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Understanding Your Metabolism Annie Rubin, MS, RDN

Updated: Nov 14, 2019

One of the most common frustrations I see in my clients is weight gain. It’s a frequent complaint among middle-aged women who have been active their entire lives. They eat healthfully, and they haven’t changed a thing in their daily eating patterns, yet they are slowly putting on weight. The solution is not always clear, but one major contributor is a slowing metabolism. What is metabolism and why is it important? Energy metabolism is a series of chemical reactions in the body that convert carbohydrates, fats and proteins (from the food we eat) into energy. The energy produced from metabolism is necessary for our entire body to function. We use this energy to do basic life-sustaining activities such as breathing, moving and sleeping. Metabolism, or your metabolic rate, determines how much energy your body needs to maintain homeostasis (balance), or your current weight. In theory, your weight remains stable as long as you consume enough calories to match what your body is using. However, it is not as simple as calories in equals calories out. The metabolic rate is a dynamic and moving target influenced by a number of factors. What does food have to do with metabolism? Food is the fuel that generates energy for your body to use. You need to eat enough carbohydrates, proteins and fats to keep the metabolic pathways running in order to supply your body with energy. Let’s say you just finished eating lunch. Your lunch will probably contain a mixture of carbohydrates, fats, protein, vitamins, minerals and water (and perhaps alcohol). The food you eat travels through your digestive tract and eventually reaches your small intestine. There, the carbohydrates, fats and proteins, which are all complex molecules called macronutrients, are broken down into their smallest individual components respectively: glucose, fatty acids and amino acids. These are absorbed through your small intestine, shuttled to your liver and eventually sent out into your blood circulation. The cells in your body absorb these nutrients from the blood and are then faced with a choice: should the cells a) use the nutrients to make energy or b) store them for later use? Ideally, you want your cells to use the nutrients for energy. Any nutrient that is eaten in excess is sent to small storage spaces in your liver or muscles, and when those storage spaces are filled up, the nutrients will be converted into triglycerides and stored in your fat, or adipose tissue. Weight gain occurs when, over time, consumption exceeds your body’s energy requirements.

What influences your metabolic rate? Total metabolic rate is composed of the following factors: Resting Metabolic Rate (RMR), Activity Energy Expenditure (AEE) and the Thermic Effect of Food (TEF). RMR is the largest, and most complex, contributor to metabolic rate, accounting for 50-70% of the total energy used. It is determined by body composition, age, gender, thyroid function, protein turnover (i.e., muscle protein breakdown and synthesis), the efficiency of cells to generate energy and electrolyte balance. AEE measures the amount of energy needed to be active. This includes things like getting out of bed, standing, exercise and fidgeting. It accounts for 15-30% of the metabolic rate. TEF is the energy needed to consume, digest and absorb nutrients from food. It accounts for approximately 8-15% of the total metabolic rate.

What causes our metabolic rate to slow down?

1. Age: Age is a major factor in slowing down the RMR. In a study that looked at age and body mass index (BMI), the RMR started to decline for middle-aged women around the time of menopause, regardless of body composition or size. In men, the RMR dropped for normal weight males at a younger age than it did for obese males. This suggests that hormones and adiposity (fat tissue) levels may influence the RMR. Other studies with similar findings concluded that age, independent of body composition, caused the RMR to decline.

2. Thyroid Hormones: The thyroid hormone is a major regulator of RMR. It influences the rate of energy generation in cells and the use of energy to control electrolyte balances inside and outside of the cell; it also plays a role in appetite regulation. Abnormally low thyroid hormone levels can interfere with these processes and lead to a slowdown in RMR and an increase in body weight and appetite. To learn more about thyroid hormones and metabolism click here.

3. Calorie Restriction: Calorie restriction, or eating less energy than your metabolic rate requires, can shift your body into adaptive thermogenesis, which is a state that causes your RMR to drop in order to conserve energy and muscle tissue to prevent further weight loss. Over time, the RMR can remain at a depressed level, increasing the chances of weight regain. Adaptive thermogenesis was confirmed by a study that looked at the long-term RMRs of 14 contestants from “The Biggest Loser” competition. The participants’ RMRs dropped significantly over the course of the competition due to calorie restriction, despite an increase in physical activity. Even six years after the completion of the show, the contestants’ RMRs remained at depressed levels.

What causes our metabolic rate to increase? Increasing lean muscle and fat-free mass. Since body composition is a major driver of RMR, it makes sense that increasing energy-burning tissues like muscle can increase RMR. This is supported by the notion that lean muscle tissue is a metabolically active substance, more so than adipose tissue. Muscle proteins are continuously breaking down and rebuilding (protein turnover), and this requires energy. The more muscle mass one has, the more protein turnover will occur, and the more energy will be burned. Some studies have shown that engaging in resistance training as well as high-intensity interval training can reduce body fat and may increase RMR. However, other studies have tried to explain the relationship between fat-free mass and RMR with conflicting results, most likely due to study design, methods and timing of RMR measurements.

What about food? There is limited research and conflicting evidence on the role of food in increasing metabolism. Chili peppers have been shown to increase the TEF component of the total metabolic rate and can increase fat oxidation (or fat metabolism for energy) in some people. However, the dosage, mode of consumption (whole chili pepper vs. capsule) and safe consumption of the chili pepper need further research. Green tea extract has been another food item pushed by companies and so-called “experts,” including Dr. Oz, as a way to boost metabolism. However, there is no compelling research to support the alleged metabolic boost green tea extract provides. How do we prevent our metabolic rate from falling? Unfortunately, we cannot reverse the aging process, but there are some things individuals can do to maintain their metabolic rate.

1. Engage in daily activity. Include resistance training to build lean muscle. Even if it is not guaranteed to increase the RMR, lean muscle tissue is essential as we age to prevent sarcopenia (loss of muscle tissue due to aging) and malnutrition from falling ill.

2. Be an intuitive eater. Your body knows what it needs to satisfy its energy requirements. If you listen to your internal cues for hunger and satiety, you will eat the correct amount of energy to maintain homeostasis.

3. Try not to restrict calories. Eating too little will cause your RMR to drop.

4. Get your thyroid levels tested. They may be abnormal, which may help explain a drop in RMR.


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Annie Rubin, MS, RDN

Annie Rubin is a registered dietitian and holds a Master’s degree in Nutritional Science. She completed her dietetic internship at Stanford Health Care. Before starting her private practice, Annie worked at Stanford as an outpatient dietitian in the Bariatric and Metabolic Interdisciplinary (BMI) clinic for surgical and non-surgical weight loss patients. She is currently the dietitian at The Wellness Studio at The Village Doctor, and sees clients from both within and outside of the practice. Annie is also an adjunct professor at San Jose State University in the Nutrition, Food Science, and Packaging department. In her spare time, Annie loves to run, hike and spend time with her family. Contact: Instagram: @annierubinnutrition

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