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Carbohydrate Restriction
How Low Should You Go?

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Carbohydrate Restriction: How Low Should You Go?ShareFollow us 261.1k

Quick Summary tl;dr

As long as you cut out industrial foods, all three studies suggest no further benefit of competitively lowering carbs further and further to promote weight loss. I’d say just take them to a level to which you can comfortably adhere.

A story about three studies and the evolution of our understanding of low carb diets.

When adopting a carbohydrate-restricted diet, many questions arise. Such as, how much of the deficit should be compensated? What ratio of fats to proteins is ideal?

These questions and more have been addressed in a few eloquent studies from which we can gather a lot of valuable insights.

Ketogenic Diet vs High Protein Diet

The first study ( Johnston et al., 2006) tested two high protein, low carb diets: one being ketogenic (10% carbs, 30% energy restriction); the other not ketogenic (40% carbs, 30% energy restriction).

Note that with enough of an energy restriction, 40% carbs could be ketogenic. On the other hand, 10% carbs may not be ketogenic in a large enough energy surplus. As you read these studies, make note of these details.

Carbohydrate Restriction: How Low Should You Go?

The study was fairly long (10 weeks) but only had about 10 participants in each group, mostly women with obesity.

The difference in fat loss was not statistically significant between the groups.

Also of note, at a strict 30% energy restriction, everyone is going to lose weight by definition.

Further, both groups cut carbs and junk food… this is what I call Hunger-Free Diet(s) because they didn’t mind only eating 70% of their usual fare. It turns out that when you give up industrial food, you don’t really need to replace the calories. Most industrial foods are high in refined carbs.

Ketogenic Diet vs Low-Carbohydrate Diet

The second study ( Johnstone et al., 2008) was shorter (two four-week phases), slightly larger (17 participants), but a crossover! In a crossover study, the participants are randomized to one of the diets for four weeks, then after a brief washout period, are switched to the other diet. In essence, in controls for any potential confounders.

Similar to the first study ( Johnston et al., 2006), both groups cut carbs to a degree, one down to 4%, the other 35%. Importantly, this study was ad lib. “Ad lib” means they could eat as much as they wanted.

Side note: it’s hard to make 4% carbs not ketogenic unless it was an energy surplus and a LOT of protein… which is pretty difficult, in part, because protein is so satiating.

Peculiarly this is what happened:

1) The ketogenic dieters didn’t like the food that much,

Carbohydrate Restriction: How Low Should You Go?

2) and causing them to eat less.

Carbohydrate Restriction: How Low Should You Go?

3) However, there was no big difference in weight loss (the ketogenic group lost slightly more weight than the low-carb group)

Carbohydrate Restriction: How Low Should You Go?

I attribute that oddity to having all of the food prepared by “kitchen research staff” LOL.

Carbohydrate Restriction: How Low Should You Go?

But the main point is that the degree of carb restriction didn’t matter much.

Just the fact of cutting out industrial foods worked in both groups. However, further reducing carbohydrate intake had no significant effect on weight loss.

Low-Carbohydrate Diet vs Low-Fat Diet

The last study is the DIETFITS Randomized Clinical Trial ( Gardner et al., 2018). This was the biggest and longest study of its kind. It was also ad lib and Hunger-Free Diet(s) (ie, they all spontaneously ate less).

Besides “clean-eating,” the low carb group was instructed to go as low as possible, which turned out to be ~30% carb (compared to 48% in the low fat group), whereas the low fat group cut fat down to 29% (compared to 45% in the low carb group). Both of the other studies were relatively high protein (30%, highly recommended), whereas this study was moderate in protein (about 22%).

Carbohydrate Restriction: How Low Should You Go?

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Bill Lagakos, Ph.D.
Nutritional sciences researcher, consultant and blogger

Bill Lagakos

Hi, I’m Bill. I have a Ph.D. in Nutritional Biochemistry and Physiology from Rutgers University where my dissertation focused on fatty acid-binding proteins and energy metabolism. I studied inflammation and diabetes at UCSD. And most recently, I studied circadian biology at the Mayo Clinic. I have a broad range of knowledge about health, wellness, sickness, and disease... and I’m learning more every day!

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Comments (4)

Could you explain how the recommended protein amts came into being?  (92 gms of protein seems high? I keep reading varying amts of protein for keto from various sources. Is the am for vihorous exercise or athletes in training?) Should gender, weight goals and age be taken into consideration when configuring protein amounts?
(Feeling confused)

It really depends. Keep in mind that protein varies for individuals. 92 g may be too much for some people but not for athletic/very active individuals. Gender, muscle mass, activity level - all of these are used to calculate your protein intake: Total Carbs or Net Carbs: What Really Counts?
Carbohydrate intake also plays a role in determining the "ideal" protein intake and as people reduce their carb intake or even follow a zero-carb carnivore diet, their protein intake will go up: What is the Carnivore Diet? Potential Benefits and Concerns

I don’t believe you should be using the term ketogenic diet to refer to your study- everything I have read regarding the ketogenic diet refers to it as low carb, high fat- NOT high protein!

I beg to differ. This article compares different diets and carb levels, not all of the diets were ketogenic. I'm not sure whether comparing only results achieved on a ketogenic diet would lead to any useful outcome. All of them would be very low in carbs and since we compare different carb levels ...