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Why cutting sugar out of your diet won’t work miracles

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There's no question that eating too much sugar is linked to obesity and other chronic health problems, such as high blood pressure and diabetes. But can cutting sugar immediately reverse those health issues?

That's what the news reports on a new study suggested. The research, published last week in the journal Obesity, got a group of kids to reduce their added sugar intake from about 30 percent of their daily calories to 10 percent and replace those sugary foods with starches (so potatoes instead of pastries, and bagels instead of sweet yogurts). The children saw amazing health improvements in just nine days: reductions in blood pressure, triglycerides, and LDL (or "bad") cholesterol levels, and a halving of their insulin levels.

Incredibly, as lead author and leading sugar opponent Robert Lustig described in an op-ed, "We reversed their metabolic disease in just 10 days, even while eating processed food, by just removing the added sugar and substituting starch, and without changing calories or weight." (Metabolic syndrome — a set of conditions that includes high blood pressure, high blood sugar, a large waist circumference, and abnormal cholesterol levels — increases a person's risk of heart disease, stroke, and diabetes.)

The way the researchers designed the study was interesting: They basically sent a group of obese children who have metabolic syndrome home with nine days' worth of catered food. They then measured the impact of that new diet on the various health markers mentioned above. The researchers say the kids didn't reduce their calorie intake (or the amount of fat, proteins, and carbohydrates they were eating). The only thing they changed was their added sugar intake. So the researchers were trying to isolate the effects of a sugary diet on health, and they came to those striking conclusions.

The study was extremely limited and had no control group

What got missed in many of the media reports was that the research was incredibly limited. The study ran for only nine days, and involved 43 children — and, importantly, no comparison (or "control") group.

The researchers also relied on self-reports from the kids and their families about what they were eating before they went on the diet. More often than not, people underestimate how much they're eating, so it's possible that the catered meals were actually different from participants' usual eating patterns in other ways than simply added sugar levels. As this great analysis of the study at Stats.org points out, the kids also lost an average of 2 pounds in just over a week, which suggests there may have been other changes to their diets.

That's not to mention the fact that catered meals and daily weigh-ins may have made the kids more mindful about their health, leading to the health gains described.

Without a control group for comparison, it's really hard to say whether the sugar reductions indeed caused all the health changes. If the study included another set of kids on a similarly catered diet with the usual sugar levels, it would have been a lot more powerful and convincing.

The trouble with the war on sugar

The message here isn't that a lot of sugar is good for you, or that reducing one's added sugar intake isn't a good idea. It's that we need to be cautious about interpreting miraculous results from limited and poorly designed studies that lack a control group. That's not to mention the question of sustainability of such a diet beyond nine days.

We've seen again and again that obsessions with cutting out specific foods or macronutrient groups fail us. Just a look at this low-fat diet study from last week; it finds that despite our decades-long push to cut dietary fat, there's no good evidence that low-fat diets work for weight loss — and other research has shown that these diets actually caused people to replace fat with more sugar.

Now sugar is the No. 1 health enemy. Sure, we can all cut back. But focusing on healthy eating, and figuring out how to encourage good dietary patterns at the population level, is probably a lot smarter than demonizing yet another type of food.