We all know someone with a fast metabolism. Or at least we think we do.
They have a voracious appetite and yet never seem to put on weight. Is it that some people genetically are predisposed to be fatter than others? Perhaps our hormones are to blame, the type of foods we eat, even our gut health.
There are certainly lots of competing theories for why some of us pile on the pounds, with corresponding diet advice.
And all of them are a distraction, at least according to Prof Keith Frayn, one of the world’s leading experts on metabolism. Having spent over 50 years studying metabolic health, he thinks we’ve made it all more complicated than it needs to be.
Instead of cutting out carbs, drinking kefir, or avoiding low-fat/full-fat (depending on which influencer guru you prefer), he wants us to strip our thinking right back to basics: calories in, calories out.
“Arguments against this energy balance model are simply distracting us from what must be done,” says the emeritus professor of human metabolism at the University of Oxford.
“I’m not denying there are genetic effects on body weight, but as far as we can tell they act by changing how much you eat. Not how much you burn off. I think people have to understand that.”
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His book, A Calorie is a Calorie: The Inescapable Science that Controls our Body Weight is a rigorous account of the science of energy balance, in which he dispels the myth that some people have faster metabolism than others.
If it’s an excuse you’ve ever used yourself, then that probably isn’t what you want to hear. As Prof Frayn adds: “I know this is probably an unpopular view, but I’ve come at it as a scientist. Not as a celebrity influencer.”
Why we really do need to eat less and move more
The energy balance model asserts that if we take in more energy than we expend, we will store the rest. Conversely if we expend more than we take in, our energy stores will be depleted, and hence our weight will fall.
However the theory has become unpopular. “People say it can’t be that simple, or else we wouldn’t be where we are,” says Prof Frayn.
This suspicion that the cause of the obesity epidemic must be more complicated than calories in and calories out has led scientists – and a host of social media influencers – to posit alternative theories.
Other weight loss theories
Cut carbs to lose weight
We’re often told to ditch the white bread and give rice a wide berth if we want to lose weight. The reasoning being that they quickly convert to sugar, which is then stored as fat. This so-called carbohydrate-insulin model has been widely promoted in recent years. Hence many of us worry about blood sugar spikes and follow-low carb diets.
However, Prof Frayn says: “I’m not convinced that spikes of blood sugar are bad for you. I don’t think there’s a lot of evidence for that, other than they make you hungry again more quickly. Whereas if you eat more unrefined fruit and vegetables they’re going to take longer to be processed and keep you full for longer.”
So it is not that carbohydrates are more likely to be laid down as fat, but rather they encourage you to eat more calories overall, affecting energy balance.
“A review of published studies has confirmed that low-carbohydrate diets help with weight loss because of reduced calorie intake rather than any specific effects of the lowering of carbohydrates,” he says.
Intermittent fasting makes you slimmer
Popularised by the late Dr Michael Mosley whose 5:2 diet proved immensely popular as a method for losing weight, the suggestion being that it would encourage a period of fat burn.
However research suggests that alternate-day fasting is about as effective as a typical low-calorie diet for weight loss.
“Time-restricted eating, intermittent fasting – so many of these ideas are just ways of altering energy intake: they do not change the underlying principle of energy balance,” says Prof Frayn.
Related to the concept of intermittent fasting is the idea that our gut microbiome influences our weight. Prof Tim Spector has also popularised the idea of giving your gut a rest so that it can work more efficiently.
There are clear observations to be made when you compare obese and lean people by looking at the microorganisms in their faeces, they have rather different populations. If obese people lose weight, it changes towards what we see in lean people.
“You can interpret that to say that the bacteria are causing them to become obese, but there is no proof of that,” says Prof Frayn. “We don’t have any experiment that would show that it is the different families of bacteria causing changes in body weight. It probably is so in mice, but mice are very different from humans.”
Dieting makes you fat
Can dieting damage your metabolism? If you believe in weight set point theory then you might believe that it does.
The theory suggests that when we diet after a time, our bodies will fight reduced calorie intake by dialling up hunger pangs and slowing down your metabolism to bring us back to our weight set point.
Prof Frayn does not find it a credible theory in the long term. “When you are on a restricted calorie intake, your energy expenditure does go down, that’s clear in starvation studies, and the question is how long that persists,” he says.
The evidence, he suggests, shows this slowing is only temporary. Prof Frayn cites data from the National Weight Control Registry database, an ongoing, observational study of individuals who have lost weight and kept it off. Analyses of the post-obese women found their body composition and energy expenditure was, in fact, very similar to those women who had never been overweight. This would not be the case if set-point theory was accurate.
“What people say is that it was hard, but the longer they are at a low weight, the easier it becomes. That’s partly because they get used to a different way of eating and they exercise more.”
UPFs are the devil
Barely a week goes by without a headline alerting us to the dangers of ultra-processed foods. Dr Chris van Tulleken, an author, NHS doctor and health broadcaster, is one such voice who believes they are driving obesity, thanks to their hyper-palatability.
Others suggest they might somehow be tricking us into eating more with their clever combinations of chemicals that override our satiety centres.
But is this really the case?
“My belief is that we don’t have evidence that it is anything other than they are high in calories and low in fibre, and therefore not filling enough,” says Prof Frayn. “The problem is that they are tasty and make you want to eat more.” Hence we eat more calories.
Getting back to calories in, calories out...
So how is it that some people can eat what they like, while the rest of us only have to look at a croissant before our thighs start expanding?
This, says Prof Frayn, is a fundamental mistruth. He cites the example of his two friends Bobby and Alex (names have been changed). Alex is large-framed and muscular, while Bobby is petite. When they talk about food, it seems that Bobby is the one who is continuously eating. Does it mean he has a fast metabolism? The answer, says Prof Frayn is, no. The truth is that Alex is under-reporting his true calorie intake.
‘So many of these ideas... do not change the underlying principle of energy balance,’ says Prof Frayn of popular weight loss methods - John Lawrence for The Telegraph
This is a problem that has confounded generations of scientists trying to study energy. Recent studies are finally showing just how unreliable we are at recording our own diets.
“You can find people who appear to eat very different amounts and yet they weigh the same. But when you bring them into the lab and study them it all disappears. It has to come down to the fact we’re not getting the information right about what they’re eating.”
...but not all foods are equal
When it comes to calories, some foods are denser than others. In 1992 when British explorer Sir Ranulph Fiennes and his companion Dr Mike Stroud attempted to reach the North Pole, they needed food that packed a calorific punch for its weight.
“They knew they would spend months pulling these heavy sledges and they knew they’d be expending a lot of energy and that they had to carry what they were taking. So it was ‘how do we maximise the amount of energy in those rations?’”
They did it by adding butter. To everything.
“If you add fat to things you will add energy to things without much weight. It creates very calorific foods,” explains Prof Frayn.
It means that if you want to lose weight, clearly trying to cut fat out of your diet might be useful.
Take the example of muesli, which Prof Frayn has for his breakfast. “If you look at the nutrition label on muesli and compare it to granola, you’ve added a good 50 calories with the latter [because of the added oils]. You’re just adding calories in the form of fat.”
One might argue it makes it tastier. And certainly if you don’t have a weight problem, go ahead and enjoy your granola. But in the Prof Frayn household, “it’s not worth the calories” is a well-used phrase.
Can fibre help cut calories?
Caloric density is not the only reason that a diet rich in fruits and vegetables is a sensible one.
There is strong evidence from observational studies, as well as clinical trials, that high dietary fibre intake is associated with lower body weight.
There may be several reasons for this, says Prof Frayn: “One is simply that dietary fibre is ‘filling’. It’s difficult to overeat when your plate is full of fibre-rich foods. Another effect of dietary fibre is that it interferes with our ability to digest nutrients. Adding fibre reduces the absorption of carbohydrates, fat and proteins.”
However, while you will excrete more calories in your faeces on a very high fibre diet, Prof Frayn says it should not be overstated: “I think there’s a lot of good reasons that we should increase our fibre intake but that’s probably because it fills you up. It probably does good things to your gut microflora but the effect on reducing the digestibility of the calories that you’re eating is really small.”
Protein advantage
Consuming one macronutrient does present an advantage in the energy balance model; protein.
“When we eat protein, the body will use what it needs to replenish its protein stores because we’re turning over proteins all the time. Anything else will be oxidised,” explains Prof Frayn.
“We don’t have a way of storing protein other than laying it down in our muscles and so on. And some people would describe it as a hierarchy of oxidation. Protein oxidises before carbohydrates and fat if you eat in excess.”
It also has this thermic effect. “When you eat a meal for a short while, your metabolic rate will be elevated as you process the food and digest it. And that’s slightly bigger for protein than it is for carbohydrates or fat. So you do burn off a few more calories.”
Although again, Prof Frayn says the effects should not be overstated. “It isn’t a huge effect”.
However if you are trying to lose weight you should opt for lean cuts. “Not burgers and fatty bacon. Protein sources like beans are even better because they’ve got more fibre.”
Exercise IS important
If that’s the ‘in’ taken care of, what of the ‘out’?
The role of exercise in weight loss has been underplayed in recent years, with diet doing most of the heavy lifting. Prof Frayn thinks that has been a big mistake.
Not only has our food environment dramatically changed, but so has our physical one.
“There are wonderful photographs from Oxford in the 1950s at the end of the working day at the big car factory, and the roads are just full of bicycles.”
Nowadays most people drive to work and they spend many more hours sitting, either at work or in front of the TV.
And we’re over-reporting our activity rates, too; adults in Britain think that they watch an average of less than 20 hours of television a week, or around three hours a day, but official statistics collected by the Broadcasters’ Audience Research Board (BARB) show that the true average in 2010 was more than 30 hours a week, or over four hours a day.
“There’s been much downplaying of exercise, with people saying, ‘Oh you’ve got to exercise two hours to burn a biscuit off,’ but I don’t think that’s logical,” says Prof Frayn. “It is almost impossible to maintain your calorie balance unless you do some exercise.”
People who have more muscle burn more calories. But if you’re trying to lose weight by just restricting calories, then you will lose muscle mass as well.
Building up muscle might mean the rate of weight loss is a bit slower but it will help in the longer term. “It will help you keep it off once you’ve got it down,” notes Prof Frayn.
So why has exercise fallen out of favour amongst diet gurus?
“If I’m really honest I think there are people who don’t want to exercise and want to find reasons to say that it’s not really helpful.”
How to get your energy in balance
If there is one thing that Prof Frayn would like us all to be, it is to be more cynical, both about advice we read from celebrity influencers, and also about what others say they eat.
“You can’t judge somebody’s metabolism by a chat about what they tend to eat, or by watching them having their lunch,” he says.
As well as thinking about what we eat, Prof Frayn wants us to think about how we move. “Exercise is something special. We need to all do it if we possibly can. It does something to our calorie balance in a way that we might not even yet understand.”
Far from being daunting, remembering the energy balance model can be empowering. “There’s nothing wrong with having a treat now and then. It’s ultimately all about balance.”
