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Some people can鈥檛 lose weight 鈥� and others can鈥檛 put it on. What separates whippets from walruses?

MY HUSBAND is the kind of man that most people hate. He can eat three helpings of dinner, yet stays steadfastly slender. He plays football just once a week, but throw him into a half marathon and he will complete the entire course in a faster time than most of those who have slogged through weeks of training. He once had a full body scan and was told they had never seen someone with so little internal or surface body fat. He doesn鈥檛, to my knowledge, have worms.

We all know people who eat like horses yet are as skinny as rakes, while others, like me, seem to pile on the pounds just thinking about food. At the heart of the difference is metabolism, the sum of the chemical reactions occurring in the body at any given time. Clearly there is more to it than a simple balance between calories eaten and calories burned through exercise. But many of our popular conceptions about metabolism don鈥檛 stand up to scrutiny. So what is the difference between people like my husband and those like me? And can we use that knowledge to take our metabolic fates into our own hands?

It was the who launched the study of metabolism. For three decades, he used a specially constructed 鈥渨eighing chair鈥� to record his weight before and after eating, sleeping, working, sex and excreting. He also weighed the food and drink he consumed and his waste products. Through these rigorous 鈥� and at times messy 鈥� studies he discovered that for every 3.6 kilograms of food consumed, he excreted just 1.4 kilograms of waste. The rest, he concluded, was lost through his skin as 鈥渋nsensible perspiration鈥�.

We now know that the fats, carbohydrates, proteins and alcohol we consume enter our cells and are fed into a complex network of biochemical pathways, eventually generating energy. Any excess is channelled into one of two energy stores: glycogen in the liver and muscle and, when that store is full, fat.

The central regulator of metabolism is the thyroid gland, a butterfly-shaped organ that sits at the front of the neck and releases hormones that speed up the rate at which cells generate energy. 鈥淚f your thyroid gland is overactive you鈥檒l become very hot and eat a massive amount but be as thin as a rake,鈥� says Steve Bloom, head of the division for diabetes, endocrinology and metabolism at Imperial College London. An underactive thyroid, on the other hand, causes people to become cold and clammy and lose their appetite but gain weight all the same. Both conditions only affect around 1 in 1000 men. Women are more prone, with around 1 in 100 having an overactive thyroid and 15 in 1000 affected by an underactive thyroid. 鈥淢etabolic rate can certainly control body size,鈥� says Bloom. 鈥淭he problem is that people will extrapolate and claim: 鈥業鈥檝e got a metabolic defect; that鈥檚 why I鈥檓 fat鈥�.鈥� Conversely, my husband maintains that the reason he is so thin is that he has a fast metabolism. It鈥檚 probably not that simple.

Idling takes energy

You can measure a person鈥檚 resting metabolic rate by putting them into a metabolic chamber, a small room where they live for a period of time, typically 24 hours. During this time, the amount of heat they release, the oxygen they consume, the carbon dioxide they produce and the nitrogen they excrete are carefully measured to calculate their total energy expenditure. Comparing obese and thin people in this way has produced some surprises. 鈥淚f you measure an obese person at rest and a lean person at rest, the obese person will actually be expending more calories,鈥� says David Stensel at Loughborough University, UK. What is going on?

What you eat and when you eat it can make a different to the weight you gain (Image: Westend61/Getty)

For a start, larger people have more cells to keep going. But it鈥檚 not just the number of cells you maintain; the type matters too. Muscle cells burn calories around three times faster than fat cells, but they are far from being the most energy-hungry; while you rest, 1 kilogram of muscle burns just 13 calories a day, whereas 1 kilogram of heart or kidney tissue burns 440 calories (see diagram). 鈥淎lthough obese people have more fat, they have also got a greater fat-free mass and they tend to have larger organs,鈥� says Stensel.FIG-mg29950401.jpg

This is bad news for anyone hoping to boost their metabolic rate by building muscle? Substituting 1 kilogram of fat for 1 kilogram of muscle means you will need around 9 calories extra per day 鈥� hardly carte blanche to eat whatever you choose. Still, having a relatively greater proportion of muscle to fat does go some way to explaining why men need more calories on average each day than women. It also partly explains why people鈥檚 energy requirements drop slightly as they age (see diagram). The average 20-year-old man, for instance, has some 5 kilograms more muscle than the average 60-year-old man.

Of course, resting metabolic rate isn鈥檛 everything 鈥� the more active you are, the more calories you burn. However, my husband鈥檚 weekly football session doesn鈥檛 seem to cut it, and there are plenty of skinny people who swear they never exercise. What鈥檚 their secret?

鈥淧lenty of skinny people swear they never exercise. So what鈥檚 their secret?鈥�

One possibility is that they are more active than they realise. James Levine at the Mayo Clinic in Rochester, Minnesota, recruited 20 self-proclaimed 鈥渃ouch potatoes鈥�, some slender, some mildly obese. All were fitted with sensors to monitor their posture and movements over 10 days. Even though none of them did any exercise as such, the mildly obese couch potatoes sat for 2.5 hours longer each day than the lean ones 鈥� equivalent to a difference of 350 calories per day. 鈥淭hat鈥檚 more than enough to explain how someone gains weight over time,鈥� says Stensel.

And we shouldn鈥檛 dismiss the notion that some people are just naturally more inclined to gain weight than others. Claude Bouchard at the Pennington Biomedical Research Center in Baton Rouge, Louisiana, recruited 12 pairs of identical male twins who were prepared to pile on the pounds for the sake of science. For two weeks they ate normally while Bouchard calculated how many calories they needed each day to maintain their weight. Then he overfed them by 1000 calories a day, six days a week for 14 weeks 鈥� equivalent to 84,000 excess calories per person overall.

Within twin pairs, weight gain tended to be very similar, but there was a three-fold variation between the different sets of twins. 鈥淚t ranged from about 4 kilograms to 12 kilograms,鈥� Bouchard says. A follow-up study with . Such studies strongly hint that genetic factors control our propensity to gain weight. But how?

Over the past two decades, Bouchard and his colleagues have been trying to tease out some answers. They have so far identified : low muscle mass, low overall fitness, low levels of testosterone (which stimulates muscle growth), being less responsive to the fullness hormone leptin, and burning less fat in the diet directly as fuel. 鈥淣one of them is the golden bullet,鈥� says Bouchard, although we do have some control over some of them.

鈥淪o far, they have identified five factors that consistently predict greater weight gain鈥�

What鈥檚 your fuel mix?

I am particularly intrigued by the last. It suggests that if my husband and I were to eat identical large meals, my body cells would predominantly generate energy from glucose (from the digestion of carbohydrates), whereas his would use more of the fats in the food. Having eaten more calories than we immediately needed, we would both store the excess. However, he would burn more calories doing this as he would be left with more glucose circulating in his blood than me, and excess glucose must be converted to fat for storage, a process that expends energy. 鈥淲e are only talking about small differences in the fuel mix that people use,鈥� says Bouchard, 鈥渂ut these could make a substantial difference to their propensity to gain fatty tissue.鈥�

This suggests that some people鈥檚 bodies are simply better at dealing with excess food without putting on weight. It鈥檚 an idea that finds favour with Daniel Bessesen at the University of Denver in Colorado. His studies show that 鈥渙besity-resistant鈥� people respond to overfeeding by burning more fat than 鈥渙besity-prone鈥� people 鈥� while they are sleeping. Understanding the role of sleep in weight gain is still in its infancy. 鈥淪leep is an important time where your brain assimilates information from the day in terms of memories, but I wonder if that鈥檚 true of nutrient metabolism as well,鈥� he says. 鈥淭he brain has to decide 鈥榟ow much did I eat today?鈥� and then burn those calories or adjust the food intake for the next day.鈥�

鈥淪ome people鈥檚 bodies may simply be better at dealing with excess food without gaining weight鈥�

Bessesen thinks one reason it has been so tricky to unpick differences between obesity-prone and obesity-resistant individuals is that the majority of studies compare people鈥檚 metabolisms over 24 hours, which is 鈥渏ust a snapshot鈥�. He sees obesity as a failure in the overall system of nutrient sensing, so to fully understand it he believes you have to take a holistic approach, looking at metabolism, appetite and movement over days or weeks.

Using this approach he has found that two days after overeating, obesity-resistant people report an aversion to energy-rich foods such as cake, and compared with obesity-prone individuals. In addition, , whereas their obesity-resistant counterparts remain as active as ever. 鈥淚 think it鈥檚 not one thing 鈥� it鈥檚 a whole body response to overfeeding that just works better in a thin person than in an obesity-prone person,鈥� he says.

There may yet be other, overlooked mechanisms controlling appetite and metabolism. Studies in mice suggest that , while the gut microbes from thin animals can trigger weight loss. One theory is that the amount of nutrients you extract from food depends on the microbes you possess 鈥� although some studies indicate little difference in the calorie content of faeces between individuals. Another idea is that our microbes are monitoring our food intake and releasing substances that might affect our appetites or the amount of fat we burn. 鈥淲e have to think of the microbiota as an organ, which is busy sending signals to the rest of the body,鈥� says Lee Kaplan at Massachusetts General Hospital in Boston. 鈥淚t鈥檚 talking from the position of being the first place that senses nutrients.鈥�

There are still plenty of unanswered questions about metabolism, not least whether there is a reliable way to boost it to help people lose weight. One possibility might be to use faecal transplants from thin people 鈥� but first we need a better understanding of how microbes shape our metabolism.

Then there鈥檚 oxyntomodulin, the appetite-suppressing and metabolism-boosting hormone, produced naturally whenever we eat. Pilot studies in obese people suggest that regularly injecting it might aid weight loss. Bloom is about to launch safety tests ahead of a larger clinical trial. 鈥淲e don鈥檛 anticipate anything going wrong, but it is still at least 10 years away from being marketable,鈥� he says.

What is clear is that metabolism is phenomenally complicated, so anyone who suggests a quick fix to boost it is probably being disingenuous. But don鈥檛 despair. Provided you can distinguish fact from fiction there are some things you can do that will help (see 鈥�14 myths and maybes about burning fat鈥�). Though it鈥檚 slightly annoying that people like my husband remain slim with so little effort, I know that if I adopt some new habits, over the weeks and months they should gradually add up. Then maybe, once in a while, I too can overindulge with impunity.

This article appeared in print under the headline 鈥淎re some people doomed to obesity?鈥�