Optimizing your workouts for peak performance requires going above and beyond the normal routines. Part of this process requires learning how your body behaves and responds to various challenges and modifying your plan around your unique response that might be slightly (or not so slightly) different than another person’s response to the same challenge.
Metabolism is one of these characteristics that is variable and unique to you.
At a high level, the Mayo Clinic describes metabolism simply as “the process by which your body converts what you eat and drink into energy.”
Your metabolism is affected by several parameters. For example, an obvious first thought might have simply been physical activity. Hitting the gym takes energy. Period.
And another, possibly less-than-obvious source of metabolic activity, is the food you eat. That’s right, eating food requires your body to burn calories. This process, known as diet-induced thermogenesis, refers to the body generating heat and using energy in order to metabolize the food you eat back into useable energy itself.
Diet-induced thermogenesis usually accounts for only around 10-15% of your daily caloric expenditure, but a subtle note to add is the role different foods (different macronutrient profiles, to be exact) play on thermogenesis. Protein, in particular, causes the largest thermogenic spike. In fact, eating protein increases metabolism by roughly three times that of carbohydrates, and over six times that of fats.
But these sources are still not the biggest contributors to your metabolism. Diet-induced thermogenesis and physical activity combined rarely come close to the largest source of caloric burn – your basal metabolism.
Basal metabolism is simply the calories required for life. To keep you going. To keep you alive.
All your internal organs require a constant supply of energy to keep things running like hearts beating, lungs breathing, and kidneys filtering. On average, your basal metabolism accounts for roughly 70% of your daily caloric expenditure. That means, if you simply laid on the couch and didn’t move all day, you’d still burn these calories.
But your basal metabolic rate is unique to you. And it is affect by several factors including your body composition, your gender, your age, and your genetics.
The largest variable affecting your basal metabolic rate is your body composition. In fact, gender is primarily only a contributing factor in large part due to natural differences in body composition and lean muscle distribution .
Of your body composition, the largest contributing factor is fat-free mass. Most of us have heard that muscle burns more calories than fat. Indeed, in addition to organs like your brain, your kidneys, etc., your muscle mass contributes a significant portion of these calories burned. Roughly 20% of the variability in basal metabolism between individuals can be attributed to the amount of lean muscle tissue. On the contrary, fat-mass contributes roughly 6% of the variability in caloric burn per person.
Other interesting things to note about your basal metabolism is that age only accounts for roughly 2% of the variability seen in basal metabolic rates. The decreased rate of metabolism often associated with aging is more closely linked to a loss in weight and lean muscle mass.
And unknown sources between individuals that can’t be attributed to these factors represent nearly 27% of the variability. Some of this could very well be associated with genetics. Other potential sources of variability could be derived from differences in organ size and distribution of body tissues such as brown fat, visceral fat, and subcutaneous fat.
Testing for Your Basal Metabolic Rate
To test for your basal metabolic rate, you can undergo a simple testing procedure that is likely available at many nutrition or personal training clinics. The test is simple but requires some preparation on your part. Specifically, you will likely need to fast for some amount of time (usually around 12 hours) before the test. Because, if you remember, diet-induced thermogenesis might influence your test results.
The test itself requires you to breathe into a mask connected to a gas analyzer while resting comfortably in a relaxed position. While you sit and breathe for a period of time (roughly 20-30 minutes), the gas analyzer is precisely measuring how much oxygen is going in and how much carbon dioxide is going out.
And that’s it. Assuming there were no abnormalities in the test results, you will finish and be provided with your basal metabolic rate. You’ll then have an understanding of how many calories you will burn as a baseline to build your nutrition plan, your workout plan, and help set your body composition goals.
But this test also provides another interesting value worth investigating. The same basal metabolic rate test can also determine your respiratory quotient. Your respiratory quotient will provide some insight into the substrate from which your body’s energy is derived. In other words, does your body harvest its energy more from carbohydrates, proteins, or fats?
Your Respiratory Quotient
Your respiratory quotient is almost certainly going to be a fraction somewhere between the values of 0.7 and 1.0. Typically, your results will lie between 0.8 and 0.85.
This number reflects the gas analyzer’s readings of oxygen uptake and carbon dioxide output. Specifically, the ratio of carbon dioxide to oxygen provides this decimal.
If all of your energy were being derived purely from carbohydrates, your respiratory quotient would be equal to 1.0. For every glucose molecule that is harvested for energy, 6 oxygen molecules are required to produce 6 carbon dioxide molecules (a ratio of this 1-to-1 equivalency creates a fraction of 1.0).
However, if all of your energy was being derived from fat, your respiratory quotient would be closer to 0.7. For example, metabolism of the fatty acid, palmitic acid, produces 16 units of carbon dioxide for every 23 units of oxygen, creating a ratio of 0.696.
Pure protein metabolism produces a respiratory quotient of around 0.81. However, your body does not store appreciable amounts of protein for use as an energy source. Protein turnover occurs at a fairly constant rate and would markedly change if protein was consumed before exercise and other readily available fuel sources were used first.
Your respiratory quotient can be used as a way to help understand your body composition and metabolic fuel source in order to adjust your body composition goals and direct your nutrition plan.
And these techniques can also be applied to studying your active metabolic behavior, too. Performing these tests during exercise can give you further insight into how your body adjusts its fuel sources during specific kinds of workouts (think sprinting, endurance events, weight lifting, etc.). This can further help direct your nutrition plan to optimize your fuel source for each particular type of training you are targeting.
Personalizing your training and nutrition plan around your body’s specific metabolic behavior can help optimize your performance. Some commonalities exist. For example, protein produces a higher diet-induced thermogenic response than carbohydrates and fats, and body composition (particularly fat-free mass) composes the majority of the variability in basal metabolic rate.
But variability in your respiratory quotient may provide further insight into your body composition and the composition of your body’s fuel sources. Actively applying these personal characteristics can help you clarify your goals and personalize your diet and training plan for peak results.
- Johnstone, A. M., Murison, S. D., Duncan, J. S., Rance, K. A., & Speakman, J. R. (2005). Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine, The American Journal of Clinical Nutrition, 82(5), 941–948. https://doi.org/10.1093/ajcn/82.5.941.