We all know someone who eat like a horse yet remains as skinny as a rake, while one of your other friend’s seem to pile on the pounds just by 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). But, is it more than a simple balance between calories eaten and calories burned through exercise.
But many of our popular conceptions about metabolism do not stand up to scrutiny. Two questions to ask include:
- What is the difference between obesity-resistant people and obesity-prone people?
- And, can we use that knowledge to take our metabolic fates into our own hands?
It was the 17th-century physician Santorio Sanctorius who launched the study of metabolism. For three decades, he used a specially constructed ‘weighing chair’ 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 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 “insensible perspiration”.
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. If your thyroid gland is overactive you will become very hot and eat a massive amount but be as thin as a rake. In contrast, an under-active thyroid 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.
Although metabolic rate can control body size, the problem is that people will extrapolate and claim: ‘I’ve got a metabolic defect and that’s why I’m fat’. Of course, one of the reasons thin people say they are thin is due to having a fast metabolism – it is probably not that simple.
Being Idle Takes Energy
You can measure a person’s 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. If you measure an obese person at rest and a lean person at rest, the obese person will actually be expending more calories.
Larger people have more cells to keep going – but it is 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, one kilogram of:
- Heart tissue burns 440 calories day.
- Kidney tissue burns 440 calories day.
- Brain tissue burns 240 calories day.
- Liver tissue burns 200 calories a day.
- Muscle burns 13 calories a day.
- Fat burns 4.5 calories a day.
Although obese people have more fat, they have also got a greater fat-free mass and they tend to have larger organs. But, is this 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 – 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. And, it also partly explains why people’s energy requirements drop slightly as they age
As we age, hormonal changes decrease our metabolic rate so that we need fewer calories. For example, the estimated kilocalorie requirements per day, by age, for:
- Men (Inactive/Active):
- 19-30: 2,400/3,000.
- 31-50: 2,200/2,900.
- 51+: 2,000/2,600.
- Women (Inactive/Active):
- 19-30: 2,000/2,400.
- 31-50: 1,800/2,200.
- 51+: 1,600/2,100.
It is interesting to note that, on average, a 20-year-old man will have five kilograms more muscle than a 60-year-old man.
However, resting metabolic rate is not everything – the more active you are (both through physical activity and exercise), the more calories you burn. Although plenty will state they never or hardly ever exercise, what is their secret?
- Some people are more active than they realise (a ‘couch potato’ who sits for less time than they realise, or vice versa).
- Some people are more inclined to gain weight than others (studies strongly hint that genetic factors control our propensity to gain weight).
Studies have elucidated five factors that consistently predict greater weight gain:
- 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.
So, What is your Fuel Mix?
Point 5 is interesting. It suggests that if Person A and Person B were to eat identical large meals, Person B would predominantly generate energy from glucose (from the digestion of carbohydrates). In contrast, Person A would use more fats in the food. Having both eaten more calories than they immediately needed, they would both store the excess. However, where Person A would burn more calories doing this, they would be left with more glucose circulating in their blood than Person B, and excess glucose must be converted to fat for storage, a process that expends energy. Although we are only talking about small differences in the fuel mix that people use, this could make a substantial difference to a person’s propensity to gain fatty tissue.
This suggests that some people’s bodies are simply better at dealing with excess food without putting on weight. Studies demonstrate that obesity-resistant people respond to overfeeding by burning more fat than obesity-prone people – while they are sleeping.
Although understanding the role of sleep in weight gain is still in its infancy, it is an important time where your brain assimilates information from the day in terms of memories – but is that true of
nutrient metabolism? Does the brain go to sleep thinking “how much did I eat today?” and then burn those calories or adjust the food intake for the next day.
One of the difficulties in trying to discern the differences between obesity-prone and obesity-resistant individuals is that the majority of studies compare people’s metabolisms over 24 hours, which is a snapshot. Is obesity a failure of the overall system of ‘nutrient sensing’? Taking a holistic approach, one would need to monitor metabolism over days or weeks. When using this approach, studies suggest that obesity-resistant people report an aversion to energy-rich foods, such as cake, and consequently their brains become less responsive to images of them compared with obesity-prone people.
Further, obesity-prone people became more lethargic two or three days after overeating, whereas their obesity-resistant counterparts remained as active as ever. Commentators suggest it is a whole body response to overfeeding that just works better in an obesity-resistant person than in an obesity-prone person.
Other studies suggest there maybe other, overlooked, mechanisms controlling appetite and metabolism. Studies in mice suggest that transplanting gut microbes from fat animals makes the recipients gain weight, while gut microbes from thin animals can trigger weight loss.
One theory is that the amount of nutrients that a person extracts from food depends on the microbes they possess – 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. If we think of the microbiota as an organ, which is busy sending signals to the rest of the body, it is talking from the position of being the first place that senses nutrients.
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, although we need a better understanding of how microbes shape our metabolism.
Finally, there is 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.
What is clear is that metabolism is phenomenally complicated, so anyone who suggests a quick fix to boost it is probably being disingenuous.
But, do not despair, provided you can distinguish fact from fiction there are some things you can do that will help. Although it is annoying that some people can remain slim with so little effort, with the adoption of some new habits, over weeks and months, they should gradually add up.
Metabolic Myths, Maybe’s & Truths
Can you tell fact from fiction?
- Skinny people have higher metabolisms.
- Middle-aged spread is inevitable.
- Thin people digest less food.
- Turn down the thermostat to lose weight.
- Sleep can make you thinner.
- Dieting permanently reduces your metabolic rate.
- You continue to burn calories after exercising.
- Exercise on an empty stomach to burn more fat.
- Nutritional supplements can boost metabolic rate.
- Eating more frequently boosts your metabolism.
- Some foods are actively slimming.
- Eat fat to burn more fat.
- You have to ‘feel the burn’ to get results.
- Eating at night will pile on the pounds.
01: Skinny people have higher metabolisms: Myth. Generally, the opposite is true: the larger you are, the more calories you need to burn each day just to keep your body going. But there may be some exceptions. Mutations in a gene called KRS2, which reduce the ability of cells to metabolise glucose and fatty acids to provide energy, are twice as common in obese people as slender ones. Nevertheless, they are rare.
02: Middle-aged spread is inevitable: Maybe. Ageing triggers hormonal changes in both men and women, and these can influence your predisposition to weight gain. Declining testosterone levels in men reduce muscle mass, which in turn decreases overall metabolic rate, while changes in the balance of female hormones like oestrogen can boost appetite and may dampen metabolism. However, weight gain is not inevitable if you remain active and eat a bit less as you age.
03: Thin people digest less food: Myth. When studies compared people’s stools, little difference was found in their energy content regardless of how predisposed they were to gain weight. Typically, about 3% of the calories consumed are excreted in the faecal matter, and this does not change following overfeeding.
04: Turn down the thermostat to lose weight: Maybe. Not all fat is equal. Babies are born with large deposits of brown fat, which actually burns fatty acids to generate heat. Adults were thought to have none, but we now know that slender adults have some. Volunteers exposed to 15 °C temperatures for 6 hours a day for 10 days while wearing only shorts and T-shirts showed increases in brown fat.
05: Sleep can make you thinner: Maybe. Sleep deprivation is thought to make you hungrier than usual. There appear to be several reasons for this. It decreases your levels of leptin, the ‘fullness hormone’;
increases ghrelin, the ‘hunger hormone’; and impairs normal release of insulin. However, too much sleep also increases the risk of obesity.
06: Dieting permanently reduces your metabolic rate: Myth. Dieting will depress your metabolic rate. Indeed, the very act of eating increases it, through the release of a hormone called oxyntomodulin. However, there’s little evidence that you inevitably regain all the weight you have lost because repeated yo-yo dieting permanently blunts your metabolism. In fact, crash dieters and those who lose weight more slowly ultimately regain the same amount.
07: You continue to burn calories after exercising: True. After exercising your metabolism is elevated as your body recovers and repairs itself. This ‘after burn’ lasts for between 3 and 24 hours depending on the duration and intensity of exertion. The effect tends to last longer following resistance exercises such as weight-training than endurance exercise but, even then, it is pretty small and tails off with time.
08: Exercise on an empty stomach to burn more fat: Maybe. If you eat before workouts you will have a little more glucose, which might mean you can work harder. In contrast, if you go in fasted, you will probably burn fat faster – although you might not be able to maintain the same level of intensity. Some commentators recommend combining both strategies.
09: Nutritional supplements can boost metabolic rate: Maybe. Many nutritional supplements are touted by fitness fanatics as a way to boost the burn. There is some evidence that caffeine, creatine, beetroot juice and fish oil have some effect on your athletic performance, but it is likely to be tiny, and you would be better off improving your regular diet than taking supplements.
10: Eating more frequently boosts your metabolism: Myth. While eating temporarily boosts your metabolic rate, it also affects your body’s response to insulin – the hormone that regulates levels of glucose in the blood. In a recent study, men were fed the same diet, either as three meals or foureen snacks. Those who ate less frequently had higher metabolic rates, were less hungry and had better control of blood glucose.
11: Some foods are actively slimming: Maybe. Contrary to popular belief, the caffeine in coffee and green tea will not increase the rate at which you burn fat while exercising, but it may reduce your perception of pain and exertion, allowing you to exercise for longer. Capsaicin – the substance that gives chilli peppers their heat – may increase energy expenditure by boosting the activity of brown fat. A substance in grapefruit seems to improve the body’s response to insulin. And a recent study suggests that fibre suppresses appetite.
12: Eat fat to burn more fat: Myth. A review of 10 studies found little difference in people’s overall metabolic rate, regardless of whether they ate a high fat, low carbohydrate diet – like the Atkins diet – or a low fat, high carbohydrate one. Protein might make a difference, though. People eating a high protein, reduced carbohydrate diet burned between 60 and 120 extra calories a day, possibly because protein requires more energy to digest than refined carbohyrdates. Protein is also thought to boost the feeling of fullness.
13: You have to ‘feel the burn’ to get results: Myth. Running at six kilometres per hour (KPH) burns exactly the same number of calories as walking at six KPH. What really matters is endurance. For example, a 73 kilogram person burns 74 calories per kilometre running at 11 KPH and 52 calories per kilometre walking at half that speed. By walking 2 kilometres they will actually burn 30 calories more than if they had run 1 kilometre.
14: Eating at night will pile on the pounds: Maybe. If you want to eat sugar or carbohydrate, it is probably better to do it in the morning because your sensitivity to insulin is better then. If you eat it during night time, you are more likely to store it as fat. This might help explain the finding that a high calorie breakfast and low calorie dinner produced greater weight loss than a small breakfast and large dinner.