It is a common lament of dieters: “Ugh, my metabolism is so slow, I will never lose weight”.
When people talk about fast or slow metabolism, what they really get is how many calories your body burns throughout the day. The idea is that someone with a slow metabolism simply won’t use the same amount of energy to do the same task as someone with a fast metabolism.
But does the speed of metabolism really vary from person to person? I am a nutrition scholar who focuses on biological, environmental and socioeconomic factors that influence body composition. This issue is more complicated than it may seem at first – and whatever the current speed of your metabolism is, there are things that will push you to lower or higher speeds.
Your body’s energy needs
Metabolism is a biological term that refers to all the chemical reactions necessary to maintain life in an organism. Your metabolism performs three main tasks: converting food into energy; breaking down food into its building blocks for proteins, lipids, nucleic acids and some carbohydrates; and elimination of nitrogen waste.
If you are dying with the speed of your metabolism, you are probably focused on how much energy you are getting from the foods you eat and how much your body is using. The energy value of a food is measured in calories.
Your caloric needs can be divided into two categories.
The basal metabolic rate is the minimum amount of calories needed for basic resting functions. Resting energy expenditure is the amount of calories your body uses while resting or sleeping – about 60% -65% of your total energy expenditure. It does not take into account the calories you need to feed everything you do – moving around or expending energy with the activity (25% -30%), thinking, even digesting food (5% -10%). Therefore, your total energy expenditure combines both: your resting energy expenditure plus your energy expenditure for other activities.
Reaching a number
The estimated daily caloric needs for a 126-pound adult woman range from 1,600 to 2,400 calories per day. For a 154-pound man, daily calorie needs can vary from about 2,000 to 3,000 calories per day. That’s about 13 calories per pound of body weight.
In contrast, babies burn about 50 calories per pound of weight per day. This requirement decreases continuously as the child grows. Therefore, babies have the highest metabolism of all. This requirement for extra calories is necessary for growth.
So, if two women of the same weight can have caloric needs that vary by up to 30%, does that mean that the woman whose body uses more calories has a faster metabolism than the woman whose body uses less calories? Not necessarily. A woman may spend more of her day physically active and therefore need more energy to boost her walking and kickboxing classes after work, for example.
In addition to these approximate guidelines, there are many ways to estimate total and resting energy expenditure if you want to discover your body’s specific calorie needs. A common and easy method is to use predictive formulas like Mifflin-St. Jeor or Harris-Benedict equations that are based on your age, height, weight and gender to find out how much energy your body needs just to be alive. To calculate the total energy expenditure, you also need to add the activity factor.
Indirect calorimetry is another way of estimating the metabolic rate. Energy expenditure is calculated by measuring the amount of oxygen used and carbon dioxide released by the body. Your body depends on oxygen to carry out all of your metabolism functions. For each liter of oxygen used, you consume about 4.82 calories of energy from glycogen or fat. Indirect respiratory calorimetry measures the oxygen you exhale as an indicator of how much food your body is burning. It is usually done in a doctor’s office, although small, portable and more affordable devices are increasingly being brought to the market.
Factors that influence metabolic rate
The metabolic rate and calorie requirements vary from person to person, depending on factors such as genetics, sex, age, body composition and the amount of exercise you do.
Health status and certain medical conditions can also influence metabolism. For example, a regulator of metabolism is the thyroid gland, located on the front of the neck, just below the Adam’s apple. The more thyroxine a person’s thyroid gland produces, the higher the person’s basal metabolic rate.
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Having a fever can also affect a person’s basal metabolic rate. For every 0.5 C (0.9 degrees Fahrenheit) increase in a person’s internal body temperature, their basal metabolic rate increases by approximately 7%.
Other medical conditions that influence the basal metabolic rate may include muscle loss (atrophy), prolonged hunger, low levels of oxygen in the body (hypoxia), muscle disorders, depression and diabetes.
Another important factor is body composition. For example, an overweight woman with a body composition of 40% body fat and 75 pounds of muscle mass will burn fewer calories during rest than a woman with 30% body fat and 110 pounds of muscle mass; muscle tissue is metabolically more active than body fat.
This is also why the basal metabolic rate decreases with age. As people age, they typically lose muscle mass and gain adipose tissue – which is equivalent to a decrease in basal metabolic rate of approximately 1% to 2% per decade.
If you really want to shake up your metabolism, the easiest way is to increase your muscle mass and your activity level. By increasing muscle mass, you will also increase the basic number of calories needed to maintain these muscles. Instead of complaining about a slow metabolism, you can try to increase it to be at least a little faster.
This article was republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Terezie Tolar-Peterson, Mississippi State University.
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Terezie Tolar-Peterson does not work for, consult, own shares or receive funding from any company or organization that would benefit from this article and has not disclosed any relevant affiliations other than his academic appointment.