When the scale refuses to budge, many of us reach for the “obvious” reason: our slow metabolisms. Turns out, however, most metabolisms are relatively normal. The good news: most slow-to-normal metabolisms can be boosted with lifestyle changes. Heather Huntsman, Ph.D., CSCS explains the truth about what factors determine metabolic rate and dives into what we can and cannot control when it comes to natural calorie burn.
What is Metabolism?
Metabolism is the biochemical process of converting the food we consume into a useable form of energy that our cells and organs can use to do their job ⎼ keep us alive and keep us moving.1
In order to survive we must consume energy in the form of food and drink to provide us with the energy we need to live. Without this energy, most commonly measured in calories, our heart wouldn’t have the energy it needs to beat. Our brains would not have the energy it needs to function and control all of our body’s life-sustaining systems.
Total metabolism is determined by 3 general categories.1,2
- Basal Metabolic Rate. This the total energy requirement of all of the biological processes necessary to keep us alive (e.g. heart beating, kidneys filtering blood, liver detoxifying the body, etc.) This is the largest contributor to metabolism and accounts for 60-75% of an individual’s total metabolic rate – and is also sometimes referred to as resting metabolic rate.
- Physical Activity. While this is the category we think of the most, and is the category that every treadmill, Fitbit, and exercise program tracks. The energy or calories we burn as a result of increased movement accounts for 15-30% of an individual’s total metabolic rate.
- Thermic Effect of Food. When we consume food, energy is required to digest convert that food into an energy that the body can use for other important functions. This accounts for about 10% of an individual’s metabolic rate.
What factors affect your metabolism?3,4
- We are born with a certain metabolic rate that is coded into every cell in our body.
- As we age, our metabolism slows as a result of many of the factors listed below.
- Men tend to have a higher metabolic rate than women.
- Body size. Bigger people tend to have a higher metabolic rate than smaller people.
- Body temp. Extreme temperatures stress our body and can have a slight impact on our metabolic rate.
- The levels of certain hormones determine the energy requirements for tissues/organs.
- Body composition. Lean body mass, or muscle mass, requires and uses more energy than fat mass.
- Activity level. The more you move, the more energy you need.
What factors can I control?1,4
- Caloric Intake. First and foremost, we all operate under the laws of thermodynamics. Energy is never created or destroyed, only converted from one state to another, and there is no getting around this. How does that translate to us? Every calorie we consume must be used or stored, so being aware of the energy we consume is paramount in the weight loss or maintenance game.
- Accounting for 15-30% of our total metabolic rate is a big range, and a big deal. You may not be able to control sex or age, but most of us can control how much we move.
- Body Composition. Increasing the total lean muscle mass the body has can increase basal or resting metabolic rate. While we can’t really change the size and metabolic rate of our kidneys, which pound for pound have one of the highest energy needs of any organ in our body, we can increase the size of our muscle tissue.
Is a slow metabolism the reason for your weight gain?
More than likely the answer to this question is no. While it is tempting to want to blame weight gain on metabolism, the data does not support that theory. That isn’t to say that there aren’t conditions that can affect metabolism like hypothyroidism4 and disorders that lower sympathetic nervous system activity,5 but these are relatively rare and are unlikely to be the underlying issue. If fact, individuals that are overweight or obese are more likely to have a higher absolute metabolic rate than their sex and aged matched normal weight counter parts.3
Why is that? Remember that body size and composition play an important role in resting metabolic rate. Yes, the more overweight a person is, the more overall fat mass they will have, and fat does not significantly contribute to metabolism. That said, the more fat mass a person has, the more muscle mass they need to carry that weight, and muscle does contribute to metabolic rate. Therefore, more body weight often equals more muscle mass, and more muscle mass means a higher metabolic rate.3
So, although 60-75% of your metabolic rate is predetermined and predominantly outside of our control, a slow metabolism is rarely an independent predictor of weight gain. In fact, overwhelmingly the data suggest that starting and stopping diets, exercise, and medications are more effective in predisposing individuals to weight gain.2
Curious what role your metabolism plays (or doesn’t play) in your healthy weight goals? Get it tested. The amazing thing about metabolic testing is that it’s non-invasive, highly accurate, and can help you precisely set your nutrition plan so that you are able to achieve your weight goals in a safe and healthy manner. Contact the Composition ID team and get started today!
- McMurray RG, et al. (2014) Examining variations of resting metabolic rate of adults: A public health perspective. Medicine & Science in Sports & Exercise. 47(6):1352-1358. doi: 10.1249/MSS.0000000000000232.
- Anthanont P and Jensen MD. (2016) Does basal metabolic rate predict weight gain? American Journal of Clinical Nutrition. 104:959-963. doi: 10.3945/ajcn.116.134965.
- Galgani J and Ravussin E. (2008) Energy metabolism, fuel selection and body weight regulation. International Journal of Obesity. 32(7):S109-119. doi: 10.1038/ijo.2008.246.
- Mayo Clinic Staff. (November 10, 2020) Metabolism and weight loss: How you burn calories. Mayo Clinic, Healthy Lifestyle: Weight Loss. mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/metabolism/art-20046508
- Davy KP and Orr JS. (2009) Sympathetic Nervous System Behavior in Human Obesity. Neuroscience & Biobehavioral Reviews. 33(2):116-124. doi: 10.1016/j.neubiorev.2008.05.024.