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Metabolism Matters - Part 2


In the previous article on metabolism, we explored energy balance, metabolism and emphasized the importance of measuring Resting Metabolic Rate (RMR). As you begin to incorporate RMR measurements with your clients, you may be challenged to provide additional information about the factors that impact metabolism.

Body Weight

A person with a greater body weight will have a higher RMR than a smaller person because their body has to work “harder” to support and maintain the extra weight.

As an individual gains or loses weight, their RMR can also change. With weight loss, there is often a decrease in RMR. The two main reasons for this decrease are that (1) a smaller body requires fewer calories to maintain the same physiological functions; and (2) during weight loss, the body may try to conserve energy in response to a lower calorie intake. The table below shows RMR changes associated with weight loss. Because of variability between individuals, it is not possible to predict the magnitude of RMR change in your clients as their weight fluctuates. When weight gain or weight loss occurs, re-measurement of RMR is key.

Body Composition

If two people are the same body weight, the person with more muscle will have a higher RMR than the person with less muscle because muscle is more metabolically active than fat. In fact, muscle burns two to four times more calories than fat at rest. Therefore, as a person gains muscle mass, their RMR is likely to increase. The magnitude of this increase in RMR is not clear. Literature supports an increase in RMR anywhere from 13 to 100 calories per kilogram of muscle.

A weight gain that is attributed to a gain in fat mass will not affect RMR as much as a gain in muscle mass; however, overall weight gain regardless of body composition results in a higher RMR.

During weight loss, it is not uncommon to see a drop in RMR (see table above). Researchers believe that when weight is lost as a result of moderate exercise and dieting, the composition of that weight loss is about 75 percent fat and 25 percent lean tissue.

Age

Resting metabolic rate gradually declines with age on the order of two to three percent per decade. This age-related decrease in RMR can be partially explained by a reduction in quantity and metabolic activity of lean tissue, although a large component of this decline may be due to a decrease in muscle mass. 

Gender

Men have a higher RMR than women, primarily because of greater muscle mass. After controlling for differences in muscle mass, fat mass and aerobic fitness, women only have a slightly lower RMR than men (about three percent).

Exercise

The effect of exercise on RMR is dependent on a number of factors, including the type of exercise, intensity and duration as well as individual and environmental factors. Burning calories even after the exercise session is over is referred to as excess post-exercise oxygen consumption (EPOC). With cardiovascular training, RMR can be elevated for as long as eight hours after a 60-minute session of moderate intensity. A vigorous strength training session of 90 minutes can elevate RMR 11 to 12 percent two hours after the session; RMR can remain elevated as much as nine percent the following morning. Most people, other than elite athletes, do not exercise at a high enough intensity to trigger a significant EPOC.

The chronic effect of exercise refers to the lasting impact that the exercise has on RMR. Strength training can increase RMR seven to nine percent after a 24-week strength training session in men and women. For a strength training program to have a chronic effect on RMR, a person has to gain muscle mass. This requires intense strength training. Aerobic exercise does not appear to have a chronic effect on RMR. For example, an endurance runner who does not participate in resistance training will not increase or decrease their RMR permanently. Keep in mind that their total metabolism will be increased due to the calories burned during the running sessions.

Recommendations for RMR Testing:

Food

The consumption of food creates an increase in energy expenditure called the thermic effect of food (TEF). This increase in energy expenditure is due to the energy the body needs to digest and absorb food. TEF peaks approximately 60 to 90 minutes after eating and may increase RMR for up to five hours depending on the size and composition of the meal.

TEF is nearly proportional to the number of calories in a meal. For example, a meal of 500 calories will produce a thermic effect of 50 calories, and a meal of 800 calories will produce a thermic effect of 80 calories (assuming a 10 percent TEF). The TEF of a typical mixed-nutrient meal (i.e., 60 percent carbohydrate, 25 percent fat, 15 percent protein) is approximately 10% of calories consumed. However, a meal composed of a greater percentage of protein will elicit a greater TEF (as much as 25 percent of calories consumed).

The measured thermic effects of nutrients are zero to three percent for fat, five to 10 percent for carbohydrate and 20 to 30 percent for protein for the caloric content of food.

Recommendations for RMR Testing:

Nutrition Supplements/Drugs

Research studies conclude that consuming caffeine in differing amounts increases RMR from three to seven percent. The dosages of caffeine ranged from as little as 5 mg to as much as 100 mg per kilogram of body weight. The elevation in RMR is typically no longer than three to four hours. Caffeine content of beverages (from the US Department of Nutritional Services):

Ephedrine can increase RMR three to 16 percent in doses ranging from 0.25 mg per kilogram body weight to 150 mg per day. It appears there is a greater increase in RMR in the first 30 to 90 minutes, after which RMR returns to near resting levels. It has been reported that ephedrine can increase energy expenditure 7.3 percent during “sleep” hours of the night due to activation of the sympathetic nervous system. A 50 mg oral dose of ephedrine reaches maximal concentrations in approximately two hours and has an elimination half-life of approximately nine hours.

Nicotine, in the form of cigarettes or nasal spray, can increase RMR from three to seven percent up to three hours.

Recommendations for RMR Testing:

Thyroid Levels

Untreated hypothyroidism can lower RMR by approximately 30 percent. If an individual has lower than normal thyroid levels (but not low enough to be classified as hypothyroidism), their RMR can be 15 percent lower than with normal levels. It is estimated that between 7.5 to 10 percent of the populations has low thyroid levels. Most of these cases occur in women (80 percent).

Menstrual Cycle

Changes in hormone levels during a woman’s menstrual cycle affect resting metabolic rate. The menstrual cycle is divided into three phases: follicular, ovulatory and luteal. The first day of menstruation marks the beginning of the follicular phase. The luteal phase begins after the egg is released from the follicle and ends when the lining of the endometrium degenerates and is shed (which marks the beginning of the follicular phase). RMR begins decreasing at menstruation and reaches its lowest point about one week prior to ovulation. It then rises until the beginning of the next menstrual period.

It is estimated that there may be up to a five percent increase in RMR during the luteal phase of the menstrual cycle.

Recommendations for RMR Testing:

Menopause

On average, women gain about one pound per year during menopause. Women who have undergone menopause experience a loss in ovarian function and luteal phase of the menstrual cycle. As you recall, RMR is estimated to be elevated by a magnitude of five percent during the luteal phase. The question arises whether this weight gain is due to a change in RMR and whether estrogen replacement therapy has an effect on the weight gain or RMR. It appears that the weight change tends to be associated more strongly with physiological and behavior changes associated with aging than with menopause.

Weight gains may be related to decreases in muscle mass. Research has found that the factor most related to weight gain associated with menopause is decreased physical activity. Postmenopausal women are less physically active during their leisure time than age-matched premenopausal women. Combine this factor to the age-related decline in RMR, and this weight gain can easily be accounted for.

Calorie restriction and exercise

Restrictive diets consisting of less than 1,000 calories per day can result in an acute decrease in resting metabolic rate. This decrease in RMR may be due in part to a decrease in thyroid level (triiodothyronine or T3) and the release of the hormone norepinephrine that accompanies caloric restriction. Adding an exercise program to a diet can preserve some but not all lean muscle tissue. Persons with a history of “yo-yo” dieting do not appear to have a permanent decrease in RMR.

Time of day

Resting metabolic rate has typically been measured in the morning after a 12 hour fast and 12 hours post-exercise. RMR measured in the afternoon has been determined to be approximately 100 calories higher than in the morning. When measuring clients in a real-world setting (as compared to a research setting), it is not always possible to measure RMR first thing in the morning.

In order to compare RMR measurements from one time to the next, it is important to re-measure a client at the same time of day every time. The question arises whether to deduct a 100 calories from an afternoon measurement to compensate for time of day. Unfortunately, not every person experiences a similar increase in RMR. In one study, the RMR difference between morning and afternoon measurements ranged from 27 to 171 calories, depending on the individual.

Understanding the various factors that impact metabolism allows you to appropriately counsel your clients and answer their questions. Focusing on those factors that can be changed, such as body composition, will ensure your clients stay on the right track to achieve their goals.

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