Low-Fat vs Low-Carb Diets for Weight-Loss?

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There is the age-old question of which is better for weight-loss: Low-Fat or Low-Carb diets? At the end of the day, it comes down to calories in and calories out. If calories and protein are matched, then the results should be equal. But that brings up a different question: What if one of the diets is easier to follow in the real world? Of course, you can show the results are the same if you account for all the variables in a strictly controlled study, but that doesn’t mean very much from a practical perspective for those looking to lose weight on their own. Due to this, I’m going to mainly focus this article on what I truly believe is one of the most interesting nutrition studies of the last decade: The DIETFITS Randomized Clinical Trial.

The Study

The study involved over 600 participants (which is huge for a nutrition study) and was conducted by Christopher Gardener from Stanford. It went for 12 months and the participants were eligible to attend 22 sessions with a registered dietitian.

For the first two months the participants were told (and shown how) to consume <20g fat per day and <20g carbs per day. Following this they were meant to slowly titrate up to the lowest level fat/carbs (depending on their group) that they felt they could maintain indefinitely. They weren’t told to limit any other macronutrient or target a specific calorie amount. They were also supposed to focus on high quality food choices and eat as many vegetables as they could. This is crucial as it means people in the low-fat group couldn’t be having heaps of added sugar through soft drink for example.

My Thoughts on the Protocol

It is an interesting protocol and not really a concept I had considered before. I like the idea of starting lower and then titrating up to something more sustainable. People tend to be most motivated at the start, and if they get results then the motivation is more likely to continue. From a mental perspective it is also way easier to stick to a diet long-term if you view it as something more sustainable.

Theoretically I don’t really like the idea of somebody consuming <20g of fat per day for a full 2 months as it would likely significantly decrease their testosterone levels. If somebody has lower testosterone levels during weight-loss, they may lose more muscle then necessary and then reduce their metabolic rate more significantly. This could then impact their long-term results.

From a practical perspective, there was no need for me to worry about <20g of fat causing issues. I don’t know what levels people started at, but I do know that 3 months in, people in the low-fat group were already consuming an average of 42g of fat per day, which is a much safer level to be consuming. The low-carb group was consuming an average of 97g of carbs per day at the same time, which is interesting to note since that is almost the same number of calories as the 42g of fat provides. It’s also important to be aware that 97g of carbs is likely to be too many carbs for the participants to be in ketosis, which is something I will discuss later. The last thing to consider is that people consistently underreport their intake in 24-hr dietary recalls, which was the method utilised. Potentially their intakes were significantly higher than these numbers.

Why is the study interesting?

Christopher Gardener is a smart man. He knew going into this that low-fat and low-carb get the same results when calories and protein are matched. He also knew that they also often get similar results in the real world regardless of insulin sensitivity. The big thing he was aware of that a lot of people ignore, is that people often get mixed results within groups in research studies e.g. even though the two groups (low-carb and low-fat) often get similar results as a whole, certain individuals seem to perform better on one style of diet. What he was really looking for in this study was whether specific genotypes influence this.

Nothing was found relating to the impact that genotypes have unfortunately. That doesn’t necessarily mean genotypes don’t have an impact. All it means is that the genotypes the looked at weren’t involved in the process. Potentially they looked at the wrong genotypes.

Although they didn’t find what they were looking for, the results that people got, as well as how they got to the results is fascinating.

Arguments for Low-Fat and Low-Carb

People who are in either the low-fat or low-carb camp often have strong opinions of why their diet is easier to stick to. Low-fat people might just say that it is easier to follow since it is so much more like what people naturally do and you only have to make minor modifications that add up to the big changes in total fat consumption. People in to low-carb group often point out the appetite supressing effects of a ketogenic diet (which usually requires <50g of carbs per day).

The argument for the appetite suppressing effects of a low-carb diet completely fell apart in this study. Due to it being a free-living study where individuals could do whatever they wanted with the advice they were given, most people in the low-carb group were eating far too many carbs to stay in ketosis.

Potentially it IS easier for some people to be in a calorie deficit if they stick in ketosis and have the appetite suppression related to that. Unfortunately, I’m of the opinion that this study makes it clear that if you offered a ketogenic diet as blanket advice to the general population of people looking to lose weight, it will be ineffective since they are unlikely to stay in ketosis and reap those benefits.

Low-fat diets being easier to stick to makes sense intuitively, however on average people still didn’t have a significant calorie deficit, although there were exceptions to this. By the end of the trial the people in the low-fat group were consuming 57g of fat (with the low-carb group consuming 132g at the same period), which isn’t excessive unless the underreporting is significant.

I’m pretty big on higher protein consumption for long-term success, which I thought would be a variable in this study. I have long believed that if you told somebody to go low-carb, they would naturally increase their protein intake. I also believed that if you told somebody to go low-fat, they would decrease their protein intake, unless they were given guidance around the topic. This study highlighted that the difference isn’t nearly as severe as I expected, with there only being a reported ~10% difference between the protein intake of the two groups during the trial.

The Results

Surprisingly for those who strongly believe that one of the two options would be superior in the real-world, both groups lost 5-6kg on average. The difference between the groups was not statistically significant or clinically relevant. There was a massive variance in results inside the groups however, with somebody losing as much as 32kg and somebody gaining 11kg. Dieting is difficult and some people argue that it may even do more harm than good. I believe that it can be beneficial if the right approach is taken, but the fact that some people gained weight during this trial is even more evidence that an individualised approach is needed.

Key Takeaway

Low-carb vs low-fat? It really doesn’t matter.

What fascinated me so much about this study was what the participants had to say at the end. Every participant who completed the study had to go through exit interviews. The people who lost 20kg or more had some really interesting things to say.

I expected a lot of answers like “low-fat changed my life” or “low-carb changed my life,” but what they said instead all revolved around mindful eating. They said that the sessions with the dietitians really changed their relationship with food. They were asked not eat in the car or in front of a screen, to eat with friends and family, to try to cook for them and with them more often, and to shop more frequently for ingredients at a local farmers market. A lot of them highlighted that they used to eat really quickly, but now they eat slowly, chew thoroughly and enjoy the taste, smell and texture of their food.

I personally have never believed that those things are the key to losing weight and keeping it off, but if every person who lost 20kg or more in a study of this size said those things, it is worth paying attention to. My parting words are that it probably isn’t worth too much time searching for the magic diet, and the focus should instead be on choosing high quality foods and finding a sustainable way to eat an appropriate number of calories.

Aidan Muir

Aidan has been exposed to the most recent and up-to-date evidence based approaches to dietetic intervention. Dating back to well before starting uni he has been fascinated by all things nutrition, particularly the effects of different dietary approaches on body composition and sports performance. Due to this passion, he has built up an extensive knowledge base in multiple areas of nutrition and is able to help clients with a variety of conditions. One of Aidan’s main strengths is his ability to adapt plans based on the clients desires. By having such a thorough understanding of optimal nutrition for different situations he is able to develop detailed meal plans for clients, or he can provide flexible guidance that can contribute to improving the clients overall quality of life.

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