Metabolism

In Causes of Obesity we ended by saying that the metabolic rate of obese individuals is higher than their leaner peers due to the energy required to support a greater body mass.

What is my Basal Metabolic Rate?

The metabolic rate, or basal metabolic rate (BMR), is the amount of energy the body burns up in a state of complete rest, with the sympathetic nervous system inactive (responsible for fight-or-flight, stress, anxiety), after eight hours of sleep in a darkened room and the digestive system is also inactive, which takes about twelve hours of fasting.

The resting metabolic rate (RMR) is the metabolic rate at rest, but under less rigorous conditions, without the fasting overnight in the test lab.

Most people searching on the internet looking for ways to increase their metabolism, look for BMR, but in reality their RMR is more likely to be appropriate for their needs.

What Does the Resting Metabolic Rate Tell Me?

The resting metabolic rate tells you how much energy (resting energy expenditure, REE) your body uses up just keeping the vital organs, such as the liver, lungs, brain, heart, kidneys, sex organs and the rest of the nervous system, muscles and skin up and running.

This RMR accounts for between 65 to 75 percent of daily total energy expenditure (TEE). The remainder is made up of day to day activity (about 20% of TEE) and food thermogenesis from digestion (also called The Thermic Effect of Food, is responsible for around 10% of TEE).

Factors that Affect Your RMR

Each of us is unique and so is our metabolism. There are many factors that can influence our metabolic rates.

Ethnicity – many studies suggest that black Africans and Asians have a lower RMR than white Caucasians.

Age – sadly, as we get older our metabolic rate declines.

Body Composition – the more muscle we have, the higher our metabolic rate will be. The greater our height and weight in general, the greater the metabolic rate.

Physical Activity – there is substantial evidence to suggest that both aerobic and anaerobic exercise raises the RMR (see Note). There have been suggestions that excessive exercise may also reduce metabolic rate, perhaps as a survival mechanism analogous to the fight-or-flight response observations made in animals.

Climate – sounds daft I know, but if you live in a cold climate where the body has to work to maintain a stable body temperature, then your metabolic rate increases.

Diet – there are a few factors within your diet that may improve your metabolic rate. Smaller, regular meals can increase fat oxidation (fat burning power); the thermic effect of protein consumption is greater than eating either fats or carbs; dieting with low physical activity levels reduces metabolic rate, so even successful losers have a lower metabolic rate than expected long after the end of the dieting effort (see Note).

Pregnancy – the increased demands on the body can result in an increased metabolism.

Note: Although not specifically related to RMR, the thermic effect of food is greater in those who habitually exercise than those who are sedentary.

Resting and Basal Metabolic Rate Calculators

The clinical methods for determining BMR and RMR are measured through calorimetric analysis, although a rough estimate of either basal or resting metabolic rate can be calculated. Estimation via equations is a far more common method than clinical calorimetry although less accurate.

Basal Metabolic Rate Formula

Historically, the basal metabolic rate was calculated using the Harris-Benedict equation, which was introduced in 1918, but this calculator overestimates by anywhere from 5 to 15%.

More modern equations have been introduced which are more accurate when compared to calorimetry results.

However, these even modern calculators are still fairly inaccurate.

In one study, the calculated resting metabolic rate was more than 10% different from measured in 22% of subjects using the Mifflin St Jeor equation, 33% out using the old Harris-Benedict equation and 35% using the Owen equation, so the most accurate was the Mifflin St Jeor equation, introduced in 1990.

Mifflin St Jeor equations:

For women:

(10 x w) + (6.25 x h) - (5 x a) - 161

For men:

(10 x w) + (6.25 x h) - (5 x a) + 5

Where

w is weight in Kg
h is height in centimetres
a is age in years

Source: A new predictive equation for resting energy expenditure in healthy individuals. Mifflin et al., American Journal of Clinical Nutrition, 51 (2), 241.

These equations for predicting resting energy expenditure (REE) were derived from data from 498 healthy subjects, including females (n = 247) and males (n = 251), aged 19-78 y (45 ± 14 y, I ± SD) of normal weight (n = 264) and obese (n = 234).

The values for resting metabolic energy usage can be converted into rough estimates for daily calorific requirements by multiplying by an activity factor, proposed by McArdle.

Source: Exercise Physiology. McArdle, William D. 2nd edition. 1986. Lea & Febigier, Philadelphia

Bottom Line:

The metabolism is an important area of study for slimmers, not from the perspective of using the equations to calculate a RMR using the Mifflin St Jeor equation or any of the other equations for that matter, but from the viewpoint that you can work with the factors that affect the metabolic rate to improve your slimming success.

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