Healthy living
Diet and weight loss
Metabolism: facts and fiction
Is weight gain caused by a slow metabolic rate? Is your metabolism fast or slow? Can you change it? And should you?
What is metabolism?
“Bodily functions such as digestion, the beating of your heart, the working of your lungs and all the activities you do in the course of a day all need energy. Metabolism is the process by which nutrients in the food we eat are broken down in our cells to produce energy for these functions,” explains Dr Toni Steer, nutritionist at Medical Research Council Human Nutrition Research, Cambridge.
“But when people talk about metabolism what they usually mean is their basal metabolic rate or BMR, which is the number of calories (or units of energy) that your body burns over a set period of time.”
What determines your metabolism?
Metabolism is controlled by the thyroid gland in the neck, which in turn is governed by a part of the brain called the hypothalamus. The speed of your metabolism depends on complex chemical messages, which are sent to your brain by your body, telling it how much it needs to keep going. “Your body size and composition determine how much food you need to produce energy. The heavier you are the more energy your body needs to keep going, so the higher your metabolic rate,” says Dr Steer.
This means that, contrary to popular opinion, if you’re overweight you’ll have a faster metabolism than someone who is slimmer - a big body like a big car needs more fuel.
Do men have a higher metabolism than women?
The make-up of body tissue is one of the key things that affects metabolism. “Lean body tissue or muscle is more ‘metabolically active’ than fat, which means it burns more energy, even when you are at rest,” explains Dr Steer. This is why men, who naturally have a higher ratio of muscle to fat, tend to burn up what they eat faster, although a fat man may have a slower metabolism than a slim women with more muscle tissue.
Could an underactive thyroid slow your metabolism?
If the thyroid isn’t producing enough hormones it can slow down metabolic rate, although only by a small amount (10 per cent or less),” says Dr Steer. But unless your thyroid is extremely underactive - in which case there would be other clues such as extreme fatigue, depression, dry skin and hair and a loss of sex drive - it’s unlikely to be to blame for weight gain. And once you get any underactivity treated it will return to normal.
Does age affect metabolism?
Metabolism slows down slightly with age, as a result of us having less muscle tissue as we get older, but only marginally. So a 70 kg man who stayed the same weight throughout life would need four to five per cent less energy at 50 to maintain his weight and 10 per cent less by the age of 70, which is why you need to eat slightly less or exercise slightly more as you get older,” says Dr Steer.
Does exercise speed up metabolism?
“Increased activity levels can increase metabolic rate because you breathe harder and your heart rate increases, which causes a demand for more energy,” says Dr Steer. “Any exercise, however, has to be really vigorous to make a significant difference and your metabolic rate soon reverts to normal as your heart rate recovers,” she adds.
The good news is that regular exercise – especially weight-bearing activities such as working out with free weights, weights machines at the gym or classes such as Body Pump - builds muscle and the greater your muscle mass, the higher your BMR.
Can supplements affect BMR?
Supplements containing adrenalin which is used to treat allergies, available in the US (where it’s called ephedrine) and other some countries, can speed up metabolism and decrease appetite. A side effect of speeding up metabolism, however, is an increased heart rate and blood pressure, which could be dangerous.
Caffeine and red chillies can raise BMR temporarily, but the effects are small and, like adrenalin, caffeine also raises heart rate and blood pressure – not advisable on a regular basis. If you want to raise your BMR you are better off keeping your cash and eating slightly less or going for a brisk walk instead.
Calorie count
Three things determine how many calories your body needs every day.
1 Your Resting Metabolic Rate (RMR), accounting for just over 70 per cent of the calories you burn, is the number of calories you burn just to stay alive - so that you keep breathing, your heart beats and your blood pumps around your body.
2 Around 12 per cent of your daily calories are used for what's known as thermogenesis - needed to keep you warm, digest food and fuel your response to stress.
3 The rest are burned during activity - whether it's just doing your daily tasks or more strenuous exercise.
Written by Jane Garton
Reader comments
i have had an underactive thyroid for the last 10 years , i am sick and tired of watching others eat what they want and comment on they must lose two or three pounds, after eating "normal " for two months i have put on another two stone, this now makes me 6 stone over weight, i work full time go out lots dancing , socialising, do not drink alcohol, i love healthy food fish ,salad, veg, friut, started another gym this week ,feel very depressed but will go on trying.
Posted by: jean pearce | 12/11/2008 16:06:15
How do you find out what your resting metabolic rate is?
Posted by: Marion Anderson | 25/10/2008 13:46:06
Nearly all people whom are overweight under report what they eat. What I regard as healthy eating and a good amount of exercise the next person may find preposterous. In life if you will, there are two buckets below leaking taps. Out of one tap leaks 'the goodness' and out of the other 'the badness'. It is up to us to regulate the drips out of each tap. Unfortunately for most, to close 'the bad' tap and open 'the good tap'is difficult. Also difficult to own up to. Think about it, both taps will never stop leaking - but the difference of water levels in the buckets is what we reap from our actions. Wake up.
Posted by: Scott Lonnee | 23/10/2008 23:35:55
I have had a underactive thryoid for 8 years now it is over and I have cut my medication down , but I despair as I cannot loose weight, I eat a healthy diet , nothing makes any difference [HELP] I have put on three stone and cannot loose one pound, I find it very depressing, I would love someone to guide me in what I should eat.
Posted by: Allyson Flett | 01/08/2008 20:38:24
I was speaking to two young women this morning. Both look very overweight and both are on throxin for underactive thyroid. I have put on a lot of weight round my tummy quite suddenly (I mean over the course of a year) and had been thinking of having my thyroid checked. However, seeing the results of thyroxin treatment in people I know I am very reluctant to start it as I have been told that once you do start you are on it for life. Does anyone know if this is the case?
Posted by: A Smith | 24/07/2008 13:36:41
I have an underactive thyroid and take thyroxine...inspite of this I still continue to gain weight....5 stone over sixteen years..this is worrying when I watch what I eat and do daily walking, appx 4 miles a day..my doctor does not seem worried but I am. After reading the readers comments I now know there are other people out there that are as distressed as me.
Posted by: Joan Rylands | 23/07/2008 08:28:18
I was first diagnosed with an overactive thyroid and when I visited the specialist he asked me how much weight I had lost. When I replied nothing I'm still on my 1400 calorie a day diet he told me I had a faulty metabolism. I had radio active iodine to put this right but have now had an underactive thyroid for over 5 years. If I eat normally I put weight on, even as much as 2lbs a day so I am constantly on a diet.I lose a couple of pounds and put it back it on. How I wish I some one could sort this problem out.
Posted by: Yvonne Spencer | 21/07/2008 21:14:27
I must agree, I've had what the doctors call a border line thyroid most of my life. In 2000 I found out I had thyroid cancer, a very slow growing cancer. They removed my Thyroid and I'm fine, well I don't have cancer, They put me on thyroid pills and at first they dosage was high, but did I loose weight, NO. Now they say its normal, big deal I'm still fat. this makes me wonder what REALLY cases a weight problem? 'cause it isn't the thyroid.
Posted by: Mary | 14/06/2008 17:10:26
How I agree with the others, even taking the prescribed medication for under active thyroid the weight has crept up even after two years at a slimming club, and exercise - how do you win?
Posted by: Chris Davies | 25/04/2008 09:19:27
I have to agree with connie morgan,glynis jones and annie's post. I have steadily gained weight even though i am checked regulary, and just had my medication increased...and i am still finding it very hard to loose just a few pounds..if only it were true that thins would return to normal after THYROID UNDERACTIVITY TREATMENT.....[fat chance of that]
Posted by: mary basing | 13/03/2008 15:42:22
If only things would return to normal after getting treatment for underactive thyroid. After many years of treatment, including hours spent at the gym, I am still struggling.
Posted by: Corinne Morgan | 03/03/2008 20:22:30
In response to Glynis Jones post: A great big ditto.
Posted by: Annie | 07/11/2007 20:22:30
How glibly that phrase rolls off the tongue!! "And once you get any thyroid underactivity treated it will return to normal." If only!!
Posted by: Glynis Jones | 02/11/2007 18:04:38
Information on this site is for interest only and is not a substitute for professional medical advice. You should consult your own doctor about any specific health concerns.
