Learn the ins and outs of your body’s energy regulation system

Most people are generally familiar with the term metabolism, but the meaning of the word can differ depending on whether you’re a member of the general public or a medical professional. In the general public, metabolism typically is discussed in the context of weight change. To medical professionals and scientists, however, it has a much broader meaning: metabolism refers to the diverse biochemical processes that maintain an organism’s life. These processes require energy availability — thus metabolism is essentially about energy balance.

As if that’s not complicated enough, there’s another bit of terminology that matters: energy expenditure. Energy expenditure is the more precise term to describe what the average person means by “metabolism”; specifically, the total daily energy expenditure is the exact amount of energy your body requires to function for one day. If you provide yourself with exactly this amount of energy in the form of dietary nutrition, you maintain your body weight. But if you consume deficient or excess intake compared to your body’s requirement, the scale of energy balance is tipped, resulting in a net weight change. Total daily energy expenditure has two components that are of particular importance: basal (resting) metabolic rate and activity energy expenditure.

Basal Metabolic Rate

This is the energy required to perform the bodily functions we don’t necessarily think about. For example, even at rest you need energy for your lungs to breathe, your heart to beat and your brain to process information. In other words, the cells, tissues and organs that make up our bodies are always hard at work even if we’re lying on the couch! Basal metabolic rate (BMR) is the rate at which the body uses energy while at rest to keep these vital functions working. The BMR depends on a person’s age, sex, body composition and other factors.

Activity Energy Expenditure

Another contributor to energy expenditure is activity energy metabolism, which consists of physical movement. This includes the exercise you might do at the gym or on a run, but also your many everyday exertions like walking up the stairs or picking up a baby, and even how much you tend to fidget.

Even as we age we still have the power to increase our energy expenditure. Not surprisingly, the most impactful changes can be in the activity category; this is why exercise can contribute to weight loss and a sedentary lifestyle can promote weight gain. Once you and your doctor agree on a safe, individualized exercise plan, be sure to include both cardiovascular and strength training — cardio increases activity energy metabolism directly, while strength training can build muscle that changes body composition (muscle vs. fat) to raise your BMR. Strength training is especially important as we age to help us retain some of the muscle and bone mass that might otherwise be lost.

Other strategies to maintain or increase your BMR are to eat a wide variety of healthy foods (malnutrition can decrease your BMR) and to avoid fasting, because this places your body into “starvation mode,” which also decreases the BMR. Interestingly, setting your home’s thermostat a little bit lower at night can increase the BMR as well, as your body has to use more energy to maintain its temperature.

Even though a declining BMR is generally to be expected with normal aging, there are endocrine and other disorders that can also have an impact. For example, thyroid hormone imbalances are classically known to disrupt energy balance. If you find yourself gaining or losing weight suddenly and dramatically or if you become extremely fatigued for no apparent reason, this needs to be investigated. Visit your primary care physician or an endocrinologist, who will work with you to find an answer and help devise a diagnostic and therapeutic plan to optimize your energy expenditure to meet your personal goals.

Taking charge of your energy expenditure

During different stages of life we require more energy than in others. Young children and teens, for example, require a lot of energy as they grow and develop; they have a relatively high basal metabolic rate (BMR). Our BMR gradually drops as we age, and if we don’t also decrease our food intake or increase our physical activity, this will eventually result in weight gain. So even if an individual in their 20s could eat whatever they wanted without adding pounds, that luxury usually doesn’t last forever because of the age-associated decline in BMR.

Elizabeth McAninch, MD, is a practicing physician and Assistant Professor of Medicine at Rush University Medical Center in Chicago, IL. She specializes in endocrinology and metabolism, and has ongoing research and publications in the field of hypothyroidism and metabolism.