The word “metabolism” is mostly used these days when referring to weight management, and a “slow metabolism” is often blamed for inability to lose weight.

Metabolism refers to all the body processes which convert the food you eat into the energy required to maintain life – for breathing, heartbeat, kidney function etc. – and for other functions such as walking, or other muscle use.

In the simpler view of weight management, if more energy is used by these body processes than you take in from your diet, then your body must draw from its reserves (usually stored as fat) to get the “fuel” needed to keep these body processes going. This obviously will lead to weight loss. This is the kilojoule in vs kilojoule out model of weight management.

However, the metabolism/weight management interaction is more complex than that. Input from major clinical trials and reports from reputable medical centres around the world has covered aspects such as how metabolism relate to exercise, weight loss, medication, age, diet and weight loss.

It is often observed that losing or even maintaining weight becomes more difficult as we age.

Age can slow your metabolism for several reasons. In general, people tend to be less active as they age, thus burning less energy. If intake is not also reduced this may result in weight gain.

Age related hormone changes also play a part. All tissues in the body are affected to some extent by sex hormones. In women, menopause is a relatively quick transition from normal production of Oestrogen to a state of deficiency. Men also undergo a change in hormone levels, but the decline in testosterone production is much slower, spanning many years, so the effects are not as obvious as with women and the menopause. A notable effect of declining testosterone is loss of muscle mass. As muscle tissue is very metabolically active and is a prime “fuel burner”, having less muscle means less metabolic activity and less fuel burning, and thus possible weight gain.

An obvious solution to this situation is to remain physically active to prevent excess muscle loss and counteract the effects of declining hormone levels. Studies have shown that in women especially, resistance exercises (weights as opposed to aerobics) sustain muscle mass and assist in weight management.

Thyroid hormone production can also decline over time, and this has a significant impact on metabolism. In simple terms, thyroid hormone regulates the speed and efficiency of cellular reactions in almost every tissue, so there may be an overall metabolic slowing down in persons with an under-active thyroid. This in turn has several effects including difficulty in losing weight, (and actual weight gain) and general mental and physical sluggishness, which aggravates all this.

Medication and metabolism

In general, any stimulant medication will appear to “speed things up”. Thyroid hormone – used to replace the missing hormone in those whose own thyroid gland no longer produces enough hormone – does affect intracellular metabolism. But most other medications in this category tend to make one feel “energised” and more inclined to get moving and get things done. This increased level of activity burns fuel (glucose and free fatty acids) and often leads to weight loss. And this in turn may be perceived as “speeding up metabolism”. Some daily supplements and cold medications such as decongestants contain stimulants (commonly caffeine or pseudoephedrine). These stimulants are often included to counter the sedative effects e.g. other ingredients in the mixture, e.g. antihistamines.

The opposite effect (sedation) is also found in many medications – as in the cold remedies mentioned above.

Some antidepressants can make one feel “sluggish”, often with associated weight gain. This type of effect is not due to slowing down metabolism, but rather a straightforward side effect, which may include mild sedation coupled with increased appetite.
Some of the older generation antihistamines which are very effective at drying up runny noses have accompanying sedation as a side effect. Again, this is not slowing metabolism, just a side effect.

However, the most common reason for slowing down metabolism is weight loss, especially if it a significant amount and rapid.

In simple terms, because you now weigh less, there is less energy required for basic functions such as heartbeat, breathing, and less muscle is energy needed to move around your lighter body. So for the same amount of food ingested, there will be more kilojoules “left over” and not burned. This is basically what happens when dieters reach a plateau in weight loss.

At this stage, your body almost “resets” and lowers your basal (at rest) metabolic requirements, recognising that you now need fewer kilojoules for basic functions. Strictly speaking, adaptive thermogenesis refers specifically to the heat-related response to food, but the term is now broadened to include metabolic adaptation to weight loss. A review of the participants of “The Biggest Loser” has shown that large weight loss triggered a “resetting” of the metabolic rate to very low (Adaptive thermogenesis is demonstrable following both short-term and long-term weight reduction [9-11], and patients who have reduced weight will have to eat less and/or exercise more than their never-obese peers of the same weight and body composition if they wish to sustain weight loss.http://onlinelibrary.wiley.com/doi/10.1002/oby.21559/full Rosenbaum & Liebel (July 2013)

There are two things that you can do to counter this: 1. Decrease your intake, 2. Increase your output via movement and exercise.

Decreasing input is often easier, especially for those following a LCHF lifestyle, which seems to have a “built-in appetite suppressant” effect. Increasing physical activity does help, especially weight-bearing exercises such as brisk walking, which slowly increases overall muscle mass. Skeletal muscle is very effective at burning kilojoules, which will help to resume weight loss. The more muscle you have, the more kilojoules you burn even at rest. However, there is a limit as to how much muscle can be built, so the best solution is to combine moderate exercise with adapting your intake.

If you can’t participate in regular scheduled exercise, you can still incorporate more movement into your daily activities for example by parking a little further away from the supermarket entrance, using stairs instead of lifts, doing a bit of gardening or housekeeping – just get moving, any movement burns kilojoules. Those with sedentary jobs can try to get up and move around a bit at regular intervals during the day.