When most of us think of calories, we think of how fattening a food is. In dietary terms, calories are the amount of energy that a food provides.
If we consistently take in more energy than we need, we will gain weight. If we take in too little energy, we will lose weight, fat, and eventually muscle mass.
The definition of a calorie is the amount of energy needed to raise the temperature of 1 gram (g) of water through 1° Celsius.
The type and amount of food we eat determine how many calories we consume. For many people on a weight-loss diet, the number of calories in a food is a deciding factor in choosing whether or not to eat it.
How and when we eat can also make a difference, as the body uses energy differently throughout the day. Our body’s energy use will depend on how active we are, how efficiently our body uses the energy, and our age.
According to the 2015-2020 Dietary Guidelines for Americans, women are likely to need between 1,600 and 2,400 calories a day, and men from 2,000 to 3,000. However, this depends on their age, size, height, lifestyle, overall health, and activity level.
Fast facts on calorie intake and use
- Recommended calorie intake depends on factors such as age, size, height, sex, lifestyle, and overall general health.
- Recommended daily calorie intakes in the US are around 2,500 for men and 2,000 for women.
- Eating a big breakfast could help with weight reduction and maintenance.
- The brain uses around 20 percent of the energy used in the human body.
- Factors affecting ideal calorific intake include age, bone density, and muscle-fat ratio.
- A 500-calorie meal consisting of fruits and vegetables has more health benefits and will keep you feeling full for longer than a 500-calorie snack of pop
The 2015-2020 Dietary Guidelines for Americans recommend a calorie intake that ranges from 1,000 calories a day for an infant of 2 years to 3,200 for an active male aged 16 to 18 years.
As people get older, their metabolic rate slows down.
This reduces their need for energy. From age 19 to 25 years, the recommended intake for women is 2,000 calories a day, but after 51 years, this falls to 1,600.
For the human body to remain alive, it needs energy.
Around 20 percent of the energy we take in is used for brain metabolism. Most of the rest is used in basal metabolism, the energy we need when in a resting state, for functions such as blood circulation, digestion, and breathing.
In a cold environment, we need more energy to maintain a constant body temperature, as our metabolism increases to produce more heat. In a warm environment, we need less energy.
We also need mechanical energy for our skeletal muscles, to maintain posture and move around.
Cellular respiration is the metabolic process by which cells get energy by reacting oxygen with glucose to produce carbon dioxide, water, and energy.
How efficiently energy from respiration converts into physical—or mechanical— power depends on the type of food eaten, the type of physical energy, and whether muscles are used aerobically or anaerobically.
In other words, we need calories to fuel bodily functions, such as breathing and thinking, to maintain our posture, and to move around.
Here are some tips for burning energy and losing weight more effectively.
1. Eat breakfast: A protein and healthy fat breakfast can keep you full for longer and help prevent snacking during the day.
2. Eat regular meals: This can help you burn calories more effectively and helps prevent mindless snacking.
3. Remember your “five-a-day:” Fruits and vegetables can be a tasty snack and they can bulk out your meals. They are high in nutrients and fiber and low in calories and fat.
4. Eat slow-burning calories: High-fiber carbohydrates, such as legumes, and healthy fats, such as avocado, take longer to release energy, so you will not get hungry as quickly.
5. Exercise: This can help burn off extra calories, and it can make you feel good. A brisk daily walk is easy for most people to do and costs nothing. Challenge yourself with a pedometer. For people who use a wheelchair, there are exercises that can boost heart health and strength.
6. Drink water: It is healthful, has no calories, and can fill you up. Avoid alcohol and sodas as these can easily provide far too many calories. If you crave sweet drinks, choose unsweetened fruit juices, or better still, get a juice maker.
7. Eat more fiber: Fiber, found in fruits, vegetables, and wholegrains, can help you feel full and encourage healthy digestion.
8. Check the label: Some items have hidden fats or sugars. “Ten percent less fat,” might not actually mean very much less fat, and it does not necessarily mean that you can eat more of it or that it is really more healthful. If you are counting calories, the label will help you keep track.
10. Slow down: Eat slowly and rest between courses or extra servings, as it can take 20 to 30 minutes for your body to realize it feels full.
11. Make a shopping list: Plan a week of healthful meals and snacks, list the ingredients you need, and when you go grocery shopping, stick to it.
12. A little of what you fancy: Banning foods can lead to cravings and bingeing. Spoil yourself occasionally with a favorite treat, but in smaller amounts.
13. Get enough sleep: Sleep loss affects the metabolism, and it has been linked to weight gain.
14. Avoid eating 2 hours before bed: Eating within 2 hours of sleeping can interfere with sleep quality and promote weight gain.
Here are some examples of activities and the calories they can help you burn in 30 minutes. The estimates are for a person weighing 125 pounds.
|Walking at 4.5 miles an hour||150|
|Running at 6 miles an hour||300|
Keeping calorie intake within certain limits will not ensure a healthful diet, as different foods have different effects on the body.
After consuming carbohydrates (carbs), insulin levels will rise significantly more compared with eating fats or protein. Some carbs in particular get into the bloodstream in the form of sugar, or glucose, much faster than others.
Refined flour is a fast carb, while legumes are slower. Slow-release carbs are better for body weight control and overall health than fast carbs.
A 500-calorie meal of fish or meat, salad, and some olive oil, followed by fruit, is more healthful and will stave off hunger for longer than a 500-calorie snack of popcorn with butter or toffee.
To work out how many calories you need, you need to know your basal metabolic rate and an activity factor.
Basal metabolic rate
One useful way of estimating BMR is the Mifflin-St. Jeor equation:
Men: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5
Women: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161
To calculate your BMR automatically, follow this link and enter your details into the calculator.
After calculating the BMR, multiply the result with an activity factor:
- Sedentary lifestyle: If you do very little or no exercise at all, your daily calorie requirement is BMR x 1.2.
- Slightly active lifestyle: If you do light exercise between one and three times a week, your daily calorie requirement is BMR x 1.375.
- Moderately active lifestyle: If you do moderate exercise three to five times a week, your daily calorie requirement is BMR x 1.55.
- Active lifestyle: If you do intensive exercise six to seven times per week, your daily calorie requirement is BMR x 1.725.
- Very active lifestyle: If you do very intensive exercise twice a day, with extra heavy workouts, your daily calorie requirement is BMR x 1.9.
This will give a rough idea of the daily calorie intake you need to keep your body weight where it is.
The result is still not perfect, as the equation does not take into account the ratio of muscle to fat. A very muscular person needs more calories, even when resting.
As with calories requirements, an ideal body weight depends on several factors, including age, sex, bone density, muscle-fat ratio, and height.
There are different ways of assessing an ideal weight.
Body mass index (BMI)
|18.5 to 24.9||Normal weight|
|30 or above||Obesity|
However, it does not take into account muscle mass.
Imagine a top athlete who weighs 200 pounds, or 91 kilograms (kg) and is 6 feet, or 1 metre (m) and 83 centimeters (cm) tall. They may have the same BMI as an inactive person of the same height. The athlete is not overweight, but the inactive person quite possibly is.
Researchers have found that many people whose waist circumference is less than half their height have a longer life expectancy.
An adult male who is 6 feet (183 cm) tall should have a waist that does not exceed 36 inches (91 cm).
An adult female who is 5 feet 4 inches (163 cm) tall should have a waist that does not exceed 32 inches (81 cm).
To measure the waist, measure half-way between the lower rib and the pelvic bone at the hip.
This measurement may be more accurate than BMI at determining a healthy weight. However, it is limited as it does not properly measure an individual’s total body fat percentage, or muscle-to-fat ratio.
A wide range of diets claim to help people lose or maintain their body weight.
Some of these are safe and effective and help people lose weight and keep it off in the long term. Others are hard to adhere to, or when the person stops following the diet they put weight back on quickly.
To find out more, see our article on the “Eight Most Popular Diets.”
The rankings for these diets were based on how many articles mentioned them favorably, how popular they were generally and which ones received the most positive feedback.
More important than counting calories is to eat a healthful and well-balanced diet that you can sustain long-term, for longer than 6 months. Equally important is to be physically active and to balance the calories consumed with the energy used each day.