The 1980s were dominated by low-fat recommendations from physicians and governments. In the next couple decades more people adopted low-carb diets, and studies arose that formally tested these diets out. Unfortunately many had too few people enrolled to make solid conclusions and others didn’t long enough to assess long-term efficacy.
The latest study claims to avoid some of the hangups of previous studies and show more definitely what a low-carb diet can do for your health. At first glance, it’s all positive. The researchers compared a low-carb diet against a low-fat diet, and found that those assigned low-carb lost more weight and had improved heart disease predictors. That comes off as a tidy message that is easy to translate to those looking to lose weight. While the study does show the promise of low carb diets, the details got lost in media coverage. And the details actually show that the study results are not what they first appear to be.
First things first. Why did the low-carb group lose weight? The easy answer is that they ate less carbs, but the easy answer isn’t always the right answer. The low-carb group was told to eat under 40 grams of carbs per day, which is no easy feat. Unsurprisingly, they ended up averaging about triple that number of carbs per day. That’s not to say they failed — their carb intake ended up being about half of their intake before the trial started.
So if the low-carb group ate less carbs than before, what other dietary changes did they make? Well, they naturally ate more fat and protein. And when you eat more protein, you often end up losing more fat. That means attributing the weight loss to “low-carb”, as the majority of headlines did, is not quite accurate. Sure, lower carb intake could definitely play a role, but we can’t be sure what aspect of the diet had what role in weight loss.
The low-fat group had a bit easier time, as they didn’t have to change their diets so much at all. They had to cut their fat intake a few percentage points, limiting it to 30% or less of daily calories. And by the end of the study they hit the 30% right on the dot. That percentage is not that far off from the standard American diet, whereas the carb target given to the low-carb group was drastically less than what most Americans eat.
Despite being a bit less straight-forward than it initially seems, there are some important takeaways from this study, none of which was focused on by media:
1) Targeting a much lower carb intake than you’re normally used to could be setting yourself up for failure. The low carb group lost pretty much all their weight in the first three months, then their carb and calorie intake crept up for the remaining nine months.
2) If you stay on a diet, you’re fairly likely to lose weight. The low-fat group actually lost a decent amount of weight, although less than the low-carb group. Both groups ate more than 500 fewer calories than baseline, around 1500 calories per day, so the weight loss makes sense. This finding was echoed in a meta-analysis (quantitative summary of trials) released the same week, showing that a variety of diets lead to weight loss if you stick to them. Not to mention that all study participants received a daily meal replacement bar (either a low-carb one or a low-fat one), which likely increased compliance.
3) Nutrition studies are hard to conduct, and sometimes even harder to interpret. It’s easy to be an armchair quarterback and design the perfect trial. But the researchers have to design a diet that isn’t too extreme compared to average diets, or else drop-out rates will be huge and the trial won’t be applicable anymore.
In conclusion, this study is good evidence for lower-carb, higher protein diets. It shows that you don’t have to be scared to go lower carb, in case your doctor was warning about heart health impacts. But beyond that, it’s tough to make any conclusions about exactly what led to weight loss. It’s just as legitimate to proclaim “Study shows high protein diets lead to weight loss” as it is to focus on the carbs. It just goes to show that you have to dig into the details to find out the real story behind the evidence.
