Traditional diet advice over the last fifty years, still espoused by most health authorities around the world, holds that if you want to lose weight, you need to cut down your fat intake. In the last few decades, a number of alternative diets have sprung up claiming that you should instead be cutting down your carbohydrate intake. These include the LCHF, Atkins, paleo, and more recently the ketogenic diets. But what do the randomized controlled trials say? Should you cut down on fat or carbs if you want to lose weight?
Before we get in to the meat of the article, we need to clarify one thing. There are basically four different dietary sources of energy available to humans. These are carbohydrates, proteins, fats, and alcohol. If you get less of your energy from one source, that necessarily means you will need to get a greater proportion of your energy from one or more of the others. So a low carb diet generally means you will be getting a greater proportion of your calories from fat, and a low fat diet generally means you will be getting a greater proportion of your calories from carbs.
The idea behind the traditional advice to cut down on fat is largely based around the cholesterol hypothesis, which we have recently debunked. Basically, it was thought (and still is, to a large extent) that a high fat diet made you fat and caused heart disease, which was the basis for recommending that people cut down on their fat intake.
The idea behind the low carb diets is largely based around the fact that fats and proteins are absorbed more slowly than carbohydrates, which leads to a longer lasting sense of fullness after a high fat or high protein meal than after a high carb meal. This should in turn leads to a lower overall intake of calories.
In recent years, there have been two decent quality randomized controlled trials comparing low carb vs low fat diets, one that was published in the Annals of Internal Medicine in 2014, and another that was published in JAMA (the Journal of the American Medical Association) in 2018. We will look at each in turn.
The 2014 trial included 148 participants who were randomized to either a low carb diet or a low fat diet. The trial was funded by the National Institutes of Health (NIH) and was carried out in New Orleans. In order to be eligible for the study, participants had to have a BMI of between 30 and 45 (30 is the lower limit for being diagnosed with obesity) and not have any other chronic health conditions. The average age of the participants was 47 years.
Participants were recruited through mailing lists, flyers, and TV-advertisements. This is important, because it means that the participants were motivated to lose weight and to start leading a healthier lifestyle. Something to be conscious of in light of this is that there was no placebo group – the purpose of this study was not to compare a diet versus placebo, but to compare two diets versus each other. This limits the data somewhat. It would have been interesting to have a third group that was getting some form of placebo treatment, in order to be able to compare the diets with doing nothing. Since the people taking part in the study were in general a least a bit motivated to lose weight, it is quite possible that they would end up losing weight over the coming year even without any specific dietary intervention.
Participants in the low carb group were instructed to keep their daily carbohydrate intake at below 40 grams per day. This isn’t just a low carb diet, it is a very low carb diet (also known as a ketogenic diet). In general, people enter a state of ketosis when their carbohydrate intake is below 50 grams per day. (Ketosis is a state in which the liver starts to produce large amounts of ketones to compensate for the lack of carbohydrates. Ketones are an alternative fuel produced by breaking down fats, which can be used by the heart, brain, and muscles when there is a lack of available carbohydrates).
Participants in the low fat group were instructed to get at most 30% of their calories from fats. Neither group was given a specific calorie target. From my perspective, this is a sensible approach, because people generally aren’t going to be willing to keep counting calories for the rest of their lives, so if a new diet is going to be effective over the long term, then it needs to be relatively effortless to maintain once new dietary habits have been formed.
Both groups were given handbooks explaining what to eat in order to meet the targets, and were provided with one meal replacement per day for the duration of the study, which was either of the low fat or low carb variety depending on which group they were in. Apart from that, the participants had follow-up meetings with a dietician weekly during the first four weeks of the study, and thereafter small group counseling sessions every two weeks for the next five months, and finally small group sessions every four weeks for the final six months.
The participants were followed up at three, six, nine, and twelve months and asked to recall everything they had consumed over the preceding 24 hours. They were also weighed, and had blood tests and urine tests drawn at each of these time points.
So, what were the results?
Overall, 80% of participants followed through with the trial. There was no significant difference between the groups in terms of likelihood of sticking with their respective diets. This suggests that both diets were about equally easy to maintain.
At baseline, the average intake of carbohydrates was 242 grams per day in both groups. The low carb group was able to decrease this to 97 grams at the 3 months mark, but was up to 127 grams at the twelve month mark. The low fat group also decreased its intake of carbohydrates, to 193 grams at the three month mark, which had risen marginally to 197 grams at the twelve month mark.
There are two things about these results which are interesting. Firstly, the low carb group was supposed to cut carbs down to 40 grams per day, but never even came close. To me, this shows the difficulty of maintaining a ketogenic diet. Having said that, they were able to halve their carbohydrate intake, which is still a significant change from baseline. Secondly, the low fat group also decreased their carb intake, which I guess makes sense, since the people taking part in the trial had all signed up to it because they wanted to lose weight and were motivated to do so, so even though there was no formal requirement to cut down on calories, it is reasonable to expect that many participants consciously did so anyway. This could obviously make any difference between a low carb and low fat diet seem smaller than it actually is, since both groups cut down on carbs.
When it comes to fats, the average intake at baseline was 34,7% of total calories in the low fat group, and 32,5% in the low carb group. At the one year mark, this had decreased to 29,8% in the low fat group, but increased to 40,7% in the low carb group.
The average weight reduction at one year was 1,8 kg in the low fat group, compared with 5,3 kg in the low carb group. The difference was highly statistically significant. At the same time, the low carb group increased its muscle mass by 1,3% while the low fat group decreased its muscle mass by 0,4% . This difference was also highly statistically significant.
But that’s not all. One common argument against a low carb high fat diet is that it might be more effective at causing weight loss, but that this is outweighed by the fact that it increases the risk of heart disease. Luckily, the researchers gathered lots of data to help us determine if that is actually the case, and based on this they calculated the ten year risk of cardiovascular disease at baseline and at one year, using a scoring system known as the Framingham risk score. In the low fat group, the ten year risk of heart disease increased by 0,4% . In the low carb group, the risk decreased by 1,0% . Again, this difference was highly statistically significant.
What conclusions can we draw from this study? Basically, the low carb group lost almost three times as much weight as the low fat group, while gaining muscle mass (the low fat group lost muscle mass). Additionally, the low carb group decreased its ten year cardiovascular risk, while the low fat group increased its risk, according to the Framingham risk score. These results strongly suggest that a low carb diet is more effective than a low fat diet, both for weight loss and for protecting against cardiovascular disease.
But before we draw our final conclusions, we’re going to look at the second study, published in 2018. This was a randomized controlled trial that involved 609 participants with a BMI between 28 and 40 (As mentioned before, 30 is the cut-off for obesity. 25 is the cut-off for overweight, so these people were overweight to obese). The average age of the participants was 40 years old. Apart from the excess weight, they had to be fundamentally healthy in order to be included. The trial was carried out in San Francisco and was, just like the previous trial, financed by the NIH. As in the previous trial, participants were followed for one year and were randomized to either a low fat or a low carb diet.
Over the course of the year, the participants took part in 22 small group counseling sessions focusing on ways to achieve the respective diet that they had been assigned to (more frequent earlier in the trial, less frequent towards the end). What participants were told to do in this study varied a bit from the previous one. Participants were told to either limit intake of fats or intake of carbs to 20 grams per day for the first eight weeks, depending on whether they were in the low fat or low carb group. After that, they were supposed to slowly add the fats or carbs back by 15 grams per week until they reached the lowest level that they thought they would be able to maintain over the long term. As in the previous trial, no specific calorie restriction was imposed. And as in the previous study, the actual amount of each substance eaten was determined by interviewing the participants at various time points about what they had eaten during the preceding 24 hours.
On to the results.
As in the other trial, around 80% of participants followed through to the end, with no difference between the groups. At baseline, the low fat group was consuming around 242 grams of carbohydrates per day while the low carb group was consuming 247 grams of carbohydrates per day. At the one year mark, the low fat group had decreased its carbohydrate intake slightly, to 213 grams per day, while the low carb group had decreased its carb intake significantly, to 132 grams per day.
Fat intake at baseline was 35% of total calories per day in the low fat group, and 36% per day in the low carb group. At the one year mark, the low fat group had decreased its relative fat intake to 29% of total calories, while the low carb group had increased it to 45% .
The average weight reduction at twelve months was 5,3 kilograms for the low fat diet group and 6,0 kilograms for the low carb group. The difference was not statistically significant.
That’s odd. The first study found a big difference in weight loss between the groups, while the second study didn’t find any meaningful difference. This is in spite of the fact that the studies were actually constructed quite similarly, and that the changes in carbohydrate and fat intake levels were similar. How do we explain that?
I have no idea. If anyone has any suggestions, please post them in the comment section. In general, the data from second study should be more reliable for the simple reason that it had over four times as many participants, which significantly decreases the risk that random chance will cause a result that isn’t real.
In terms of Framingham ten year risk score, the low fat group decreased its risk by 0,1%, while the low carb group increased its risk by 0,1% . At the end of the trial, participants in the low fat group had a 0,7% ten year risk, while participants in the low carb group had a 0,9% ten year risk. This difference between the groups is too small to be clinically meaningful, but it is quite a difference from the prior study, which found a significant decrease in ten year risk with the low carb diet.
One thing to note before we get to the final conclusion, is that the carb intake in the low carb group was around 100-150 grams in both studies. While this is low carb compared to the standard western diet, it is far above what proponents of a ketogenic diet recommend. It is possible that a more severe carbohydrate restriction would show even greater weight loss benefits. These two studies are not able to answer that question. However, the first study was aiming for a ketogenic diet and didn’t even come close, so it is also quite likely that ketosis is not a realistically achievable long term state for the majority of people, even if it is effective in theory.
Final conclusion: Basically, from these two studies I think we can conclude that a low carb diet is at least as effective as a low fat diet for weight loss. Additionally, the data from these studies does not support the idea that a low carb diet increases risk of heart disease compared with a low fat diet. As an aside, that is another nail in the coffin of the cholesterol hypothesis.
You might also be interested in my article about whether exercise is an effective way to lose weight or my article about whether salt is bad for your health.