Does Metabolism Really Slow as we Age?
Recently I came across a study I wanted to share with you in Science magazine entitled “Daily energy expenditure through the human life course.” In this paper Pontzer et al. studied a very large population of men and women ranging from 8-day-old babies to individuals as old as 95. They assessed total daily energy expenditure to see how metabolism changes over one’s lifespan. I’ve talked about how important metabolism is countless times, so I wanted to share the findings of this research group.
When I work with middle-aged individuals I frequently hear that the reason they have been gaining weight in adulthood is due to their slowing metabolism. It appears to be common fitness knowledge that as we age our metabolism slows and this is why it gets harder to keep fat off. However, this is only partly true. It is true that our total daily energy expenditure declines as we age on average, but the reason for this is not because of a “slowing metabolism.” When adjusted for muscle mass and activity level it actually appears that your metabolism doesn’t slow down at all.
Many of you are familiar with the fact that muscle mass is highly correlated with resting metabolic rate (RMR). This is why I focus so heavily on strength training and high protein intake with my clients. As we age past 30-35 years we start to lose a small amount of muscle mass on an incremental basis. This is referred to as “sarcopenia” and it affects all of us if we do not intervene to mitigate or reverse it. What the researchers found in this study is that total daily energy expenditure in adults from 20-60 years old is exactly the same when adjusted for total muscle mass. This means that all of the reduction in RMR you will see in an adult under the age of 60 is directly proportional to muscle mass reduction and is completely preventable!
After around the age of 60 is when metabolism actually starts to slow a bit when muscle mass is equated. This phenomenon actually appears to be a slowing of metabolic processes in the body. What this means is that a 70-year-old who has the exact same body composition as an equivalent 40-year-old will actually burn less calories per day than them. If you are below the age of 20 it appears you are in luck because your metabolism will actually be elevated over that of the 20- to 60-year-olds even when fat and lean mass is the same.
(Total Energy Expenditure (TEE) and Basal Energy Expenditure (BEE) over the life course of humans when adjusted for total lean body mass)
So what does all this mean for you? It means that if you are under the age of 60 a “slowing metabolism” is likely not the reason for fat accumulation. Individuals can have slow metabolisms because genetic variation is still a factor and hormonal imbalances still exist that can slow metabolic rate, including hypothyroidism. However, for any given individual their resting metabolism will not start slowing down until around the age of 60 or later. When this slowing does occur it will be small, incremental, and over time.
Luckily for us our basal metabolic rate isn’t the be-all and end-all to how healthy we are. It accounts for about 50% of our total daily energy expenditure with the remaining 50% being completely under our control. So how do we go about increasing the rest of our calorie burn? I’m glad you asked.
1) Move a lot- The more you move the more calories you will burn and the more you can eat. However, avoid too much intense cardio because this will inhibit muscle growth and increase stress. I prefer you increasing your movement by walking more and adding in more low-intensity cardio. Studies have shown that even small constant movements like fidgeting burn significant amounts of calories and could help you stay lean.
2) Build muscle- Muscle is what consumes energy at rest which is the key to burning calories around the clock and staying lean year round. As stated before if you are over the age of 30 you are losing muscle on a yearly basis and this will hurt your metabolism. Follow a training program designed to build muscle and you will reap the benefits, especially if you are older.
3) Increase Protein- I’ve said it a million times. Protein will help support muscle growth and will generally not be stored as fat if overeaten. In addition it has a high thermic effect (TEF) which means it uses more calories to break down than the other macronutrients.
4) Eat whole foods- Whole foods are higher in protein, fiber, and micronutrients than processed foods. All of these will help keep you full longer, make your body function better, and increase your caloric burn by boosting your TEF.
5) Stay hydrated- Your body functions better and at a higher rate if you are hydrated. In addition you will feel more energized if you are hydrated which will allow you to move more!
In strength,
Mark Kolding, M.S.
Personal Trainer/Triathlon Coach
ACSM CPT
510-734-2817