It’s very common for individuals to gain weight, lose muscle, and have more difficulty staying in shape as they age. But why is this? The answer lies in our metabolism, which tends to start slowing down each passing decade after the age of 20. This is a natural phenomenon that we must be aware of if we hope to maintain a stable weight and a healthy lifestyle.
When we talk about metabolism, we are referring to all anabolic (synthesis) and catabolic (decomposition) reactions in the body. Metabolism encompasses virtually every biological reaction in the body- from breaking down nutrients into energy to using that energy for building new tissues (2).
Most of the time, when we hear about metabolism it is in relation to weight-loss or weight gain. This is because the metabolism has a direct impact on how many calories we get from our food and how our bodies will use those calories. For the most part, people with a slow metabolism gain weight more easily while those with a fast metabolism don’t. But, there is more to it than that!
The metabolic rate -the rate at which we yield and utilize energy- of an individual is determined mostly by genetics. Other factors do play an important role, including age, gender, muscle mass, daily activity, sleep, and hormone levels. Metabolism is a complicated system with many variables that could affect its efficiency.
A variety of metabolic analytics exists to help better gauge energy expenditure and its effect on weight -loss/-gain. Such examples include Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR), which are used interchangeably.
Basal Metabolic Rate (BMR)- This is a measure of the minimum amount of energy required to stay alive. In other words, how many calories it takes to maintain all basic functions of the body.
Resting Metabolic rate (RMR)- This is a measure of energy expenditure at rest- with no physical activity. In other words, how many calories we burn by just sitting there.
While these terms are often used interchangeably, BMR is considered a more accurate measure of metabolic rate.
How Does Age Affect Metabolism?
Simply put, at some point in our 20s our bodies shift from a “growing” stage to an “aging” stage. Every year thereafter the metabolism becomes exponentially slower. This is because as we age (especially noticeable after turning 30 years old or so), our bodies go through a series of changes that slow down metabolism. This includes a loss in lean muscle mass, lower levels of anabolic hormones, and less gene expression associated metabolic and biosynthetic processes (3).
The result is a gradual decline in how many calories the body burns daily. Even if one consumes the same number of calories as before, the decline in metabolism leads to an excess of calories getting stored as fat. Some would describe this phenomenon as the body becoming more effective at utilizing energy, requiring fewer calories to carry out the same functions with age.
First and foremost, watch your calories! It is important to focus on eating nutrient-dense foods as opposed to energy-dense foods. Nutrients-dense food provide a large number of nutrients per calorie, while energy-dense foods provide a large number of calories per gram of food.
Fruits and vegetables are great examples of nutrient-dense foods, while bread and potato chips are very energy-dense. Sticking to nutrient-dense foods ensures that you are getting the benefit of nutrients -including fiber and antioxidants- while limiting calories. Stay active!
Preventing Metabolic Decline
Metabolic decline due to aging is inevitable, but we can still slow it down. The best way is to exercise plenty and to build lean muscle mass. Lean muscle is very metabolically active and burns lots of calories even when we are resting. For more help with weight-maintenance and preventing metabolic decline with age, consult a registered dietitian and exercise expert. These individuals are very knowledgable in lifestyle interventions that will help keep you healthy throughout your life.
- Galgani, J, and E Ravussin. “Energy Metabolism, Fuel Selection and Body Weight Regulation.” International Journal of Obesity (2005), U.S. National Library of Medicine, Dec. 2008, www.ncbi.nlm.nih.gov/pmc/articles/PMC2897177/.
- “Biochemical Principles of Energy Metabolism.” Coursera, www.coursera.org/learn/energy-metabolism.
- Sohal, Rajindar S., and Richard Weindruch. “Oxidative Stress, Caloric Restriction, and Aging.” Science, American Association for the Advancement of Science, 5 July 1996, science.sciencemag.org/content/273/5271/59.