce the amount of processed ingredients. (3) Too little fibre — swap white pasta for wholemeal; add lentils or beans; include more vegetables. (4) Too much free sugar — reduce the amount of added sugar; use fresh fruit to sweeten instead of syrup or jam. Each modification should be linked back to a specific health benefit.