There’s a common perception surrounding our metabolism and how it’s affected by age - specifically that our metabolism tends to slow down as we hit our 30s. Today, we’re about to explore how scientifically accurate this is, with the help of clinical evidence.
A quick recap on what we mean by ‘metabolism’:
Metabolism, as a term, is colloquially used to describe our ability to use up energy and/or fat in order to stay fit. While that’s partly true, metabolism has little to do with our weight, and more to do with our existence as living beings. It can simply be defined as the sum total of all the chemical reactions and processes happening in our body for us to sustain life . (We’ve covered the concept of metabolism in more detail in one of our blogs, which you can read right here.)
And what we mean when we say our “metabolism slows down”
Let’s start with the basics - every cell in our body is constantly working, growing, and repairing itself. And to continue to do this, it needs energy. Similarly, there are various other functions in our body that actively need energy to keep us going.
The minimum energy required by our body to carry out all these chemical processes (cumulatively called our ‘metabolism’) while it’s at rest is called the basal metabolic rate or BMR .
Our BMR accounts for up to 75% of our body's daily energy requirements , depending on our age and lifestyle - a reduction in this requirement is what we can refer to as ‘the slowing down of metabolism’ .
Our energy requirement is affected by the basic changes that our body goes through in general (even something as simple as a shift in our sleeping patterns) .
Let’s take a look at the factors that affect our BMR, and whether these are affected by the ageing process.
So, is there any evidence of our metabolism slowing down with age?
The short answer to this question is yes, our metabolic rate (or BMR) declines as we age . However, there’s no real set date or age for it. It’s a gradual process that varies for each one of us, and depends a lot on our lifestyles and diet.
Our BMR is mainly dependent on two factors - fat-free mass (FFM) and fat mass (FM) [7, 8]. Fat-free mass is the total mass of our muscles and organs. Fat mass, on the other hand, is, well, the mass of our total body fat .
Our fat-free mass tends to take up a lot more of our energy requirements, compared to our fat mass. Our organ mass itself makes up an entire 77% of our BMR. In an ideal situation, where a person has built sufficient muscle, their total muscle mass can contribute as much as 20% towards their energy requirements, whereas their total fat mass would contribute only 3% . Even when there's a significant increase in fat mass (which is not ideal), it would not use even half as much energy as the same amount of muscle mass would.
As we age, however, our organ mass tends to decrease naturally (because our cells shrink), which consequently lowers their contribution to our BMR. This decline promotes changes in our body that also favor an increase in our fat mass, and a decrease in our muscle mass.
Given that the largest contributors towards our BMR (our organ and muscle mass) have now decreased, and the lowest (fat mass) has increased, we can see why these changes would affect the body’s total energy requirement and reduce our BMR .
When does this happen?
If we have to put a number to it, research suggests that adults start to lose 3-8% of their muscle mass per decade after the age of 30 (this decline may begin then, but doesn’t happen overnight) .
The loss of muscle mass is further amplified by a decline in testosterone (which otherwise promotes muscle growth in our body) that begins after the age of 30 as well, at the rate of about 1-2% a year [13, 14].
Note: Other factors that affect our metabolism do exist, but they aren't significantly altered by our age. For example, the thermic effect of food (the increase in BMR that takes place after we eat a meal) is only ~1% lower in people over the age of 60, as compared to people who are about 35 years of age .
To conclude, the available evidence does confirm that our metabolism decreases as we age, with a loss in our muscle and organ mass being a significant reason behind this.
Can we still maintain a healthy metabolic rate as we age?
Of all the factors that affect our metabolism, we have control only over our muscle and fat mass. And since our lifestyle affects both these factors, there are quite a few ways to maintain a healthier BMR.
Here are a few evidence-based steps:
Leading an active lifestyle, as we know, is a solution to a lot of lifestyle-related health problems. In this case, exercising helps by increasing our energy requirements and our muscle mass, while reducing our fat mass .
Here are two approaches that have been proven to improve our metabolism (evidence suggests that a combination of both can give us best results):
In addition to its general health benefits (which, essentially, come down to improving our physical and mental health), resistance training (AKA strength training or weight training) also helps us preserve and build muscle.
That’s because resistance training makes our muscle fibers undergo trauma, or what’s called muscle injury, which triggers our body’s repair mechanism and gives it the opportunity to build more muscle as it repairs the damage . (In case you’re wondering, this is why it’s a good idea to drink protein shakes after training - the body uses the protein for this muscle-rebuilding process.)
And we know that more muscle mass = a higher BMR. In fact, a small-scale study showed that men who were between 50-65 years of age, and performed resistance training exercises three times a week, increased their BMR by 7.7%. This suggests that resistance training, when done right, is effective at maintaining and increasing our metabolism, even at a higher age .
Activities that increase our breathing and heart rate, such as walking, running, swimming, skipping rope, etc., come under ‘endurance exercise’. These physical activities keep our body and its circulatory system healthy, and their consistent practice may help reduce the amount of fat our body would otherwise accumulate in the long-term [19, 20].
Studies suggest that individuals who are able to sustain and maintain a high level of endurance exercise (eg. brisk walking or cycling for an hour straight), even as their age increases, may maintain a better BMR in the long run (pun intended) [21, 22]. This is because long-term endurance training activates our muscle cells, which promotes muscle growth; it also helps burn intermuscular fat - the kind of fat that's around our muscles and prevents them from functioning at their best .
If you are someone who intends to work on your BMR - 45 minutes of cycling or running five days a week is a good place to begin .
Lack of sleep can adversely affect our metabolism. A study on sleep found that those who slept for only four hours a day had a 2.6% lower metabolic rate, as compared to those who slept 10 hours a day . Poor sleep quality may also lead to a decrease in our muscle mass , and since our muscle mass influences our BMR, this may exacerbate the decline in our metabolism .
Fortunately, good and consistent sleeping patterns (seven to nine hours of sleep every night) can reverse these effects.
Protein has multiple positive impacts on our metabolism. Consuming it increases our metabolic rate by 15-30% (as compared to a 5-10% increase through carbohydrates, and 0-3% through fats). That’s simply because our body burns more calories while digesting and absorbing protein, as compared to other nutrients .
Secondly, consuming protein promotes muscle growth and preservation (irrespective of age), which is great for our BMR . (If you’d like to understand how much protein you should consume daily, based on your level of physical activity, you might find this article helpful).
Ageing is the most natural phenomenon, and every living being experiences it. We should embrace our body as it changes, grows, and ages. Having said that, taking care of ourselves by maintaining a healthy lifestyle, and giving our body the right nutrition, are the foundational steps to good health. And good health is something that we believe we all deserve to have - at any age!
And, as always, if you have any health-related queries, feel free to drop them in the comment section below. We’d love to answer them for you. :)