Are low-carbohydrate diets really better for weight loss?

The world is, rather worryingly, becoming increasingly tribal. Two or more groups, fuelled by the echo chamber of social media, become entrenched in their views and biases, refuse to hear reason or fact, and vilify all other opinions. It’s not a recipe for growth and progress.

Nowhere do we see a more ferocious tribal war than in the world of diets and weight loss. The health community is obsessed with arguing about which diet will result in the slimmest body. For decades it was all about low-fat diets, and an entire low-fat food industry was created in the process. More recently, low-carb diets have come to the fore, with a committed following who’ll stop at nothing to crush the low-fat ‘myth’.

So, is it true? Are low-carb diets better than low-fat diets for weight loss?

They don’t appear to be. The evidence suggests they are equally as effective, but no better.

Both low-carb and low-fat diets result in weight loss, on average. But there is no meaningful difference between the two. Perhaps more importantly, weight regain is a problem with both low-carb and low-fat diets – after a few years quite a lot of the weight is put back on. This is thought to be due to physiological adaptions (which counteract weight loss) and declining diet adherence in the face of our fattening food environment.

There is a ton of research on this subject. But there is one particular study that provides us with several important insights.

It was a well-designed randomised controlled trial done by a team at Stanford in 2018, which compared the effects of a low-carbohydrate versus a low-fat diet on weight loss over a 12 month period. So far, so familiar. But there were a few features that made this trial particularly interesting:

  • Firstly, it was much larger than most diet trials, with over 600 participants, making the results more reliable
  • Secondly, both the low-fat and low-carb diet prescriptions were ‘healthy’. Participants were instructed to, “focus on whole, real foods that were mostly prepared at home,” including, “as many vegetables as possible…lean grass-fed and pasture-raised animal foods as well as sustainable fish”, and excluding, “processed food products, including those with added sugars, refined white flour products, or trans-fats.” This is, believe it or not, very unusual in diet comparison trials
  • Thirdly, participants were specifically instructed not to reduce calorie intake. In other words, they could eat as much as they wanted of the prescribed foods 

Here’s a summary of the results:

  • Both the healthy low-carb and healthy low-fat diet resulted in the same weight loss (5-6 kg), on average
  • Individual weight changes in both groups were highly variable, ranging from 30 kg weight loss to 10 kg weight gain
  • The individual weight loss response to the low-carb or low-fat diet was not linked to either suspected genetic predispositions or insulin sensitivity (as has previously been suggested)
  • Both diets were equally favourable to other markers of metabolic health, including body fat, waist circumference, blood pressure, blood sugar, and blood lipids

What can we glean from all this?

The main point here is that the quality of your diet matters way more than the proportion of carbohydrate to fat. In fact, on average, the proportion of carbohydrate to fat doesn’t seem to matter at all.

But, we’re not averages, we’re individuals. Both low-carb and low-fat diets work for some people and not others, and it’s possible that certain people are better suited to one or the other, but we don’t yet know why.

The best starting point for weight loss is to eat more whole, real foods prepared at home, and less processed foods prepared in a factory. That may be enough for many people. But if you want to optimise further, try overlaying carbohydrate or fat restriction. Tinker with it, experiment, and see what happens.

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