Every week there’s another scientific study telling us something new (or old) about our diets. They often seem to contradict each other, not to mention the various fad diets that are constantly shifting emphasis from one food group to another.
So how do you know whether that article you’ve just read is true? It’s in a trustworthy paper, and it’s quoting all sorts of qualified professionals, but it’s saying the opposite of the one you read last week.
Let’s take a look at one of the recent studies just published in the Annals of Internal Medicine. 148 men and women were studied over 12 months. Half were asked to cut down on carbohydrates in their diet (eg white breads, sugar, processed foods). And the other half were asked to cut down on saturated fats (eg meat, whole milk, butter). Other than that, there were no calorie limits, although both groups reported eating 500-700 fewer calories per day on their new diet.
After 12 months, the low-carb group had lost an average 5.4kg, while the low-fat group had lost an average of only 1.8kg. The low-carb group also showed a greater reduction in cholesterol and body fat, as well as a higher proportion of lean muscle mass.
It sounds pretty conclusive. But studies like this rely on self-reporting, which is notoriously unreliable. In describing this study, The Guardian reports that 80% of participants stuck to their diets for the whole year, but look at the report and you see that “82% of the low-fat group and 79% of the low-carb group completed the intervention”. That doesn’t mean they stuck to the diet, just that they carried on handing in their reports. They were probably being paid to take part in the survey, so if they didn’t complete their reports, they wouldn’t get paid. But does having an authority figure assessing the results of your dieting make you more or less likely to be honest about how well you’ve stuck to it? How would we know if the carb group have sneakily indulged in a little extra fat now and then?
The number of people in the study is also far too low to come to any solid conclusion. With only 148 people, of whom about 20% dropped out, that’s a tiny number to rely on, when so many other factors could be at play. Since the participants were randomised, they came from all walks of life. That means they can’t be separated for things like lifestyle, wealth, cooking habits, education, nutritional knowledge or any number of other differences in their lives.
And that’s the problem with dietary studies. They can only give us vague indications of possible effects. It’s impossible to do a properly controlled study, because it would require bringing up large numbers of people in confinement from birth to old age, controlling every aspect of their entire lives just to study one tiny difference in treatment between groups. It’s obviously completely inhumane, but to be able to rule out lifestyle factors, you have to at least use much larger sample groups.
This is often done by conflating studies together and finding an average figure for the results. Indeed, just days after this study, an amalgamation study was completed of 48 different trials looking at different diet types. This study concluded that over 12 months, there was no significant difference between a low-fat or low-carb diet. The problem with amalgamated studies is that they take studies which may have been done under different circumstances with different levels of control, and treat them the same.
There’s no satisfactory answer, really. The studies do have value, and over time may be able to gradually account for the variables, but we need to be aware of their shortcomings and understand why they don’t provide a final answer yet.
In the meantime, what are you supposed to do about your diet? Well, as the BBC reported recently, any diet works, as long as you stick to it, so experiment a bit until you find the one that you feel most comfortable on. Of course, that advice was based on a study!
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Diet Studies – Who to Trust? | Total Diet Food - London's Fresh and Personalised Diet Delivery Service
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