Plant-based vs ketogenic diet for weight loss

The cause of the obesity epidemic continues to be ferociously debated.

Thankfully, the idea that gluttony and laziness are individually or jointly responsible has long since been discredited.

It’s also becoming apparent that declining levels of physical activity is not the major contributor.

There are, then, two viable theories:

  1. Excessive carbohydrate intake (Carbs theory) 
  2. Availability of ultra-processed food (Junk theory)

The question is, which is it? And does it matter?

Well, a recent study appears to contradict the Carbs theory. But I’m not sure it matters, at a practical level, to our everyday lives, because the dietary recommendations that spring from the two theories don’t differ a great deal.

The Carbs theory of obesity states that excessive consumption of processed, high-glycemic-load carbohydrates elevates insulin, which directs calories preferentially to fat storage, which, in turn, creates a state of ‘internal starvation’ in active tissues (like muscle) thereby inducing overeating and fat gain. 

The Junk theory posits that the ubiquitous availability of highly rewarding, calorie-dense, convenient, heavily-advertised, inexpensive, ultra-processed food overrides normal regulation of appetite and eating behaviour, inducing spontaneous and nonconscious overeating, which, in turn, drives fat gain.

It’s difficult and expensive to rigorously test these theories, but a group of researchers had a good crack at the Carbs theory earlier this year.

They recruited 20 volunteers and kept them in a research facility for four weeks. Ten people were randomised to a high-carb/low-fat plant-based diet for two weeks, followed by a low-carb/high-fat ketogenic diet for two weeks. And 10 were randomised to the same diets but in the opposite order. Importantly, the participants could eat as much or as little as they wanted. The primary outcome was calorie intake (the main driver of changes in body fat).

If the Carbs theory is correct, you would expect the high-carb plant-based diet to increase calorie intake compared to the low-carb ketogenic diet (by increasing insulin levels).

It didn’t.

In fact, even though the high-carb diet increased post-meal glucose and insulin (as you would expect), it decreased daily calorie intake by 600 calories compared to the low-carb diet (every single participant ate fewer calories on the high-carb diet), and caused fat loss (which the low-carb diet did not). This suggests that the Carb theory is incorrect, and that high-carb/low-fat diets may to be better for appetite control and fat loss than low-carb/high-fat (ketogenic) diets.

That being said, the study had a very short duration. Extending it to months or years may produce different results (as physiological adaptions to the diets progress), but I suspect there wouldn’t be many volunteers for this. It was also in a very artificial environment. In the real world, diets are much harder to stick to because the brain fights weight loss, and we are literally surrounded by the very foods we are trying to avoid. In fact, in almost all weight loss trials with a duration of more than 6 months, results peak at 6 months and then most of the weight is regained.

Should you care about any of this?

Yes and no.

Yes, because obesity (and its associated diseases) is nothing short of a public health crisis, and identifying the true cause will catalyse more action, at the government and food industry level, to tackle it.

No, because I don’t really think it changes sensible dietary advice for most people.

Include whole, fresh foods. Exclude refined, processed foods*. That’s the basic advice of both the Carbs and the Junk theorists. Every sensible nutritionist, wellbeing consultant, and obesity researcher, I’m sure, would agree.

(* To clarify a common point of confusion, a subset of “refined, processed foods” is flour-based products such as bread and pasta, including their wholemeal versions.)

Beyond this, experiment with the ratio of carbs to fat to see what you like better and what you can sustain as a lifestyle. Adherence is much more important than macronutrient ratios. But, for what it’s worth, this study indicates that a (whole food) high-carb/low-fat diet may be better for weight loss, and a (whole food) low-carb/high-fat diet may be better for controlling blood sugar and insulin.

Now, this study wasn’t specifically designed to test a plant-based versus an animal-based diet, but, in effect, that is what it did.

And the (high-carb/low-fat) plant-based diet came out on top. It was better than the (ketogenic) animal-based diet for controlling appetite (with the same caveats that I outlined above).

I’m not a huge fan of plant-based diets (yes, I have done my time as a vegan). They are not as nutrient-dense as omnivorous diets and so increase the risk of nutrient deficiencies. But for appetite control and fat loss? This study tells us that a plant-based diet works well…if you can stick to it.

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