The Sugar Diet Works—But Not for the Reason You Think

People are slamming hundreds of grams of sugar and reporting weight loss. I’m going to explain to how the sugar diet can work, and debunk it. Let's understand the "fad" at a deeper metabolic level.

The sugar diet! Yes, people are literally slamming hundreds of grams of sugar per day, while restricting protein, and are reporting weight loss and improved energy and workout performance.

I’ll concede, when this “fad” first reared its head, I was resistant to covering it. Is it crazy? Absolutely! But I’m the last person to equate crazy to wrong.

And I just came across incredible research that changed my mind about the sugar diet.

I’m going to explain to you why – to my saccharine surprise – the sugar diet can work for weight loss. It makes metabolic sense. And I’m also going to debunk it at the same time.

This isn’t a “gotcha” video, or tribal response narrative. It’s a data-driven story about how to understand a bizarre trend on a deeper level.

The Sugar Diet Trend

First, to get those of you up to speed, if you haven’t been following. The sugar diet is defined by eating low-protein, low-fat and lots of carbs. As an example, a friend of mine, Mark Bell has been on the sugar diet for several weeks and eating ~0.5 grams of protein per pound of body weight (~100 grams at 209 lbs), keeping fat <30 grams and eating as much as 800 grams of sugary carbs per day.

If we use the numbers 100g protein, 30g fat and 800g carbs that’s 3,870 Calories, with 83% from carbs.

He’s also including “sugar fasts” on top of his sugar diet, where for days at a time he’ll consume only these six foods: fruit, fruit juice, maple syrup, honey, sugar, and candy.

But he’s reporting rapid weight loss. And others are reporting similar. So, should you believe them, or are they just lying on behalf of Big Sour Patch?

Let’s discuss some important data, then you can decide for yourself.

New Human Controlled Trial Data

The study that captured my attention was recently published in Nature Metabolism. It investigated the effects of a low-protein, high-carb diet on energy expenditure. The subjects were healthy young men in their mid-20s, mean BMI ~25 kg/m², who were placed on a diet that was 9% protein and 70% carbs as percent of calories for five weeks, before reverting to a higher protein diet (18% protein) for the following five weeks.

Remarkably, the participants on the low-protein, high-carb diet needed to increase their energy intake by 19%, or 574 Calories per day on average, to maintain body weight. And, despite the increase in Caloric intake, they still lost 1.0 kg.

This occurred without a significant change in muscle mass and without an increase in physical activity.

They also replicated the low-protein, high-carb diet results on another set of young men. Again, energy intake needed to be increased by 20% to maintain weight, without any increase in physical activity.

High Carbs or Low Protein?

Finally, they asked whether swapping some of the carbs for fat changed the result by conducting a third study. This study was similarly low-protein (9%), but and fat was more than doubled to 50%, while carbs were chopped down to 41%.

Pause and make a prediction.

• If it was the power of carbs and sugar accounting for the metabolic boost, you’d expect swapping carbs for fat would reduce the benefit.

• But, if it were the protein restriction that was responsible for the increased energy expenditure, you’d expect the same results as in the other studies.

Question: Can you guess what happened? Was it the high carbs or low protein?

Answer: On the protein-restricted higher fat diet, the results were the same, namely that participants needed to eat 21% more Calories per day by the end of week five to maintain their weight. And, again, there was no change in physical activity.

Thus, something about the protein restriction was causing these folks to burn off more energy.

FGF-21 is Key Hormone

The next obvious question we need to tackle is: What is the mechanism?

This research makes a compelling case it has to do with a hormone called FGF-21, and how this hormone alters metabolism in fat cells.

In the studies, protein-restriction increased FGF-21 levels substantially, up to 361%. This increase was mostly attributable to the post-prandial period. Otherwise stated, each low-protein meal spiked FGF-21 levels. This was true whether the non-protein calories came from carbs or fat.

Simply stated: Low protein feeding spikes FGF-21.

*Nuance Note 1 (No noted change in other energy metabolism hormones): there were no notable changes in other hormones with known impacts on energy metabolism, including norepinephrine, thyroid hormone, and glucagon.

*Nuance Note 2 (Carbohydrate Overfeeding): Some Sugar Diet Influencers have raised that carbohydrate overfeeding, even without protein restriction, can spike FGF-21. This is true; however, comes with a catch. Carbohydrate overfeeding spikes FGF-21, in association with a large spike in the generation of new liver fat (de novo lipogenesis) and >100% increase in triglycerides. The increase in triglycerides is most likely secondary to the increase in liver fat induced by carbohydrate overfeeding, even as compared to fat overfeeding. This conclusion is supported by data that FGF-21 is positively associated with liver fat, likely in the context of “FGF-21 resistance,” as we review later in this letter..

Changes in Fat Cell Mitochondria in Humans

Next question: What was the effect of this increase in FGF-21?

When they studied fat cells taken from participants, they found increases in mitochondrial proteins involved in energy production. Specifically, they noted increased in proteins of the electron transport chain, including components of complexes I, III and IV, which are involved in pumping protons from the inside of mitochondria (the matrix), across the mitochondrial inner membrane and into the intermembrane space.

Stepping back and setting aside as much scientific jargon as we can (not all of it), this means the low-protein diets increased levels of proteins that are essential for turning the stored energy in the chemical bonds of food – what many people call “Calories” – into the potential energy “waterfall” that mitochondria use to make energy.

But, funny enough, there was also a decrease in the outlet for the energy waterfall, ATP synthase.

All this suggests that the low-protein diets increase FGF21, which changed the function of mitochondria in fat cells such that they generate energy less efficiently.

This “mitochondrial uncoupling” – Oops! There I go again with the jargon! – explains why protein restriction increased energy expenditure so substantially.

In the rest of this letter, we tackle:

• Data showing FGF-21 is, in fact, causal

• The Sugar Diet Explained

• The Sugar Diet Busted

• Who should NOT do the sugar diet, and why

• Obesity as an FGF-21-resistant state

• Genetics of FGF-21

• Summary, Study limitations, Open questions

And - quickly - a HUGE Thank you! We’re #1 in the “Rising Science” Category on the entire Substack App!