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Fact vs Fiction: I have a slow metabolic rate

Blog #3 in Metabolism Series

“I have a slow metabolism” …. This is a common statement to hear from people when you are talking about metabolism or “My husband/partner’s metabolism is so much higher because he is a man”, but is this true? Most people believe there is a difference in metabolism between males and females, non-obese and obese, and the young and the old. Let’s take a look at each of these scenarios.

Resting Metabolic Rate (RMR) Differences

Let’s review what RMR is… it is defined as the energy required (calories burned) by the body in a resting condition and it typically accounts for the largest portion of total energy needs. Studies have shown differences in RMR between men and women, between obese and nonobese adults and even possibly racial/ethnic differences. BUT, let’s look at this more closely. In a 2015 meta-analysis (McMurray et al.), researchers looked at almost 400 metabolism publications and found that the difference between individuals is not that great.

Let’s start with differences between males and females. The average RMR of all women (young, old, non-obese, obese) tends to be about 0.839 kcal·kg−1·h−1 and all men tend to be about 0.892 kcal·kg−1·h−1…is this a big difference? If we compare a 150 lb. female to a 150 lb. male we can see the difference is less than 100 calories per day.

What about non-obese versus obese individuals? The RMR of a normally weighted woman is about 0.926 kcal·kg−1·h−1 compared to an obese woman which is about 0.721 kcal·kg−1·h−1. We see some definite difference in numbers there but do we really see a difference in RMR? Yes, we do! The obese female has a higher RMR than the normally weighted female.

Ok – what about young versus old? Older adults (>70 yr) have been shown to have lower RMRs than younger adults by as much as 20%–25%. But once again, the difference is not that great. Research has shown that the majority of the population (from young to old, normal weighted to obese, male to female) exists in a range of 200-300 kcal/day from each other and do not possess hugely different RMRs.

Now, there has been a recent study, published in August of 2021, that has given us pause and we may need to rethink all of our previous research in this area. Metabolic research is expensive, and so most published studies have had very few participants. But this new study was quite unique in that 80 researchers agreed to share their data from a span of over 40 years! Using data from nearly 6,500 people, ranging in age from 8 days to 95 years, researchers discovered that there are four distinct periods of life, as far as metabolism goes. Central to their findings was that metabolism differs for all people across four distinct stages of life. There’s infancy, up until age 1, when calorie burning is at its peak, accelerating until it is 50 percent above the adult rate. Then, from age 1 to about age 20, metabolism gradually slows by about 3 percent a year. From age 20 to 60, it holds steady. And, after age 60, it declines by about 0.7 percent a year or about 20% from age 60 to age 95. What is interesting is that energy requirements of the heart, liver, kidney and brain account for 65 percent of the resting metabolic rate although they constitute only 5 percent of body weight. A slower metabolism after age 60, may mean that crucial organs are functioning less well as people age. It might be one reason that chronic diseases tend to occur most often in older people.

The researchers found that there are no real differences between the metabolic rates of men and women after controlling for body size and muscle mass. The group also expected that the metabolism of adults to start slowing when they were in their 40s or, for women, with the onset of menopause, but that didn’t happen either! The four periods of metabolic life depicted in this study show “there isn’t a constant rate of energy expenditure per pound,” The rate depends on age. That runs counter to the longstanding assumptions that have been widely held.

Where do we land with this ‘slow metabolism’ notion? Fact or Fiction? Obviously more research is needed in light of this newest study, but in general RMRs are not a cause of extra pounds, but it is more the influence of NEAT (Non-Exercise Activity Thermogenesis) and TEA (Thermal Effect of Activity or Exercise).

Next month – we will look at genetics and how they influence metabolism.

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