It's the 'science stuff' behind maintaining a healthy weight and body, but what is it, what does it do, and how can we affect it? Hannah Ebelthite breaks it down...
You've tried every diet, embraced every fitness trend and still you're not losing weight. There's only one explanation: your slow metabolism. That friend who never exercises and is best friends with Ben & Jerry's? She must have a super-speedy one. Life's so unfair.
Or is it? Is metabolism really a sliding scale of weight fate? Is our calorie-burning potential predestined and unchanging? Does our metabolism affect our risk of certain diseases? And, come to think of it, what is metabolism exactly?
"It's a confusing concept," agrees Dr Michelle Harvie, lead dietitian at Genesis Breast Cancer Prevention. "Scientists are constantly trying to understand more about metabolism, so we can offer the right advice - not just to help people stay a healthy weight, but to become healthier overall. And we're discovering new things all the time." So we bring you a masterclass in metabolism: what it is, what it does, what you can do to affect it - and what you can't.
Metabolism: the science bit "It's the biochemical processes that transform the food we eat into useful energy, or fuel," says Dr Jules Griffin, reader in human metabolism and nutrition at the University of Cambridge. "It's responsible for all the chemical processes that go on continuously inside your body, which require energy.
It keeps you breathing, digesting food, keeps your organs and nervous system functioning normally." Basically, it keeps you alive. And metabolic reactions occur constantly in all your cells. Your pancreas secretes the hormones insulin and glucagon to tell your organs how to function. Your thyroid gland controls your rate of metabolism by releasing the hormone thyroxine.
What's this got to do with my weight?
"If you don't burn through all the calories you consume, your body stores them as fat," explains Dr Harvie. "It's an evolutionary survival mechanism, so you'd still have an energy source to draw on if food were to become scarce."
However, our metabolisms are still in caveman mode - they haven't really evolved since hunter-gatherer times when hunger was a big issue. So if we eat too much and move too little, our bodies store more and more fat - which we know is bad news for our health. That's why it pays to find out a little about how your metabolism works, and how you can keep it on side.
The 3 types of metabolism
Your metabolism can be broken down into three levels. First comes your basal metabolic rate, or BMR. This is measured in calories and indicates the minimum amount of energy your body requires to tick over. That's without you lifting a finger. It typically accounts for 40% to 70% of your daily energy requirements.
"On top of this comes your active metabolism," says Dr Griffin. "This accounts for ten to 30% more, depending on how much exercise, daily activity, even how much fidgeting you do."
"Then there's diet-induced thermogenesis." Forget the confusing name, this third level just refers to the calories your body burns while digesting food (yep, you burn calories eating calories). It makes up around 8% to 15% of your total metabolic rate.
While it's not good to become obsessed with calorie counting, experts do believe it's wise to have an understanding of these figures. "I'm all for patients finding out their vital statistics, like BMI and BMR," says Superdrug's health and wellbeing ambassador, Dr Pixie McKenna. "Knowing your BMR is useful in that it gives you a good indication of the minimum number of calories you should have daily."
So is BMR different for everyone?
Yes, some people have a higher metabolism than others - for many reasons. This means they burn fuel faster and burn more calories at rest. But here's where metabolism gets confusing. Because most of us could be forgiven for thinking that larger people, who find it hard to lose weight, must have a slow metabolism. Yet the opposite is true. "Overweight people tend to have a higher metabolism," says Dr Griffin. "This reflects the energy requirements of a larger size. The leaner you are, the less energy you need."
How to work out your BMR:
Aside from lab testing, the best way to work it out is with a simple calculation. You can do it easily online, here: . Or, for a more accurate reading that takes into account your muscle-to-fat ratio and not just your weight, invest in Tanita BC-730 Innerscan Body Composition Monitor Scales, £44.99,
5 factors that affect metabolism
"This is the number one factor and one you can influence," says Paul Hough, lecturer in health and exercise science at St Mary's University, Twickenham. "Muscle is more metabolically active than fat. The higher your muscle-to-fat ratio, the higher your BMR."
Men tend to have a faster metabolism, as they are naturally heavier, with bigger frames, more muscle mass, less body fat and heavier bones to carry around.
Experts think metabolism may be genetically determined in part (your genes affect muscle size, for example), but they don't yet fully understand how.
"Metabolism tends to slow with age, because we become less active and have more fat, less lean muscle mass," says Dr McKenna.
These do a good job of keeping metabolism in balance. "Metabolic disorders, such as an underactive thyroid, are relatively rare and will cause symptoms such as fatigue, irregular periods, constipation and general aches and pains - not just weight gain," adds Dr McKenna.
The GLAMOUR metab-o-meter
We looked at some of the most hyped-up 'cures' for a sluggish metabolism, to see what actually helps...
"Any diet that drastically cuts calorie intake can reduce your BMR by forcing your body to start breaking down your muscles for energy, because you're not supplying it with enough food," says Dr Harvie. It's called 'starvation mode'.
Often they'll contain ingredients that boost thermogenesis, such as chilli, but in amounts unlikely to give any obvious health change. "Very few pharmaceutical drugs are proven to help with weight loss, let alone over-the-counter remedies," says Dr Harvie.
A high-calorie fat that contains medium-chain triglycerides. These are burned off as fuel in the liver and raise metabolic rate. Some studies claim this makes it a good weight-loss food. Others argue it's still calorie dense. Conclusion? More evidence needed.
"Your body uses energy to get the water to body temperature, but not enough to notice a significant benefit," says Peta Bee, author of The Ice Diet. GP Dr Dawn Harper agrees: "It would be fantastic if this was the solution to the obesity crisis. Sadly, it's not."
"While there is evidence that your metabolic rate gets a small boost after consuming capsaicin (a compound in chili peppers), this will not have a great impact on your weight if you change nothing else," says Dr Harvie.
Caffeine will shift your metabolism up a gear for about three hours. It's why athletes use it pre-race or in training, to increase endurance performance. But some people are more sensitive to caffeine, and one person's wake-up buzz is another's anxiety-inducing palpitations. Avoid caffeine in sugary drinks (energy drinks, flavoured lattes, cola) or you'll negate any added burn.
Studies have shown that two to three cups of (non-decaf) green tea will burn an extra 80 calories. It's down to the compounds called catechins, which are also heart-healthy.
Digesting food is part of the metabolic process, so the more effort required to digest a food, the more calories you'll burn. Favour protein, which requires more energy to digest than carbs or fat.
5 metabolism boosters That work
"Metabolism is a product of your activity levels," says Hough. "When you're at rest, your metabolism is low. Get moving and it increases. The higher the intensity, the better, but it's all relative to your fitness levels. If you're new to exercise, a brisk walk might count as high intensity. This is why it's important to keep challenging yourself. Once it gets easy, up the intensity to get the same fat-burning effects."
Then there's frequency. "Metabolism doesn't go back to its resting level as soon as you stop exercising," explains Hough. "It stays raised for several hours, meaning you'll be burning more calories, even though you're at your desk. It's called excess post-exercise oxygen consumption (EPOC) or 'afterburn'. Do ten minutes' HIT, three times a day, and you'll get three times more afterburn than from a pre-work gym session."
With all that exercise, you're going to want some shut-eye. And that'll be giving your BMR a push in the right direction, too. Bonus. Getting quality sleep doesn't just affect mood, energy and productivity the next day. A 2010 review published in the International Journal Of Endocrinology found that when study participants slept for only four hours a night over six nights, the rate at which their bodies processed calories from sugar dropped by over 40% - in other words, a significant alteration in normal metabolic function.
Turn down the thermostat
<p class="BodyBembo">"Most of us inhabit far too warm an
environment these days, with heated cars and homes," says Bee.
"Evidence shows that reducing temperatures can cause 'non-
shivering thermogenesis', which activates brown, 'good' fat." Dr
Harper says: "Unlike the white fat you can feel beneath your skin,
which stores excess calories, the purpose of brown fat is to keep
the body warm, which it does by burning calories. Being colder
won't impact weight if you're eating whatever you want. But if you
have a healthy diet, there is evidence that being a bit chillier
could raise metabolism and switch on more brown fat."
Say no to sugar and saturates
"The best diet for a healthy metabolism is simply a balanced one, with nothing in excess," says Dr Griffin. "It's important to match your diet to your expenditure - in other words, not to eat more calories than you can burn off (or burn off more than you've eaten)." Don't let your body get too used to storing excess high-carb and high-fat foods as fat, or that may become its default.
"Most people are aware that too many saturated fats are bad, but we're still consuming too much sugar. This is stored as fat in the liver, which can contribute to fatty liver disease and cirrhosis. Diets high in saturated fats and refined carbohydrates are also closely linked with insulin resistance, a key predictor or Type 2 diabetes."
Here's a stat we like: for every pound of fat you transform into muscle, you'll burn around an extra 14 calories a day, doing nothing. Resistance training, strength training, weight lifting - call it what you like, anything which helps you turn fat into lean muscle mass is going to improve your metabolic health.
"It's not just about calories and weight loss," says Hough, "although muscle takes up less space than fat, so you will be slimmer and firmer. The greater your muscle mass, the better your glucose control - important for staving off insulin resistance and Type 2 diabetes. Getting and maintaining a lean, strong physique now will help prevent your metabolism lowering with age." If free weights in the gym aren't your thing, try circuit training, studio classes such as BodyPump, or get to grips with equipment like kettle bells, medicine balls or the TRX.
"If you eat late at night, more of the energy will be stored in fat cells than mobilised in muscle cells, ready to be burned off," says Dr Griffin.
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