Have you ever wondered why some people can seemingly eat whatever they want and stay the same weight, while all it takes for you to gain a few pounds is indulging in a couple of takeout orders?
The answer lies with your metabolism, which is influenced by a variety of factors—some that you can control, and others you can’t. Before learning how (if at all) you can speed up metabolism, here’s what it is and how it works.
What Is Metabolism?
In very basic terms, metabolism is the way your body breaks down food for energy, says Audra Wilson, M.S., R.D., L.D., C.S.C.S., a bariatric dietitian at the Northwestern Medicine Metabolic Health and Surgical Weight Loss Center at Delnor Hospital in Geneva, Illinois.
Your body digests different macronutrients at different rates. For example, carbohydrates start to break down in your mouth while you’re chewing, thanks to salivary enzymes, says Wilson, but the bulk of absorption happens in the small intestine. Each macronutrient must be broken down to its most basic component—carbs into glucose, fats into triglycerides, and protein into amino acids—before it can be absorbed and used by your body for different processes, she adds.
In order to understand how this impacts your metabolism, first, you need to know your total daily energy expenditure or TDEE. Essentially, this is the total amount of calories you burn in a day portioned into two categories: Thirty to forty percent of these calories are based on your activity level (a long-distance runner is going to need more energy/calories than an office worker), and 60-70 of those calories are just the basic necessity to keep your body alive and functioning properly. Both vary from person to person.
The latter is something called your resting metabolic rate, or RMR. Basically, a higher RMR (or required calories to remain in homeostasis at rest) means you burn more calories at rest (i.e., you can eat more without gaining weight), whereas a lower RMR means your body burns fewer calories at rest (i.e., you have a “slow” metabolism, which can lead to weight gain if you’re eating more calories than you’re burning). Knowing your TDEE is important because it helps you determine how many calories you should be eating to meet your goal (whether that’s to lose, gain, or maintain your weight).
Many different factors go into your individual RMR: Your age, weight, gender, lean body mass (muscle), calorie intake, and activity levels. Hormones can have an impact, too, says Wilson. For example, if you have a hypoactive (underactive) thyroid, your RMR will be lower, but a hyperactive (overactive) thyroid can increase your RMR.
There’s no doubt about it: Metabolism is necessary for life. You just don’t have control over many of the factors that influence it.