The truth about your metabolism

Healthy Food Guide nutritionists answer your metabolism questions — and bust those metabolism myths.

Why is it that you could eat whatever you wanted when you were 20 — and stayed slim — but now just looking at a chocolate bar makes the scales jump? You eat healthily and exercise regularly, yet you are still struggling to keep your weight steady, let alone lose those extra few kilos. Is it fair to put the blame on a slow metabolism?

‘It’s one of the most common reasons women give for their expanding waistlines, especially once they hit their 40s,’ says nutritionist Juliette Kellow. ‘And, although it’s not the whole answer, it’s a big factor — our metabolism does start to drop as we get older, making  it harder to control our weight.’

Your body’s engine

Understanding how your metabolism works and what affects it is therefore crucial if you want to give it a boost and stop those pounds piling on.

‘Your metabolism determines how many kilojoules you burn throughout the day, even when you’re resting. This, in turn, determines how slim you remain,’ explains Juliette. ‘Think of your metabolism as your body’s engine. It is constantly burning fuel — or kilojoules — to keep it ticking over. Most people are surprised to discover they actually burn most of their kilojoules at rest.’

In fact, our resting metabolic rate — the amount of kilojoules our body burns simply to keep us alive (things like breathing and keeping our heart beating) — accounts for 60–75 per cent of our metabolism. The other components are the kilojoules we use up when we eat, digest and process food (which accounts for around five to 15 per cent of our metabolism) and exercise.

There are several factors that influence how efficient your metabolism is. ‘Some of these, such as your age, gender and genes, are beyond your control,’ says Juliette. As you get older, your muscle mass decreases, and the less muscle you have, the slower your metabolism. That’s why men, who naturally have more muscle than women, tend to have a faster metabolism. Also, some of us are lucky enough to be born with a fast metabolism, while others have to put in more effort to speed it up.

You can’t blame your genes entirely, though. ‘Research has shown that the wrong diet can slow down your metabolism by as much as 30 per cent,’ adds nutritionist Amanda Ursell.

True or false?

Eating lots of small meals keeps my metabolism going

During the hours you are awake, it’s helpful to eat regularly to give your body a slow and steady supply of fuel which it can use right then and there. Eating food also stimulates your metabolism. However, researchers who have tried to find whether more frequent meals are better than a three-meals-a-day regime haven’t come up with a definitive answer. It seems any effect on body weight is the result of total energy intake rather than the different meal patterns.

Bottom line

There is no set number of meals and snacks you should eat in a day — we are all different — but regularly eating healthy, balanced meals and snacks can help regulate your body glucose levels and help your body be more efficient.

Certain foods, drinks or supplements can boost my metabolism and help me burn fat

You may read that foods such as coffee, green tea and chillies can speed up your metabolism. While this is true, the increase is quite small and unlikely to produce a noticeable difference in your weight. Include them in your healthy diet, but don’t rely on these foods to help you lose weight.

There are many supplements that claim to speed up the metabolism but the truth is if you are looking for long term results, there is little these products can do for you. As well as being expensive, in some cases they can have harmful side effects.

Bottom line

No food will significantly alter your metabolic rate. A proven and safe way to boost your metabolism is to increase your muscle mass. Include resistance or strength training into your exercise routine several times a week and be sure to eat well after exercise to help muscle repair and recovery.

Stress boosts my metabolism

The stress response temporarily alters your body’s metabolism and suppresses appetite. Sudden stress initiates the ‘fight or flight’ response: a rush of glucose and oxygen go to your brain and active skeletal muscles so that you are ready to go fight the wildebeest, or at least run from it. If stress continues your body releases an array of hormones including cortisol which can stimulate your body to start the breakdown of fat and muscle in case they are needed for fuel. This might sound good but it is a short-term solution to help your body cope in particularly challenging situations. Cortisol also appears to stimulate appetite, and long-term exposure to high levels of stress hormones suppresses the immune system and your general health starts to decline.

Bottom line

Stress gives your metabolism a short-term boost to cope with life-and-death situations but prolonged or chronic stress can lead to increased appetite and weight gain. Learning to manage stress is important for weight management as well as long-term health.

I’m overweight because I have a slow metabolism

It’s unlikely. Larger people generally have a higher metabolic rate than smaller people because the bigger you are, the harder your body has to work with every movement you make. On the other hand, people who are slim and very fit will have a relatively faster metabolism because increasing muscle mass increases metabolic rate.

Bottom line

A small percentage of people have a medical condition causing a slower metabolism, so they carry more weight than you might expect, but most of us can’t blame our metabolism for any excess weight.

Some people have naturally faster metabolisms

We are all different, and so are our metabolic rates; they vary depending on our genetics, gender, age and body composition. So it is true that two people may eat the same amount of food and it will affect them differently.

Bottom line

Those with a high muscle mass have a faster metabolism. Appearances can also be deceiving. Most of the time, the classic ‘super-slim, eat what you like’ person who appears to eat and eat is not consuming as many kilojoules as you think.

Are there conditions that can slow down or speed up metabolism?

There are many medical conditions which can affect your metabolism, two of the more common being hyperthyroidism (where the thyroid gland is overactive and metabolism is faster) and hypothyroidism (where the thyroid is underactive and metabolism slower). Some medications may also affect appetite or the way your body uses fuels or stores body fat. These include steroids, insulin and drugs for psychological health issues. Consult your GP if you think you could have a medical problem or have concerns about medications you are taking.

Five ways to boost your metabolism

1. Move it to lose it

Studies show that fidgeters burn more kilojoules than people who happily sit still for long periods of time. So if you work in an office, crank up your metabolism by moving regularly. Walk to a colleague’s desk instead of sending an email and take the stairs instead of the lift. At home, stand up to do chores and talk on the phone.

2. Do strength training

Using weights or doing lunges and squats that use your own body weight will increase your muscle mass, and the more muscle you have, the more kilojoules your body burns — even when it’s doing nothing. This partly explains why men have higher kilojoule needs than women — having more muscle means they burn more kilojoules. Fat cells, on the other hand, are a dead weight — they don’t burn many kilojoules. So include strength training in your workout regime and for the best fat-burning results combine it with aerobic exercise.

3. Go for the burn

There’s an enormous amount of evidence that regular cardiovascular or aerobic exercise such as running, jogging or swimming (anything that gets the heart and lungs working harder) maintains lean muscle mass, hence a more efficient metabolism. Research also shows our metabolism stays raised even after we exercise and can remain elevated for up to 24 hours after a workout. For this to happen, we need to be exercising for more than 50 minutes at a fairly energetic pace.

4. Ditch the crash diets

Don’t severely reduce your food intake in the hope you will quickly become slim. You won’t. In fact, drastic dieting will probably have the reverse effect. In response to an inadequate number of kilojoules, your body simply goes into survival mode and so starts to use muscle rather than fat to provide it with energy. This has the effect of slowing down your metabolism and conserving energy so that you need fewer calories to survive — ideal if you are in a famine situation, but a disaster if you have excess kilos to lose. Unfortunately, when you go back to eating normally, your metabolism will have become slower so you will pile on weight more quickly and then struggle to lose it. This can lead to a binge/starve cycle that further slows down your metabolism. It recovers, though, so puta stop to the cycle and watch your weight stabilise.

5. Don’t skip breakfast

According to numerous studies, people who eat breakfast tend to be slimmer than those who don’t. Eating in the morning can actually prevent weight gain, too, as researchers believe it gets our metabolism into gear for the rest of the day. When you wake up, your body may have gone around 10 to 12 hours without food, depending on when you had your evening meal, so if you don’t eat breakfast, it thinks you are denying it kilojoules and goes into ‘starvation mode’. Not eating breakfast mimics a potential starvation situation, which is your body’s way of saving itself in the event of a real famine. When your body is starved, its metabolism slows down to conserve kilojoules.

Lose fat, not muscle

The downside to shedding the kilos is that most of us lose lean muscle as well as fat — studies have found that when we lose weight, only three-quarters of it is fat and the remainder is muscle. This is a problem because your overall muscle mass is important for boosting metabolism and helping you keep weight off. The solution is to reduce your kilojoule intake moderately, rather than excessively — for example, up to 2000kJ a day – and build exercise into your weight-loss plan. Combine healthy eating with a mix of aerobic exercise and strength training to help boost the number of kilojoules you burn.

What are those supplements

Carnitine is an amino acid required for fat metabolism. It occurs naturally in protein-containing foods and can be bought in liquid form or found as an ingredient in sports bars and shakes. The theory is that if you take carnitine on top of an exercise programme, it may help you increase the amount of fat you burn. Although considered safe for most people, the Australian Institute of Sport doesn’t think this is worthwhile and any benefit is likely to be very small or even non-existent.

Fat burners / thermogenics contain ingredients which suggest they can help your body use more energy (kilojoules) and encourage your body to burn fat. They generally include plant extracts and stimulants such as green tea, guarana, ginger or bitter orange. Products vary widely in their composition and also their safety. Some may have no side-effects, while others can send you into a cold sweat, make your heart pound and come with a long list of potential negative side-effects.

For more on metabolism:
How to speed up your metabolism
How can I boost my metabolism?