How Many Calories Does My Body Need to Survive at Rest?

Even without exercise or physical activity, your body needs calories in order to carry out basic functions like breathing, circulating blood and maintaining your body temperature. Known as the resting metabolic rate (RMR), it is the number of calories needed to sustain the body at rest.

When you think of the word "calories," you probably think of it in the context of how fattening a certain food is. However, calories are actually a unit used to measure energy. A calorie is defined as the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius.

In dietary terms, when you say an apple has 95 calories, that is the amount of energy the apple is giving your body. Just as a car needs fuel to power it, your body needs this energy to stay alive. It burns calories to fuel all your actions, some of which you're aware of and many of which you aren't.

Read more: What Are Calories — and Should You Be Counting Them to Lose Weight?

For example, your body burns energy when you perform actions like walking, talking, typing, sitting down, standing up and lifting things. However, your body also burns energy as it performs a number of involuntary actions, like breathing in and out. All the processes that are constantly going on within your body, like digestion and circulation for instance, also require energy.

Even your brain requires considerable energy to function. A study published in May 2017 in Frontiers in Neuroscience notes that even though the adult human brain only makes up 2 percent of your total body weight, it consumes up to 20 percent of the total energy your body metabolizes from glucose under normal resting conditions.

According to the American Council on Exercise, the total number of calories you burn in a day is known as your total daily energy expenditure (TDEE) and it is made up of many sources, including RMR. Your TDEE is calculated by adding up the following factors:

• Resting metabolic rate (RMR): This is the number of calories needed to sustain the body at rest and it covers only your basic bodily functions. It is not to be confused with the basal metabolic rate (BMR). Your BMR is the number of calories your body requires to just exist and be awake. It is usually achieved only under clinical or laboratory settings when you're in a fasted state so your digestive system is inactive. RMR is therefore a more realistic way to measure the number of calories needed to sustain the body at rest.
• Thermogenic effect of food (TEF): This is the number of calories your body uses to digest and absorb the food you eat and eliminate waste.
• Nonexercise activity thermogenesis (NEAT): This is the number of calories you burn every day doing activities other than sleeping and eating. Exercise is not counted in this.
• Excess post-exercise oxygen consumption (EPOC): These are the additional calories your body burns after a workout, or the afterburn, if you will. The more strenuous your workout, the more calories you burn afterward.
• Exercise (Ex): This is the number of calories you burn while exercising.

Among these, your RMR and TEF are fairly constant, with minor fluctuations due to muscle conditioning and dietary changes respectively. However your NEAT, EPOC and Ex components can vary quite a bit according to your lifestyle and can have a substantial influence on your total calorie expenditure.

Read more: Can Cardio Boost Metabolism?

There are two main ways to calculate RMR, according to the American Council on Exercise. The first is more accurate but more difficult; it involves measuring the gases you expire and using a calculation known as the Weir equation to assess how much fuel and the types of fuel your body is using.

The second way to calculate RMR is easier but less accurate and involves applying parameters like height, weight and age to certain formulae. One example of a formula to calculate RMR is the Mifflin-St Jeor Equation.

Per this formula, a 48-year-old man who is 5 feet, 8 inches tall and weighs 176 pounds would have an RMR of 1,694 calories per day. A woman of the same age, height and weight would have an RMR of 1,528 calories per day.

While there are multiple other formulae that can help you estimate your RMR, they all give different results that can vary by up to 1,000 calories. Various factors can affect RMR, so even if two people have the same age, height, weight and gender, their RMR may not be the same. A study published in August 2015 in Medicine and Science in Sports and Exercise concluded that no single RMR value is appropriate for all adults.

Read more: How Many Calories Should I Be Eating for My Height?

According to the American council on Exercise, several factors can affect your RMR. Genetics, for example, can determine whether you have a fast metabolism or a relatively slower one. Gender plays a role because men tend to have more muscle mass and a lower body fat percentage than women, which means they tend to have a higher RMR.

Age matters because your BMR drops by up to 3 percent every decade as you get older. Height makes a difference because taller people's bodies tend to have more surface area and more lean body mass. Weight is a factor because the more a person weighs, the higher their RMR tends to be.

Your body temperature also matters because if you have fever, the chemical reactions in your body take place faster in the higher temperature environment. Your BMR goes up by 7 percent for every 0.5 degree Celsius that your body temperature increases. The external temperature also affects your RMR because when it's cold, your body works harder to keep you warm. Prolonged exposure to heat can also cause your RMR to increase.

Exercise also helps because it not only contributes to your overall calorie expenditure but also helps you build muscle. The more muscle your body has, the higher your RMR.

Health.gov's Dietary Guidelines for Americans recommends that adult women consume 1,600 to 2,400 calories per day and that adult men consume 2,000 to 3,000 calories per day, depending on how active their lifestyle is. This is just an estimate; you can use a physical activity and food calorie calculator or consult your doctor to better understand how many calories you need to be eating per day.

Read more: Non-Starving, 1200-Calorie Diet

If you're trying to lose weight, you need to cut 500 calories a day to lose 1 pound of body weight a week. Starvation is not the answer to weight loss and can even cause you to gain weight, cautions the UK's National Health Service. Eating 1,000 calories per day or less is the same as total starvation, according to UCLA Health.