Robert Lustig is a Californian scientist and anti-sugar campaigner who became a minor internet sensation with his YouTube video, The Bitter Truth, which portrayed sugar — and fructose, in particular — as the driver of obesity and diabetes. Today, he published a study which, he says, proves his highly controversial hypothesis that a ‘calorie is not a calorie’. He has also been given a column in the Guardian in which he calls for a sugar tax. Unfortunately, the evidence he presents is just awful.
The aim of Lustig’s nine-day experiment, which involved 43 obese children, was to see what happens when someone maintains their calorie intake but reduces their sugar consumption. Mainstream scientists would expect to see no change in body mass. Lustig expected to see weight loss, and he did.
There’s a simple way to conduct a study like this. Give half the kids a 1,500 calorie per day diet that is high in sugar and give the other half a 1,500 calorie per day diet that is low in sugar. Make sure they eat it (don’t just send them home with it, as Lustig does) and see what happens.
Alas, that’s not how Lustig went about things. Instead, he asked 43 fat kids what they usually ate and then gave them an equivalent number of calories in low-sugar meals for nine days. At the end of the period, he observed an average weight loss of 0.9 per cent. Bingo! A calorie from sugar is more fattening than a calorie from pizzas, crisps, hot dogs, popcorn and burritos (for that is what the low-sugar meals consisted of). Tax sugar now!
Not so fast. There are at least two critical flaws in this experiment which render it worthless. First, Lustig took the kids’ evaluation of their usual diet on trust, despite it being well known that everybody — especially obese people — under-report how many calories they consume. If the kids said that they usually ate, say, 1,600 calories when they really ate 2,200 calories then the 1,600 calorie diet Lustig put them on would be bound to induce weight loss diet regardless of its sugar content. As Tom Sanders, a professor of nutrition and dietetics, has pointed out, the kind of weight loss seen in the nine days of this experiment would require an energy deficit of over 600 calories per person per day. Either the laws of thermodynamics are wrong or Lustig’s child subjects under-estimated how much they normally eat. The latter explanation is vastly more probable than the former.
Secondly, there was no control group. This is such a basic requirement of a scientific experiment that its absence is astonishing. We need to know what would happen if a group of similar children were given a high-sugar diet containing the same number of calories as the low-sugar diet. If, as is highly likely, the kids under-reported their normal calorie intake, the use of a control group would have exposed this flaw because both sets of kids would have lost weight, regardless of how much sugar was in their diet.
This small, uncontrolled study, which made no attempt to verify what its subjects normally eat, tells us nothing useful whatsoever. The claim of the Guardian‘s headline writer that ‘the science is in’ would be laughable if it were not so depressingly typical of the standard of scientific analysis in the popular press.