Do I have a slow or fast metabolism?

Metabolism is a frequently occurring word in the arena of health, nutrition, and exercise. There are few of us who can say that we have not heard (or even uttered the words ourselves) “slow metabolism” or “fast metabolism”, usually in the context of comparing two people and their ability to gain, maintain, or lose weight. This begs the question(s): what is a metabolism and is yours fast, slow, or, God forbid, normal?

What is a metabolism?

In its simplest form, metabolic rate describes the chemical processes naturally occurring in a living organism to maintain its status as “living”.  These chemical processes have energy demands; therefore, a human’s resting metabolic rate (RMR) refers to the amount of energy required to maintain its homeostasis and continue living at rest (e.g., while sitting). This is the most common definition of metabolism and usually what people are referring to when they use the term metabolism in conversation.

What drives metabolic rate?

From the description above you might think that a metabolism is not very elastic and a body will use energy as needed to keep the sum of its living tissues alive. This, in many ways, is true. That’s not to say that one person’s metabolism cannot be different from another’s but the differences may be smaller than you think especially when taking into account the main drivers of metabolism. The variables which appear to play a large role in metabolic differences are age, weight, gender, and fat free mass.

Weight is one of the biggest drivers of metabolism – if a person has a large amount of mass to keep alive, they will have greater energy demands and a higher resting metabolic rate. This equates to a larger person burning more energy at rest compared to a smaller person.

After total mass is taken into account, fat free mass plays a key role in a human’s energy demands at rest. Stored fat does not require much energy whereas muscle and organs (fat free mass) demand greater amounts of energy expenditure to maintain homeostasis. Organs are the biggest drivers of metabolic rate and require more than 50% of energy demands at rest but only weigh roughly 8% of total body weight. Muscle mass accounts for 20% of resting energy expenditure and fat mass only 3%.

It has long been thought that as we age our resting metabolic rate (RMR) declines. This is true to an extent but the phases of life when these metabolic changes occur might surprise you. There are significant changes in RMR between infancy and 20 years of age but then very little change (when total mass and fat free mass are taken into account) between the ages of 20 and 60 years of age. This means that for most of our adult life our metabolism stays relatively constant and mass and fat free mass are accounted for. After 60, RMR declines about 0.7% each year so a 90 year old will burn ~20% less calories per day compared to a 20-60 year old.

Gender is the other factor when assessing metabolism. Although it is true that on average males burn more calories than females, this is largely explained by differences in body composition – males generally have a greater amount of fat free mass. When fat free mass is taken into account, men and women are very similar in their energy demands at rest.

What is normal?

Metabolic literature has shown that 1 standard deviation of the population (~68%, think the meaty part of the bell in a bell-curve) will sit within 6-8% of the average RMR and 2 standard deviations are within 10-16%. This means 95% of the population will burn within 200-300cals of each other at rest if the average energy expenditure for an adult is ~2000cals. To put this in perspective, 200cals is equivalent to a medium sized banana and one tablespoon of peanut butter. For most adults this will be the difference in calories burned over 24 hours between themselves and someone with a faster or slower metabolism within the 95% average range.

There will still be people who sit outside of this range and burn far fewer or greater calories per day compared to the average, but they are the outliers and statistically most people will sit much closer to the average. There will also be people who burn a lot more or less than 2000cals because they are bigger or smaller and have more or less mass to keep alive at rest.

To get an estimate of your own basal metabolism (basal metabolism = lying down ~ RMR/1.1), use an equation such as the Harris-Benedict:

Male kCal/Day: BMR = 66.5 + (13.75 x wt in kg) + (5.003 x height in cm) – (6.755 x age in years)

Female kCal/Day: BMR = 665 + (9.563 x wt in kg) + (1.850 x height in cm) – (4.676 x age in years)

You can then use an activity factor to estimate daily energy expenditure with physical activity.

Table 1. Activity factors for daily energy expenditure

What is best for a metabolism?

As the initial description of metabolism alluded to, resting metabolic rate is not very flexible and the main drivers are not easily changed. The main driver that can be affected is fat free mass. If the percentage of total mass taken up by fat free mass can be increased then RMR should increase with it. This would be achieved by decreasing fat mass and/or increasing fat free mass. Nutrition and exercise are the environmental factors we can use for this to occur. For example, if someone started completing strength training on a regular basis along with adequate nutritional intake and rest, this would be a message to the body to increase muscle mass which would equate to a greater % of total mass being fat-free, meaning greater energy demands at rest. Exercise can also be used to acutely to impact your metabolism. As demonstrated in Table 1., if you are more active then you will expend more energy on a given day. For most of us, to maintain a healthy metabolism we can focus on nutrition, exercise, and rest.