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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
Recommended intake
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.
Burning calories
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.
Tips
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.
9. Use smaller plates: Research indicates that portion sizes have increased over the
last 3 decades, and this may contribute to obesity. Using a smaller plate encourages
smaller portions.
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.
Activities
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.
Activity
Calories burned
Lifting weights
90
Aqua aerobics
120
Walking at 4.5 miles an hour
150
General swimming
180
Running at 6 miles an hour
300
Computer work
41
Sleeping
19
Food choice
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 weightcontrol 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.
Daily needs
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.
Activity factor
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.
Ideal body weight
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)
Body mass index (BMI) is one way of working out what a person should weigh. If
you know your height and weight, you can use this calculator to find out your BMI.
BMI
Definition
Below 18.5
Underweight
18.5 to 24.9
Normal weight
25-29.9
Overweight
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.
Waist-hip ratio
Researchers have found that many people whose waist circumference is less than half
their height have a longer life expectancy.
People with a smaller waist to height size have been found to have a lower risk
of diabetes, cardiovascular disease, stroke, and other health conditions that are related
to obesity.
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.
References
Appendix 2. Estimated calorie needs per day, by age, sex, and physical activity level. (2015).
Retrieved from https://health.gov/dietaryguidelines/2015/guidelines/appendix-2/
Benton, D. (2015, June 7). Portion size: What we know and what we need to know. Critical Reviews
in Food Science and Nutrition, 55(7), 988-1004. Retrieved
from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4337741/
BMR calculator. (n.d). Retrieved from http://www.myfitnesspal.com/tools/bmr-calculator
Calculate your body mass index. (n.d.). Retrieved
from https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
Calories burned in 30 minutes for people of three different weights. (2017, March 17). Retrieved
from https://www.health.harvard.edu/diet-and-weight-loss/calories-burned-in-30-minutes-of-leisure-
and-routine-activities
European Society of Endocrinology. (2017, May 22). Sleep loss affects your waistline [Press
release]. Retrieved from https://www.sciencedaily.com/releases/2017/05/170522081109.htm
Fitness advice for wheelchair users. (2016, June 28). Retrieved
from https://www.nhs.uk/Livewell/Disability/Pages/fitness-for-wheelchair-users.aspx
Sharp, D. E., Sobal, J., & Wansink, B. (2014, September 16). Using plate mapping to examine
portion size and plate composition for large and small divided plates. Eating Behavior, 15(4), 658-
663. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4441217/
12 tips to help you lose weight on the 12-week plan. (2016, December 5). Retrieved
from https://www.nhs.uk/Livewell/weight-loss-guide/Pages/successful-diet-tips.aspx
