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Why do some people put on weight easily while others seem to eat what they like and stay slim? We don’t fully understand but the evidence has a positive message: in health and fitness, nearly all of us have the power to take our destiny into our own hands.

It’s worth getting babies into healthy eating habits as early as possible: overweight children are more likely to stay that way into adult life.

The energy equation

There is no getting away from it: we gain weight when we eat more calories than we expend, and vice versa. This ‘energy equation’ is a basic law of physics and it is important not to lose sight of it, especially if you are weighing up some of the commercial weight-loss plans on offer. Any diet that promises that you can slim without changing your diet or activity level in anyway is one to strike off your list!

However, the simple truth of the energy equation hides a multitude of complex factors that can affect, on the one side, how and why we eat, and on the other, how efficiently our bodies burn fuel. These factors include our genetic make-up; our age; our metabolism; our environment; our state of general health; and our emotional response to food.

Very often you will read about a new scientific breakthrough that appears to lay the blame for our weight problems solely at the door of one of these factors: ‘it’s all in our genes’ or ‘how hormones make you fat’. In reality, however, it is unlikely that any single discovery will ever give us all the answers or provide a complete solution. And in a way this is good news because it means that we have more control than we might think over the many factors that affect our weight, and there is a lot that we can do to improve our chances of staying in good shape throughout our lives.

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Getting heavier?

There is now evidence that our environment can seriously damage our size. In 1955, fewer cars and more housework meant that the average woman could expend up to 800 calories a day more in activity than she does today. According to a recent survey, the average woman now weighs 65 kg (10 st 3.5 lb) and measures 38-34-40.5, but in 1951 she weighed 62 kg (9 st 10 lb) and she measured 37–27.5–39.

Family matters

A child with two obese parents has a 70 per cent chance of becoming obese, compared to 20 per cent for a child of two slim parents. That alone suggests strongly that our weight is predetermined by what we inherit from our parents. But how far is that inheritance down to genes, and how much is due to the family’s lifestyle? And if the link is genetic, which side of the energy equation are the inherited genes affecting?

Much of the current research into the link between genes and weight concerns the behaviour of individual genes and how they affect our appetite or the way we turn food into energy. There have been some exciting developments: for instance, it has been shown that in very rare cases, severe obesity is caused by a genetic mutation that causes the failure of a hormone called leptin, which regulates appetite in the brain. Children born without leptin will eat uncontrollably, but their appetite and weight will return to normal once they receive leptin injections.

Our caveman inheritance

Eventually, research into leptin deficiency and some other specific gene mutations may lead to effective treatments for what scientists now call ‘common obesity’. However, whereas only a very few people are unlucky enough to have faulty genes, none of us can escape the genetic pattern that has been handed down to us over millions of years.

Our earliest human ancestors evolved in an environment where food supplies were scarce and sporadic. Our strongest food preferences and cravings – for sweet, fatty and salty foods – are a reminder of the times when sources of glucose, essential fats and minerals were hard to come by. Energy conservation in time of famine was vital, so those human beings who had ‘thrifty genes’ –who were most able to convert food to fat and store it effectively – were the most likely to survive, breed and pass on those genes.

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Body shapes

Basic body shape is inherited and can only be modified, not completely changed, by diet and exercise. Typical body shapes are as follows:

Endomorph (soft and rounded, with a higher than average percentage of body fat).

Mesomorph (well proportioned with a higher proportion of muscle to fat).

Ectomorph (taller and narrower than average with lower than average body fat).

Old genes, new environment

Unfortunately, the genetic make-up that suited us so well for survival in prehistoric times is very unhelpful today in our modern environment. Instead of eating fatty, sweet or salty foods only rarely, we now have access to them 24 hours a day, seven days a week, if we want. When we do eat excess fat and sugar, our ‘thrifty genes’ ensure that we store it very efficiently as fat. And instead of foraging and hunting to find our food, we only have to drive to the supermarket, pick up the phone or log on to the internet.

So how can a history lesson help us to lose weight? One vital thing it teaches us is that weight is not a moral issue. People do not put on weight because they are essentially greedy, lazy or sinful: on the contrary, gaining weight is the natural human response to life in a food-rich and activity-poor environment. If our ancestors had had the choice between hunting for their food or going to the supermarket, which one do you think they would have chosen?

From generation to generation, it’s hard for us to escape the genetic patterns that are handed down through the ages.

As we are discovering, the fact that our genes have not yet caught up with our environment is bad news for our health. So the other lesson that history teaches us is that, as we cannot change our genes, in order to stay slim and healthy we need to adapt our environment-with small changes to diet and lifestyle that can swing the energy balance back in our favour.

Metabolism

This is the process by which the body converts the food that we eat into everything it needs to function, and as such it plays a key role in regulating our weight. Between 20 and 30 per cent of the energy that our metabolism produces is spent on exercise, and between five and 10 per cent is used to digest our food. The remaining energy – which is between 60 and 75 per cent – is consumed as ‘resting metabolic expenditure’ (RME), and this can vary widely between individuals. Medical problems that can affect the metabolism include an under-active thyroid and, in very rare cases, conditions such as Prader-Willi Syndrome.

If you’re stressed, try to relax or have a massage rather than comfort eating.

People with weight problems often believe they have a slow metabolism, but in fact overweight people have a faster metabolism than average as they need to burn energy more efficiently to move a heavier body around. This explains why slimmers often find it hard to lose the last few pounds towards their target; with less weight to carry, the body requires slightly fewer calories to support the metabolic process and therefore their diet and exercise plan might need a slight adjustment. This does not mean though that dieting ‘damages’ the metabolism; it is a natural process that re-sets the body’s energy balance over time.

One major factor that influences your metabolic rate though is the amount of lean muscle tissue in your body, as lean tissue is a more efficient fuel-burner than fat. Becoming more active is a great way to boost your metabolism – exercise burns fat, and builds muscle, which burns fat more efficiently. . . a win–win situation for your body.

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Fast food

It’s official: fast food causes weight gain. A 15-year study in the US showed that people who ate at fast food restaurants more than twice a week gained an extra 4.5 kg (10 lb) compared to those who ate fast food less than once a week.

A NEAT trick

Scientists are also excited about the way that we move and use energy when we’re not consciously exercising: a process they call Non–Exercise Activity Thermogenesis (NEAT). Studies at the Mayo Clinic in Minnesota, USA, have shown that obese people expend fewer calories in all areas of daily life than slim people; they sit still for longer periods and even fidget less. Researchers estimate that slim people could ‘naturally’ burn up to 350 calories more per day in this way than overweight people – the equivalent of a 30–minute run. When slim volunteers were overfed so that they put on weight, their energy output remained the same, however, suggesting that the overweight people were not being lazy’: there is a bio–chemical process at work that predisposes them to be less active.

While it may take years to identify exactly how and why this happens, it is further evidence that making activity an everyday habit is key to losing weight and staying slim –wherever you start on the road from completely sedentary to fighting fit.

Eating for two

A 2005 survey of 2,000 new mothers found that nearly two years after giving birth, their average weight was a stone heavier than they had been before their pregnancy. Of course, putting on some weight is expected and desirable in pregnancy. However, many women who have never had a weight problem find that the pounds pile on and then prove very hard to lose long after their baby is born – so much so that they despair that pregnancy has somehow changed their natural weight forever. The reassuring news is that there seems to be no fundamental, physical reason why this should be the case. It appears more likely that any weight gain is a side-effect of the upheaval in lifestyle that can come with the arrival of a new baby: lack of sleep, snatched meals, no time for exercise and more time spent at home can all alter the energy balance sufficiently so that weight goes on and stays put.

Monitoring your weight in pregnancy and not ‘eating for two’ so that you do not gain too much is sensible; around 13 kg (28 lb) is an average amount. Breastfeeding is good for the baby and for you, as it uses up around 500 calories a day. Crash diets and intensive exercise are not the way to regain your pre-pregnancy shape, whatever you might read in celebrity magazines. Instead, go for a slow and steady weight loss, with a healthy diet and gradual increase in activity, and aim to lose any surplus pounds over a period of months rather than weeks.

It’s my age

Is it inevitable to gain weight as we get older? Surveys suggest that we are at our heaviest in our 40s and 50s, and that the main reason for this is that we become gradually less active, rather than eating more. One large research project found that most adults will gain 9 kg (20 lb) between the ages of 20 and 55 years unless they take specific steps to avoid it.

So eating as if you are still playing football twice a week, when these days you are watching it on television twice a week, is a relatively effective way to score an own-goal as far as your weight goes.

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Drugs and medication

Weight gain can be a side-effect of some drugs and medical treatments. These include cancer drugs, such as tamoxifen; some steroids and antidepressants; and some drugs for diabetes and epilepsy. Ask your GP about the side-effects of any medication you are prescribed and whether you need to be careful about your diet while you are taking it.

The menopause

Women going through the menopause often feel they have more trouble managing their weight and may accept gaining the ‘menopause 10’ as an inevitable side-effect. However, as with pregnancy, that other major hormonal event, research suggests that the menopause does not necessarily lead directly to weight gain, although increased levels of testosterone and decreased levels of oestrogen may mean that the distribution of weight will change slightly, so that ‘pear’ shapes become ‘apples’ – the classic middle-age spread.

As we saw earlier, carrying excess weight around the waist represents a higher risk of developing health problems than on the hips and thighs, so it is well worth aiming to get your weight into the healthy range and your waist below 80 cm (32 in).

want to know more?

Take it to the next level. . .

Work out your own energy balance

Boost your metabolism with exercise

Build activity into daily life

Other sources

Buy a pedometer to monitor how active you are each day.

Swap to low-fat, low-sugar versions of the foods you crave to enjoy the taste without the calories.

For more on healthy family eating: www.nutrition.org.uk

Spot the obesogens in your daily life: are there ways to combat them?

Join a reputable slimming club for support in healthy eating and managing your weight.