Calories are the unit of measurement which determines the amount of energy the food we eat gives us. You'll find the number printed on food packaging as Kcal, often next to a similar unit of energy called kilojoule (kJ).
A simple analogy to understand calories is to imagine the body as a car. Calories are the fuel which keeps the body moving. Even at rest, the body is burning calories to keep the blood pumping around the body and maintain the ideal temperature. When you exert yourself, - equivalent to pressing down the accelerator - the body uses up the energy faster.
If you regularly exert yourself, you will need to consume more calories to ensure your body has enough energy to power you through your workout. Similarly, you should consume fewer calories if you do not exercise as much.
How many calories should I be consuming in a day?
According to Public Health England, men should consume 2,500 calories and women should consume 2,000 calories per day.
"20% of daily energy and nutrients should come from breakfast and 30% from both lunch and dinner. All other snacks and drink consumed between meals should make up the difference." ~ Public Health England
While this is a good guideline to follow for most people, these numbers are contested as they do not factor in a persons' energy expenditure. The rate at which a person uses up or "burns" calories differs from person to person depending on their metabolism.
Metabolism is the speed at which your body can process chemicals in your body to keep it alive. For example, a person with a higher, more active metabolism may need to intake more calories than someone with a slower metabolism. As you age, your metabolism typically slows down.
By combining the rate of your metabolism with the recommended daily intake of calories, you can fairly accurately determine the amount of calories your body needs to stay alive - this is referred to as your basal metabolic rate (BMR).
What is Basal Metabolic rate (BMR)?
Basal metabolic rate is the speed at which your body consumes energy at rest. So, if you were lying in your bed all day, your BMR is the amount of calories your body would need to maintain its normal function. These calories are used to keep the heart beating and remain a constant ideal temperature through homoeostasis.
Your basal metabolic rate is not the amount of calories you should consume daily. However, you will need your BMR to work out your daily calorie intake.
How do I find out my basal metabolic rate (BMR)?
Note: The following information is not medical advice. The equations below determine a close approximation of the amount of calories a person should consume based on their age, height, weight and activity level. We encourage you to speak to your GP or a qualified dietician for further advice on daily calorie intake.
To work out your BMR and recommended daily intake of calories, you will need:
- A tape measure (cm)
- Scales (kg)
- A calculator
The following equations are based on The Schofield Equation. They have been simplified.
For men:
| Age | Equation (kcal per day) | SEE (standard error of estimation) |
|---|---|---|
| 18 - 29 | 15.057 x Weight (Kg) + 692.2 = BMR | 156 |
| 30 - 59 | 11.472 x Weight (Kg) + 873.1 = BMR | 167 |
For women:
| Age | Equation (Kcal per day) | SEE (Standard error of estimation) |
|---|---|---|
| 18 - 29 | 14.818 x Weight (Kg) + 486.6 = BMR | 120 |
| 30 - 59 | 8.126 x Weight (Kg) + 845.6 = BMR | 112 |
For example, a 25-year-old man who weights 60 kg would have a BMR of 1595.92. This means that they would need about 1,596 calories per day in order for their body to function at idle.
The standard error of estimation (SEE) is used to account for the muscularity or leanness of an individual. If our example above was relatively muscular, he would add 156 to his BMR, raising his BMR to 1,752. Likewise, if our 25 year old male was fairly lean, he would deduct 156 resulting in a BMR of 1,440 calories.
Alternatively, you can use one of the following equations which account for your height:
- BMR for Men = 66.47 + (13.7 x weight [kg]) + (5 x size [cm]) − (6.8 x age [years])
- BMR for Women = 655.1 + (9.6 x weight [kg]) + (1.8 x size [cm]) − (4.7 x age [years])
Using a combination of your height, weight and age can be more accurate.
How do I work out how many calories I should be consuming in a day?
Once you work out your basal metabolic rate, you will be able to work out how many calories you need based on your activity level. To do this, you multiply your BMR by a number which represents your level of activity.
Note: There are other equations and formula which you can use to work out how many calories you should be consuming. For the sake of simplicity, we chose to use the Harris Benedict Formula.
Use the following to calculate the calorie intake required to maintain a suitable weight:
| Activity level | Equation |
|---|---|
| Little/no activity | BMR x 1.2 = Recommended calorie intake |
| Light activity | BMR x 1.375 = Recommended calorie intake |
| Moderate activity (3 to 5 days a week) | BMR x 1.55 = Recommended calorie intake |
| Very active (6 to 7 days a week) | BMR x 1.725 = Recommended calorie intake |
| Extra active (Very active and physically demanding job) | BMR x 1.9 = Recommended calorie intake |
Taking the example from before, a 25-year-old male, weighing 60 kg (~9 stone), and whom goes to the gym 3 days of the week would need to consume ~2,473 calories a day to provide his body with enough energy to maintain his current weight. You may notice this number is very similar to the guideline set out by Public Health England and this is simply because our example represents a very average 25 year old male.
If our example male lived a sedentary lifestyle, he would only need ~1,915 kcal per day to maintain a constant weight.
Note: This final number represents the amount of calories your body needs to maintain its current weight - not to lose or gain weight. For people with a body mass index (BMI) over 25, who are classified as overweight, may find that their daily intake of calories, determined by working our basal metabolic rate (BMR), may be higher than the recommended daily intake of calories set out by Public Health England.
I worked out my daily calorie intake. What should I do if I want to lose weight?
If you are looking to lose weight, the NHS recommend deducting between 500 and 600 calories from your daily intake. Of course, you should speak to your GP or a dietician before making drastic changes to your diet.
BMR vs BMI
Basal metabolic rate (BMR) is not the same as body mass index (BMI).
Body mass index (BMI) is a scale which determines whether you are underweight, overweight, obese or the right weight for your age and height. BMI does not factor in your rate of energy expenditure or your body's composition (muscularity).
Likewise, basal metabolic rate (BMR) is an equation which is used to determine the necessary daily intake of calories to maintain a constant weight. Basal metabolic rate does not determine whether you are underweight, overweight, obese or the ideal weight for your age and height.
Using both body mass index (BMI) and basal metabolic rate (BMR) in tandem can be useful in dieting. Start by working out your BMI = [Weight] ÷ [Height in m²]. You can use the NHS' Online BMI Calculator to work out your BMI. Make a note of whether you are obese, overweight or underweight. Then calculate your basal metabolic rate (BMR) and your daily intake of calories. If you are a healthy weight, you shouldn't need to adjust your calorie intake. If you are overweight, you can adjust your daily intake of calories to lose weight.