TIME 4 FAT LOSS SERIES – PART 3 – Is your metabolism making you fat?

16 April 2019


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This is the third in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.

PART 3 – Is your metabolism making you fat?


What is this thing called metabolism and how does it influence our level of body fat?

The term ‘metabolism’ simply refers to the sum of all the biochemical reactions which occur within the body. The aspect of metabolism which people are referring to when they talk about “slow” and “fast metabolisms” is the amount of energy or calories they expend at rest.

A number of terms are used to describe this.  These include resting metabolism, resting metabolic rate, basal metabolic rate and resting energy expenditure. Although they have slightly different meanings, in short, they refer to the minimal amount of energy needed to maintain the body’s essential physiological functions, such as breathing, circulation and temperature regulation, while at rest (1).

The average total daily energy expenditure (TDEE) of a person engaged in normal activity ranges from 1,800 to 3000 kcal per day (2).  Our resting metabolic rate accounts for approximately 50-75% of this with the energy we expend through physical activity and the digestion of the food we eat accounting for the remainder (3). Consequently, if you expend greater amounts of energy at rest you will typically find it easier to maintain a leaner physique than those who expend less, as fewer excess calories are available to be stored as body fat.


How can I find out what my resting metabolic rate is?

The most accurate assessment of resting metabolic rate (RMR) is achieved using specialist equipment in a laboratory setting. As this is impractical for most people, a number of formulae have been developed to estimate RMR with reasonable accuracy (plus or minus 10%) (4).


Method 1

The first method simply requires your body weight in kilograms and is calculated in the following way.

Men: RMR kcal/day = body weight (BW) in kilograms (kg) x 24.2

Women: RMR kcal/day = BW (kg) x 22 (4)

For example, a male client weighing 100 kg (220 Ib) would have an estimated RMR of 2,420 calories per day (100 x 24.2 = 2,420 kcal/day)

This means that this person would need to consume 2,420 calories just to maintain their body’s essential functions at rest!

Method 2

Our resting metabolic rate is influenced by the amount of muscle (fat free mass) we have; the greater muscle mass, the greater the energy expenditure at rest to support it. Therefore, a relatively easy and more accurate estimate of RMR can be made based on fat free mass (FFM). To use this method, you’ll need to have your body composition assessed and then use the following equation.

Unlike the first method, this is a generalised equation which can be used for males and females over a wide range of body weights.

RDEE= 370 + 21.6 (FFM, kg) (3)

Using this formula an individual weighing 90.9kg (200 Ib) with 21% body fat would have an estimated FFM of 71.7kg (158 Ib).  This translates to an RMR (using 72 kg) of 1925 kcal (370+ 21.6 x 72 =1925 kcal).


What factors influence our resting metabolic rate?

  • Muscle mass:

Our RMR is directly related to the amount of muscle we have. This is because muscle burns more energy than fat at rest; 7 to 10 kcal per day per pound in comparison to a pound of fat, which requires approximately 2 kcal/day.  Therefore, more muscular people have a higher RMR than fatter people of the same weight (3).

  • Gender

The RMR of women is on average 5-10 % less than for men of a similar size. This is primarily due to men having more muscle and less fat than women of a similar size (2).

  • Surface area

The greater the surface area an individual has, the greater the heat loss that occurs across their skin, which increases RMR, as more energy is required to maintain their body temperature (3). Therefore, as big people have a larger surface than small people, they also tend to have higher RMRs.

  • Age

In sedentary men and women, RMR tends to decrease by about 2-5% per decade after the age of approximately 25 years (5). This is largely due to age related decreases in muscle mass, with sedentary adults losing on average 0.5 pound of muscle per year during their 30s and 40s, which may double to one pound per year past the age of 50 years (6).

  • Stress

Stress can increase our RMR by increasing the activity of the sympathetic nervous system (2).

  • Hormones

As we saw in the first part of this series, our hormones can exert a considerable influence on our RMR. For example, thyroid hormones can increase RMR by 60-100%, while underproduction of the hormone thyroxine can reduce RMR by 30-50%. Growth hormone, epinephrine, norepinephrine, and various sex hormones may elevate RMR by as much as 15-20%. These hormones increase during exercise and may be responsible for the elevation in RMR that continues after exercise (5).

  • Drug induced thermogenesis

Certain substances, such as caffeine, have a stimulating effect on our metabolic rate and cause an increase in heat output, which is referred to as drug induced thermogenesis (2).

  • Temperature

Environmental temperature can have significant impact on our RMR.  For example, people living in warm climates typically have an RMR 5 to 20% greater than those living in more temperate regions (3). On the other hand, during extreme cold, a person’s RMR can double or triple as shivering occurs in an attempt to maintain a stable core temperature (3).


Do fat people really have slower metabolisms?

As RMR is related to body size and surface area, taller, heavier people tend to have a higher RMR than shorter, lighter people. Also obese people often have a considerable amount of muscle mass due to the fact that they have to carry more weight around (7).

Interestingly, a number of studies have shown that some obese people actually eat less than people of the same age and sex with average body fat levels. However, they are much less physically active (2).

People tend to overestimate their levels of spontaneous physical activity with increasing levels of fatness. This includes day-to-day activities such as walking to the shops, vacuuming, and even fidgeting and changing posture.

A study by Levin et al (8) found that lean people burned about 350 more kcal each day than their obese counterparts through spontaneous physical activity. It also showed that the obese subjects were on average seated for two hours longer per day than the lean subjects.


How does exercise effect resting our resting metabolic rate?

As resistance training can increase our muscle mass, it can directly increase our resting metabolic by 7 to 10 kcal per day per pound of additional muscle. This may not seem much but over a year this equals the energy equivalent of 1 pound of fat. Over 10 years, the additional energy expenditure of just 1 pound of muscle may help to prevent the accumulation of 10 pounds of fat! This is in addition to the calories we expend doing the exercise, which is approximately 5.9 kcal per kilogram of bodyweight per hour.


EPOC: the ‘after burn’ effect

Even after we have finished exercising, our metabolism remains elevated for some time afterwards, as the body recovers from its exertion. This involves the replenishment of energy stores, removal of lactate from the blood stream, and the return of circulation and temperature to normal (2). This is referred to as excess post exercise oxygen consumption (EPOC) or ‘after burn’. As a guide, we expend approximately 15 kcal during recovery for every 100 kcal spent during exercise (7).

Returning the metabolic rate to resting level after exercise can take several minutes after light activity and up to 38 hours after more exhaustive exercise (9).

The intensity of the exercise has a significant effect on the amount of energy we expend during the recovery period, with higher intensities inducing greater metabolic responses that take longer to dissipate (2).

EPOC can create a significant expenditure of energy when considered over the entire recovery period. For example, if EPOC is 15 kcal/hr and metabolism remains elevated for 5 hours this produces an additional energy expenditure of 75 kcal. If you train 5 days per week, this equates to an energy expenditure of 375 kcal. In 10 weeks the recovery period alone could a create calorie deficit equivalent to 0.5 kg or 1 Ib of fat (3).


When we try to find the cause of our weight gain and when devising a fat loss programme, we tend to place the greatest emphasis on our energy intake, i.e., our diet. Whilst this is important, we must also be mindful of our energy expenditure and its influence on our levels of body fat. The more we can increase our energy expenditure and particularly our RMR, the greater the amount of fat our bodies will be breaking down even when we are at rest. So when we are sleeping our body will be nibbling away at our fat stores.

Later in this series, we will be looking at the most effective forms of exercise for enhancing energy expenditure both during the activity and the recovery period.

CLICK HERE TO READ: Part 4 – How much fat should I aim to lose and how quickly?