My slim friend eats way more than me
Overweight people are not the only ones who are bad at estimating their caloric intake; slim people seriously misjudge their calories, too. However, they tend to err in the opposite direction. One of the reasons for this seems to lie in our desired goals: people who want to lose weight tend to underestimate their food consumption, while those who want to gain weight, are more likely to overestimate it (Johansson et al., 1998).
I have often asked thin people who claim to eat huge amounts to weigh out their food precisely and record it. Funnily enough, just minutes before I sat down to write this chapter, the following message appeared in the comments section of my blog:
I always thought I ate like crazy (and probably unhealthily), but when I stumbled upon this, I thought I’d try counting kcal. Sure enough, even when I bought a multipack of chocolate bars and ate them all within 2 hours, I was surprised to find that they contained far less than 8000 kJ (~2000 kcal). When I added up everything else I ate that day, I still only reached the normal daily requirement (actually a bit less than that, but I don’t exercise much). Altogether, my energy balance is relatively normal — to my surprise — and even a little below the daily requirement. From my personal experience, I can confirm that it’s easy to overestimate what we eat.
So far, no one willing to do my little experiment has reported anything different.
A recent study by Kuhnle et al. (2015) yielded similar results when it investigated the sugar consumption of overweight and non-overweight people. It used both self-reporting diet diaries and objective measurements. The study found that those with the greatest objective sugar intake were, unsurprisingly, 54 per cent more likely to be overweight.
However, the exact opposite was true when it came to self-reported intake: those who reported consuming the most sugar were 44 per cent more likely not to be overweight. The slimmer subjects were generally of the opinion that they ate more sugar (and more in general) than their overweight peers, although in reality the precise opposite was true.
Another example of the distorted perception of underweight people is this one I came across in an online forum:
Hello friends! I’m searching for people with the same problem as me. The problem being that I’m soooo skinny! I’ve always been able to eat whatever I want without putting on weight. I had a full medical check-up at my doctor’s and everything is fine. He told me I just had to change my eating habits. Okay, fine, but after my gynaecologist asked me yesterday whether I have an eating disorder, I realised something really needs to change. Today, I’ve been at home all day just eating: nuts, oatmeal, muesli … I’ve already got a stomach-ache and I’ve just worked out that I’ve still ‘only’ eaten 900 kcal. Is there anyone else out there who’s trying to gain weight, and how do you do it? What do you eat? I’m soooo full, but I really want to gain weight and stop being so thin.
She can eat ‘whatever she wants’, but clearly that’s not very much if she’s only managed to consume 900 kcal after a whole day of desperately ‘stuffing herself’. So, once again, we can see that people with small appetites perceive even a small amount of food as a lot — and they express that perception, too (‘I’ve eaten soooo much, again!’).
In the case of overweight people, this problem of perception could be at least partly explained by the fact that we often see others in situations where they are eating, for example in restaurants, at the cinema, at birthday parties, or at other festivities. These are situations in which even thin people often eat very large amounts. Research shows, for example, that the presence of other people increases the amount people eat by up to 72 per cent (Guyenet, 2014). This can quickly lead to the impression that other people eat far more than we do ourselves: while we often experience ourselves eating small meals, we only see others in social contexts in which people generally eat more.
Unlike overweight people, slim people often offset that increased consumption unconsciously and unintentionally by eating less in the following few days, simply because they feel less hungry than normal. Or a slim person will do considerably more exercise. Differences in between-meal snacking behaviour can also have a huge effect.
Of course, there are medical conditions that can lead to some of the nutrients ingested by an affected person not being absorbed and being excreted undigested (including fructose/lactose/gluten intolerance, diabetes, Crohn’s disease, certain parasites, and a surgical shortening of part of the digestive system). In most cases, though, symptoms related to these conditions cause serious problems and so the conditions themselves rarely go undetected. (An extremely fast metabolism — or an overactive thyroid — is also such a condition, which I deal with separately later in this book. It is likewise associated with such serious symptoms that it would not normally remain undetected — and contrary to popular belief it doesn’t cause an immense increase in calorie consumption.)
Ultimately, perception is the deciding factor: someone with a naturally small appetite and someone with a large appetite will perceive the same portion of food completely differently. Most of us know this from personal experience: a portion seems bigger when we don’t really feel hungry, or if we don’t like the food. The opposite is true when we are ravenously hungry, and wolf down a whole plate of food and only start to feel full after a second helping. This is exacerbated by the fact that the stomachs of people who regularly eat large portions become stretched in comparison to those of people who eat only small meals.
In most cases, a person who can ‘eat anything she wants without gaining weight’, simply cannot and/or does not want to eat more than she does, and she doesn’t have the feeling she is denying herself anything. According to her perception, she always eats as much as she wants, while her overweight acquaintances complain of constant self-denial. Often, it is the occasions when she completely pigged out that lodge in her memory, while the fact that she forgot to eat breakfast, or that she didn’t eat a thing during the eight hours between breakfast and dinner, are simply forgotten.
An overweight person, by contrast, will be more likely to underestimate portion size and to forget about the between-meal snacks they had. That piece of cheese from the promotion lady in the supermarket, or that handful of nuts offered by an office colleague can often be full of calories which add up through a day, but they are usually forgotten.
The only useful way to get a realistic idea of your eating habits is to weigh absolutely every mouthful with a set of kitchen scales and write it down. Estimating the calorie content of food is so inaccurate that it’s as good as useless. This is even truer for people who see themselves as too fat or too thin and want to change their weight.
While I was writing this book, an article appeared on CBC News (2015) titled ‘Diet research built on a “house of cards”? Nutrition studies depend on people telling the truth. But they don’t.’
The article describes how for many years researchers gathered and published confusing or erroneous data based on self-report studies about nutrition. For decades, the article goes on, scientists have carried out research into the supposed paradox that obese people eat less than people of normal weight, leading them to investigate all kinds of theories to do with metabolism, genetics, or other possible explanations. But, CBC explains, scientists now realise that obese people actually eat more than people of normal weight and that their self-reported intake simply does not correspond to the objective data.
Archer et al. (2013) analysed the data gathered over decades from more than 60,000 respondents as part of a large-scale nutritional study in the United States. The researchers came to the conclusion that two-thirds of female respondents and nearly 60 per cent of males made physiologically impossible claims about their caloric intake. In the most recent survey, respondents of normal weight underestimated their calorie consumption by around 150 kcal, overweight respondents’ estimates were around 180 kcal too low, and those of obese respondents were an average of 590 kcal below the real value. On the basis of those figures, it’s easy to see how the impression could be created that overweight people eat less than people of normal weight.
The amount of confusion and mystification that surrounds this issue is actually absurd. The results of the Archer et al. study emphasise the huge importance of handling the results of nutrition studies with caution: how was the data gathered, and was it based on self-reporting or objective measurements? If the Kuhnle et al. study on sugar intake mentioned earlier had been based solely on questionnaires, it might well have appeared in the media under the headline ‘Sugar makes you thin! Researchers discover that people who eat more sugar are more likely to be slim’.
That would be doubly wrong: because the data is wrong, but also because the media tend to confuse correlation with causation. In other words, they will use the headline ‘Sugar makes you thin!’ rather than ‘Thin people report that they eat more sugar’.
I have recently been coming across many similar claims about chocolate — and googling ‘chocolate makes you thin’ will bring up no end of reports to that effect. Just one example is an article that appeared on Spiegel Online in 2012. Although it does point out that the relation between cause and effect is not clear, it says, ‘The team led by Beatrice Golomb interviewed around a thousand people about their dietary habits […]. In summary, it can be said that regular consumption of chocolate is associated with a lower BMI.’
The operative word here is ‘interviewed’, so the information was apparently self-reported. Later in the article, the author puzzles over possible causes ‘since the sweet-toothed respondents were not any more active and, on average, they even consumed more calories than other subjects’. The author goes on to speculate whether there could be substances in cocoa that could influence metabolism.
At the time of writing, the first page of Google search results comes up with the following list of headlines from various news providers, which appear to be based on the above study:
So, a person who didn’t know that self-reported data on nutritional habits is, to put it bluntly, complete bullshit, would see all kinds of astounding research results on the subject and would almost be forced to conclude that the secret to staying svelte is to spend all day stuffing yourself full of high-calorie confectionery.