It’s the battle between keto versus low fat – the Diet Wars have been raging for years now, all over social media and everywhere else. On one side, we have the low-carb/keto group, and on the other, pretty much everyone else. 

I jest, of course, about the ‘everyone else’ part, but really, we’ve been conflicted for a very long time about which way of eating is best.

Previous studies have found that there is little difference between low-carb and low-fat diets for weight loss, and that a ketogenic diet may significantly improve health markers in type-2 diabetics.

But the debate rages on, especially about something called the carbohydrate-insulin theory, which is popular among low-carb evangelists such as Aseem Malhotra, David Ludwig, and Gary Taubes, to name a few. 

This theory claims (and I’m simplifying as much as possible here) that the overconsumption of processed, high-glycemic carbs results in an overload of insulin in the bloodstream, which leads to fat storage and weight gain.

They also hypothesize that these high insulin levels drive appetite – so theoretically, they say, people eat more on a high-carb diet.

I’ve sure you’ve heard this sort of talk all over social media; it’s extremely popular among influencers, as well as ‘nutrition coaches’ and other unregulated professionals. 

Conversely, these people believe that low carb diets keep insulin low, leading to less hunger, greater fat burning, and more weight loss.

The ‘fat burning’ claim of the carbohydrate-insulin theory was disproven in a 2016 study in the American Journal of Clinical Nutrition. It found that while a keto diet did result in weight loss, it did not perceptibly change the body’s rate of energy expenditure (aka ‘fat burn’).

Kevin Hall and other researchers at the National Institute of Diabetes and Digestive and Kidney Diseases, wanted to put this theory to the test.

When allowed to eat as much as they want, would subjects on a ketogenic diet be less hungry and eat less than subjects on a low-fat, high-carb diet? Would there be any difference in blood insulin levels after meals?

They decided to do some research, which was released in the journal Nature, in January 2021.

The reason why I’m writing about this study instead of just rolling my eyes and scrolling by it thinking it’s another nutrition study with overhyped headlines, is because these researchers actually kept people in a metabolic ward. And fed them pre-measured, controlled meals. And did bloodwork regularly. And calculated everything on a daily basis, from calories eaten to insulin levels and everything in between.

This doesn’t happen every day, folks. Usually, we’re looking at data from populations and making hypotheses aka educated guesses from that. And as you know from my previous writings, these hypothesis can be deeply flawed. 

How was the study designed?

There were 20 participants: 11 men, 9 women. One person dropped out in the middle of the study.

The participants were randomized into one of two diet groups: animal-based keto, or plant-based low-fat. 

They were kept in a metabolic ward for 28 days – each subject spent the first two weeks on the diet they were initially randomized to, then the second two weeks on the other diet.

They got regularly scheduled meals, and all the snacks they wanted.

The two diets were matched evenly in calories, non-starchy vegetables, and protein. They differed as follows:

Keto diet: 75.8% fat, 10% protein

Low-fat, plant-based diet: 75.2% carb, 10.3% fat (I also refer to this group as the low-fat, high-carb diet in this article. 75% carbs is HIGH.)

What was measured?

The main metric that the researchers were looking to get was the total average calorie intake by members of each diet group. 

A ton of other things were measured too, like cholesterol levels, glucose levels (participants wore continuous glucose monitors), ketones, energy expenditure, and triglycerides, to name a few.

This is one of the many benefits of having the study subjects in a metabolic ward: they could be continuously monitored in a controlled environment. Again, not a regular occurrence in diet studies.

What did the study find?

According to the carbohydrate-insulin hypothesis, a low-carb or keto diet should decrease insulin in the blood, leading to less hunger and more weight loss, especially from fat. 

The keto people lost more weight, but more of that weight lost was muscle.

Fat loss (versus muscle) was greater in the low-fat group. Weight loss difference between the groups was not significant. 

In other words, both groups lost a comparable amount of weight. Check! Confirming what we already suspected from previous studies.

Amazingly, that people on the low-fat diet ate significantly fewer calories a day – 500-700, in fact. The keto group’s intake did go down by 300 calories a day in their second week on the diet, probably due to the presence of ketones in their blood.

But there was still a net loss of calories in the low-fat group versus the keto group. 

Researchers attributed the lower calorie intake of the low-fat group to higher energy density and greater fibre than in the keto foods. But the carb-insulin theory’s assertion that high-carb equals increased intake? Blown to bits. 

The study found that people on the keto diet had lower blood glucose levels after meals and as an overall average, and lower insulin levels after meals. This, we expect. But both blood glucose and insulin levels were within normal in both groups.

Eating a low-fat, high-carb diet didn’t spike participants’ sugar and insulin levels to the moon for days. Again, nothing to see here, folks. 

What’s better – keto or low fat?

According to Kevin Hall, ““Interestingly, our findings suggest benefits to both diets, at least in the short-term. While the low-fat, plant-based diet helps curb appetite, the animal-based, low-carb diet resulted in lower and more steady insulin and glucose levels,” Hall said. “We don’t yet know if these differences would be sustained over the long term.”

Fair enough. So, both diets have good and bad, as we already know. Here’s the thing though: why do we have to look at the extremes?

This is a good place for me to say what I always say: the best diet is the one you can stick to for the long-term. It doesn’t have to be 75% carbs, or 10% carbs. It can be somewhere in the grey area, which might be easier to sustain.

As I always used to say to my clients, there are a million different ways to do this – we just have to find the one that works for you. With your lifestyle, your preferences, your finances, your social life, and your health.

There’s always someone who will lose weight or achieve whatever goal on any one diet, but for the majority of people, diets are unsustainable. 

What if we aren’t eating a ketogenic diet? Or on the flip side, a diet full of highly processed carbs? What if we have a moderate-fat diet that contains a lot of plants, a few less carbs, and some meat and fish? 

We wouldn’t have to worry about keto versus low-fat, high-carb. That’s what. We’d just EAT.

I think it’s important to understand that we don’t exist in a vacuum – or a nutrition lab. We aren’t rats, or lab dishes. Every one of us is different, in not only metabolism and genetics, but in preference, lifestyle, socioeconomic status, education, and all the other things that impact food choices and the way our bodies respond to them.

Honestly, if you’re going to eat to nourish yourself physically and emotionally, I recommend rejecting the counting, restriction, and constant on-off lifestyle of any prescriptive way of eating.

Getting to know which foods make you feel good, as well as the ins and outs of your relationship with food, eating, and your body, can have more of a positive effect on your health than just jumping on another diet. 

If you need help with this, my book Good Food, Bad Diet can help.