The amount of energy contained in food can be measured in two different units – the kilocalorie (kcal) and the kilojoule (kJ). To a physicist, a standard calorie is the amount of heat energy needed to raise the temperature of 1 gram of water by 1 degree centigrade (from 15 degrees C to 16 degrees C). Food is so full of energy that the energy it provides is measured in kilocalories, each of which is equivalent to 1000 standard calories.
This can cause confusion. If you read that a large egg, for example, provides 78 calories, this should actually be 78 kilocalories (kCal) which is equivalent to 78,000 calories – a scarily high number!
Because physicists (bless them) often get confused (or annoyed) by what they see as these calorie labelling mistakes, many scientists prefer to use the more modern unit, the kilojoule (kJ). In summary:
- 1 kilocalorie (kcal) = 1000 calories
- 1 kilojoule (kJ) = 1,000 joules
- 1 kcal = 4.2kJ
- 1 kilojoule (kJ) = 0.239 kilocalories
We aim to use the term kilocalories or kcal when describing the calories found in food, but for the avoidance of doubt, if we say that an egg provides 78 calories, we actually mean kilocals.
How many calories in food?
Having muddied the waters with those confusing explanations, the next thing to consider is that different foods provide different amounts of energy depending on their chemical makeup.
- Pure carbohydrate provides 4 kcal (16.8 kJ) energy per gram
- Pure protein provides 4 kcal (16.8 kJ) energy per gram
- Pure fat provides 9 kcal (37.8 kJ energy per gram
- Alcohol provides 7 kcal (29.4 kJ) per gram (annoying, right?)
When you eat more calories (or kilocalories) than your body needs, over a prolonged period of time, you will store the excess energy as fat.
How many calories do you need?
Around half your daily energy needs are used to fuel your body’s basic functions of life, such as maintaining your body temperature, keeping your heart beating and your lungs breathing, digesting food, making new proteins, fats and glucose, moving sodium and potassium in and out of your cells, and generating electrical messages in the brain. The energy needed to keep all these going is known as your basal metabolic rate (BMR). The higher your basal metabolic rate, the more energy you will burn at rest and during sleep.
The other half of your daily energy needs are used to fuel your physical activity such as walking, dancing and running. When these are assessed, the average person, of average weight, and average level of physical activity needs the following number of calories (kcals) per day:
|Age||Males (kcal)||Females (kcal)|
|11 – 14 years||2220||1845|
|74 + years||2100||1810|
How many calories to lose weight?
If you consistently eat around 500 kilocalories less than you need, per day then, over seven days, you should lose around 1 pound of fat (half a kilogram) per week, on average. That’s because one pound of body fat stores 3,500 kcals. It’s not quite as simple as that, as a kilocalorie provided by protein, for example, has different effects in the body than a kilocalorie eaten in the form of carbohydrate, but we will address that in another post (look out for A Calorie Is Not Just A Calorie).
If an adult eats significantly fewer caloreis than they need, hoewver, your body goes into survival mode and your metabolism becomes a lot more efficient. You will ‘waste’ less energy as heat.
So, we don’t recommend that a woman goes below 1000 kcals to 1,200 kcals per day, or that a man goes below 1,800 kcals per day, long term, unless you are following a regimes such as intermittent fasting.
If you are interested in how scientists estimate your daily calorie needs, then read on. If not, you might like to check out these Calorie Counter and Diet Tracker fitness watches and digital smart scales on Amazon.co.uk or Amazon.com.
Your basal metabolic rate (BMR)
Scientist define your BMR as the energy you burn when lying in bed, at complete physical and mental rest, 12-14 hours after last eating, in an ambient room temperature of 26oC to 30oC.
Your basal metabolic rate depends on factors such as your age, gender, weight, muscle bulk and the genes you’ve inherited which dictates the efficiency of your metabolic reactions, and the amount of brown fat your own.
The type of food you eat also plays a role as energy is used up, and heat produced, during its metabolism. This effect is known as dietary-induced thermogenesis and accounts for ten per cent or more of the energy provided by foods – especially protein-rich foods.
Even so, it’s possible to get a good estimate of your BMR, based on your weight, age and gender, using mathematical formulae known as the Schofield equations. In case you’re interested, these are as follows (don’t despair, you don’t need to do these calculations!)
Table: Basal Metabolic Rate (Kcal/day)
W = Your body weight in kilograms
|0-3||BMR = 60.9W – 54||BMR = 61.0W – 51|
|3-10||BMR = 22.7W + 495||BMR = 22.5W + 499|
|10-17||BMR = 17.5W + 651||BMR = 12.2W + 749|
|18-29||BMR = 15.3W + 679||BMR = 14.7W + 496|
|30-59||BMR = 11.6W + 879||BMR = 8.7W + 829|
|>60||BMR = 13.5W + 487||BMR = 10.5W + 596|
A 40 year old woman weighing 60kg has a BMR of (8.7 x 60) + 829 = 1351 kcal/day.
A 50 year old male weighing 70kg has a BMR of (11.6 x 70) + 879 = 1691 kcal/day.
Your physical activity level (PAL)
Having estimated your basal metabolic rate, scientists also need to estimate how many additional kilocalories you need to fuel your level of activity. By adding these two together, they can assess your total daily calorie (kcal) needs.
Scientists estimate your daily energy requirements by multiplying your basal metabolic rate (BMR, predicted from the Schofield equations above) by a factor known as your physical activity level (PAL) which takes into account all the activities you do during the day.
So, your total energy requirement = BMR x PAL
- If you are a sedentary person who sits around all day in an office, your PAL is about 1.2
- If you are an average lightly active person who does a bit of walking, PAL works out at 1.4
- If you are moderately active during work and leisure times, your PAL is around 1.6 (females) or 1.7 (males)
- If you have a high level of physical activity during both work and leisure, your PAL is estimated as 1.8 (females) or 1.9 (males)
- If you have a vigorously active lifestyle you could have a PAL of 2 to 2.4
These types of calculations suggest that the averagely active adult male needs around 2550 kcal (10.6 MJ) per day to maintain body weight, while the average adult female needs around 1,940 kcal (8.1 MJ).
If you are of average weight, then your daily calorie needs will vary depending on whether you have a sedentary day, a moderately active or a very active day, as follows:
|Age (years)||Sedentary||Moderately Active||Active|
|19 – 30||2,000 kcals||2,000 – 2,200 kcals||2,400 kcals|
|31 – 50||1,800 kcals||2,000 kcals||2,200 kcals|
|50+||1,600 kcals||1,800 kcals||2,000 – 2,200 kcals|
These are only estimates, of course. The only way to know exactly how many kilocalories you need is to live in an insulated chamber in which scientist can directly measure the heat loss from your body. This is known as direct calorimetry. An alternative, known as indirect calorimetry, involves breathing in and out of a special mask which allows scientists to measure the amount of oxygen you consume, the amount of carbon dioxide waste gas you exhale, and to donate your wee, so they can measure the amount of nitrogen you excrete.
Another way is to accurately measure the number of calories present in the food you eat, and see what level of food intake allows you to maintain your body weight without losing or gaining any pounds.
Why do calorie needs vary with age?
Most excess weight is gained after the age of 35 as so-called, middle age spread. Weight is put on more easily in later years because of changes in your body and lifestyle. The most significant change is loss of lean muscle tissue, which is mostly replaced with fat. This process, known as sarcopenia, will occur naturally unless you continue to follow a muscle building regime and obtain sufficient protein in your diet to build new muscle.
Resting metabolism also slows by around 5% every ten years after the age 25. As a result, your daily need for calories goes down. By the time a woman is 75, she needs around 300 kcals less per day than when she was 18, and 130 kcals per day less than when she was 50. The difference is even greater in men, who need around 655 fewer kilocalories per day at the age of 75 than when they were 18 years old.
So, to avoid gaining weight as you get older, stay active and cut back on the amount of food you eat, favouring a higher protein, lower carbohydrate type of diet.
Do you need to count calories?
Low calorie diets used to be the ‘in’ thing, recommended by doctors and dietitians alike. But counting calories is a bit of a faff (although there are apps to make these easier). Low calorie food tends to be low in fat, and high in carbs, which fights you body’s ability to lose weight too (by increasing insulin) and stimulating hunger. In our experience, they also encourage you to think you are ‘on a diet’ which will eventually come to an end, rather than retraining your eating habits long-term.
We believe it is better to follow a healthy, lower carb diet (eg a low glycemic diet), a Mediterranean-style diet (go easy on the pizza and pasta, please) or even the new Atkins Diet, which favour healthy fats and protein, vegetables and fruit, with less starchy foods, sugar and salt.
This will help you lose weight naturally as foods rich in protein and fibre help to curb your appetite more quickly, while protein and healthy fats stimulate fat burning rather than fat storage.
Do you find counting calories is helpful or a bit of a chore?