A new study has revealed surprising insights into how the super-lean stay trim. Here, we hear their secrets and day-to-day routine. Photo / Getty Images
We all know one – or know of one: that lucky person who stays catwalk-model skinny, despite eating anything they want. They may even complain that they'd love to put on weight, gain some muscles or curves, but simply can't.
Katreen Hardt is one of those annoyingly fortunate people. "I've been thin all my life," says the 54-year-old writer, who weighs 9st 8lbs (55kg) at 5ft 10, giving her a BMI of just 17.4.
"At school people made fun of me for being tall and skinny, but being thin has also been helpful. In my twenties I was an aspiring actress and was hired as Gwyneth Paltrow's body double for the film Great Expectations. The wardrobe department was elated to discover I could easily fit into Gwyneth's tiny costumes. I met her, and we were very similar – two tall, gangly blondes. But I've never dieted in my life, and I never go hungry."
Hardt tends to avoid processed foods, obsessively reading labels to avoid trans fats and sugar – "I feel better if I eat well" – but still enjoys French fries, the odd packet of crisps and a glass of wine. So far, so normal. So what's her secret?
Studies over the past few decades have suggested that many naturally thin people eat a lot and burn it off by being highly active, but a new study has blown that theory away. It turns out natural skinnies are actually pretty lazy. And while they may eat whatever they like, it turns out that they eat a lot less than the rest of us.
The study was conducted by an international team of scientists, led by Prof John Speakman of the School of Biological Sciences, University of Aberdeen and the Shenzhen Institute of Advanced Technology in China. It is the first research objectively to test how much "super-lean" people eat and how active they are, rather than examine their genetics or report what they claim to do.
The results, as Speakman told The Telegraph, came as "a big surprise". He says: "The classic explanation of the healthy underweight individual is that they can eat what they want and then burn this off, somehow. So, we expected to find just that: high levels of food intake, matched by high levels of physical activity." But, he says, "this seems to be a sort of urban myth".
The study, newly published in the journal Cell Metabolism, looked at 150 "super-lean" volunteers with a BMI of less than 18.5. This means they are technically underweight. A 5ft 4in woman would weigh 7stone 10lbs (48kg) or less, while a 6ft man would weigh no more than 9 stone 11lbs (62kg). They then compared them to 173 other people with a normal BMI of between 21 to 24.
The research found that, on average, healthy but naturally skinny people who didn't restrict their eating ate about 12 per cent less than normal-weight people. And, to the astonishment of researchers, they were also about 23 per cent less active. This was measured via an accelerometry device worn around the waist which senses movement and is regarded as the gold standard measure for activity measurement.
Speakman told The Telegraph that he suspects the main driver is that skinny people have skinny genes. "We found several common genetic mutations that were enriched in the super-lean group relative to the normal weight group," he says.
And these genes may not just be associated with smaller appetites but also may cause people to have low muscle mass, as well as low body fat. He says that their lower overall levels of activity may be because having less muscle means "they get tired more quickly".
Not that they always realise it. "Four of the super-lean subjects told us that they exercised up to 10 times a week to control their weight. When we checked, however, the physical activity of these four subjects was not higher than the activity levels of normal people," says Speakman.
Yet, despite their lifestyles, thinner people were on average healthier than more active normal-weight people, with lower LDL – or "bad" cholesterol levels – and less fat in their blood. And while they might enjoy the odd packet of crisps, if you dig a little deeper, you'll likely discover that effortless skinnies really don't eat that much.
Take Hardt, who always eats breakfast, but usually muesli and fruit with oat milk "as I'm lactose-intolerant". Lunch is a Caesar salad with shrimp or a salad nicoise, or just an apple, a handful of olives, and some hummus with carrots. "It depends on how much time I have. I never have bread in the house – it just makes me feel bloated – so I'll never make myself a sandwich." Dinner is either a salad or salmon with roasted vegetables.
To the rest of us, that might sound suspiciously like a diet. But not to Hardt, or likely the other 1.7 per cent of the population who are in the same effortlessly skinny cohort, who just forget to eat when they're busy. Or take forever to eat a meal, or who just don't like junk food and never even think to turn to Haagen Daaz in times of need.
"I never go hungry. I just like to eat healthily," says Hardt thoughtfully. "Sometimes I see people eating a lot of junk food. I can see how it affects their weight and mood and I wonder why they do it."
Perhaps less surprisingly, natural skinnies were also found to have supercharged metabolisms. They had higher-than-normal levels of thyroid hormones, which contributed to a resting metabolic rate about 22 per cent higher than the low rates expected, given their body size and muscle mass.
Like most effortless skinnies Hardt has never consciously watched her calorie intake. Nor does her weight yoyo. She has only gained weight once. "Years ago, I gained five kilos while going through unsuccessful IVF. Shortly after, my relationship ended, followed by my father's sudden passing. The trauma caused the weight to fall off and I've never regained it," she says. Weight is just not an issue. "I'm extremely comfortable in my body now. If I put on 10lbs (4.5kg) it wouldn't feel right, but I don't think that's likely to happen."
Speakman points out that it's too early to judge if there's anything special about the diets of these naturally thin people, but he says: "We asked the subjects to photograph all their food. That should give us a clearer picture of what they eat and how that potentially differs from people with normal BMI.
"The basic argument is that if you want to know how to make people thin then you should study thin people, not those already living with obesity."
You can't buy yourself skinny genes, nor, sadly, can you supercharge your metabolism. But previous studies have suggested there are certain habits that naturally slender people tend to adopt.
In 2015, researchers at the Cornell University Food & Brand Lab presented the results of their Slim by Design study, which asked 852 lifelong slim people, 72 per cent of whom were women, about their daily habits. Around 75 per cent said they never or rarely dieted. You might think this obvious – why would slim people diet? But studies dating back to the 1940s have found that even in people who start at a normal weight, dieting can cause them to "overshoot" their previous fat levels when the pounds pile back on, as they usually do. A 2013 review found that in 15 out of 20 studies, dieting predicted future weight gain, not loss. A decision to eat mostly healthy food in moderation may be a better strategy.
In the Slim By Design study most people said they exercised, but 38 per cent exercised between one and three times a week, while 10 per cent never exercised at all. Exercise is vital for health and helps slow weight gain over time, but studies show it is unlikely to make you lose weight. And rewarding yourself for tough workouts with unhealthy treats can even result in weight gain.
The same participants reported that their favourite snacks were fruit (39 per cent) and nuts (20 per cent). Giles Yeo, an obesity researcher at Cambridge University is the author of Why Calories Don't Count. He says that while nuts have a bad rap for being fattening: "Studies by US researchers showed that a quarter of the calories in whole, unroasted almonds were not absorbed after participants ate them. Similarly, participants absorbed only 80 per cent of the total calories in walnuts.
"It's because the fats and protein are contained inside the nuts' rigid outer coating which is tough for the body to break down." A 2011 study found that eating nuts, even more than vegetables, was associated with slight weight loss over time.
Studies show that vegetarians and vegans tend to weigh less than meat-eaters, but only seven per cent of the Slim By Design respondents said they were vegetarian, though 61 per cent preferred chicken to red meat. More than 80 per cent drank alcohol.
"One important take-away from this study is that a very high rate of slim people actually eat breakfast instead of skipping," said lead author Anna-Leena Vuorinen. Nearly half of the respondents ate fruit at breakfast (46 per cent) while eggs were chosen by 31 per cent. A wise choice, says Yeo: "A calorie of protein makes you feel fuller than a calorie of fat, because protein is more complex to metabolise."
Nearly two-thirds of the respondents eat vegetables with dinner every day (63 per cent) and more than a third ate salads for lunch most days. Not only are vegetables naturally low in calories, but the fibre they contain also feed a diverse gut microbiome, which is associated in multiple studies with lower weight and even increased weight loss. Many studies show that a higher fibre diet equals a lower weight. A 2019 US study found that people who ate meals consisting of ultra-processed foods ate 500 calories a day more and gained 2lbs in just two weeks.
A study of Japanese people published in the journal BMJ Open found that people who ate slowly were a staggering 42 per cent less likely to be obese than people who gulped down their food. What's more, people who deliberately chose to slow down their eating speed lost weight and had slimmer waists. Why? Eating slowly causes your body to produce more "feeling full" hormones.
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