Low-carb vs. low-fat diets: surely one of the most vicious rivalries of the contemporary fitness world, full of zealotry and quasi-religious belief. Add to that the ‘calories are all that matters’ crew and you have the makings of an epic Hollywood screenplay… or perhaps not.
In any case, this aspect of nutrition has to be one of the more difficult areas in which to tease out the truth from the nonsense and the science from the quackery. Today, we’re going to compare low-carb and low-fat diets in one very specific context: fat-loss. Is there any difference in the effectiveness of low-carb and low-fat diets when it comes to losing weight?
The DIETFITS study sought to answer exactly this question in a randomised controlled trial (RCT). 600 overweight participants were randomised to either low-fat or low-carb diets at the beginning of the study. Over a 12-month period each cohort was educated about the principles of a healthy low-carb (LC) or healthy low-fat (LF) diet and how to implement the appropriate nutritional strategy. At 12 months, the researchers re-measured participants’ weight to see if there were any differences between the groups.
The headline: there was no statistically significant difference in weight-loss between the LC and LF subjects. On average the LC participants lost 6kg while the LF group lost 5.3kg. In addition, both groups showed a similarly large range of individual weight-changes. In both groups, individuals ranged from a 30kg weight-loss all the way up to a 10kg weight-gain.
There were two particularly fascinating aspects to the study. First, the trial was designed to emphasise food quality in both groups:
“Both diet groups were instructed to (1) maximize vegetable intake; (2) minimize intake of added sugars, refined flours, and trans fats; and (3) focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible.”
After all, food quality could surely be a significant confounding variable if it differed between groups…
Secondly, even though no explicit instruction was given to reduce caloric intake, participants in both groups ended up eating significantly less:
“Despite not being instructed to follow a specific energy (kilocalorie) intake restriction, the mean reported energy intake reduction relative to baseline was approximately 500 to 600 kcal/d for both groups at each time point after randomization.”
This tells me two things: first, that food quality and nutrient density is the primary driver when it comes to weight-loss, over and above macronutrient designs; and second, that we don’t need to necessarily measure caloric intake in order to significantly decrease it and consequently create an energy deficit. If you focus on micronutrient dense foods, you will likely eat less food overall and lose weight as a result.
So that’s it then? Is there really no difference between low-carb and low-fat diets? I would suggest that although the study found no statistically significant differences, all fat-loss metrics subtly favoured the low-carb group. In the first instance, weight-loss was greater (-6kg vs. -5.3kg). Additionally, the low-carb group’s body-fat % dropped further as did their waist circumference (see below).
It’s also important to point out that there is always a limitation to studies which find no observable effect. It’s related to the concept of statistical power (stick with me here, it’s not that technical!). Most of you are probably familiar with the idea of statistical significance, which is generally set at 5%. A significance level of 5% means that there is a 5% chance of a false-positive; in other words, a 5% chance that an observed effect doesn’t really exist but randomly materialised out of the data-set.
Power relates to the other side of the coin: false-negatives. A power of 95% would mean that there is a 5% chance of a false-negative; in other words, a 5% chance that despite no effect being observed, there really is an effect that randomly did not show up in the data. You can limit the chances of a false-negative if the effect sizes are large or if your sample size is large. Now, the researchers didn’t publish the power level but it could range from around 68% for smaller effect sizes up to 99% for larger effect sizes, given the sample size. In other words, the study was not powered to rule out smaller differences between low-carb and low-fat weight-loss strategies so could well have missed a subtle difference (and a subtle difference is suggested in the data that consistently favours low-carb).
Beyond this, the study did highlight some differences between low-carb and low-fat groups, particularly with regard to blood lipids. Over the course of the 12 months, HDL cholesterol increased significantly more and triglycerides decreased significantly more in the low-carb group compared to the low-fat group. That sounds positive. But conversely, the low-carb group saw a roughly equivalent increase in LDL cholesterol while the low-fat group saw a small decrease. What to make of that? Cholesterol and blood lipids are complex and nuanced. I’ll need to return to the subject in more detail but I should point out that the changes in cholesterol were too small to be terribly significant (LDL-C increased by about 0.09 mmol/L in the LC group, where total LDL-C should sit below 3 mmol/L) while the changes in triglycerides was larger (triglycerides decreased by around 0.3 mmol/L where total triglycerides are normally below 2.3 mmol/L). Again, this looks to be slightly favourable to the LC approach.
Interestingly, both groups saw a marked and statistically significant improvement in important health biomarkers: blood pressure reduced and fasting glucose and insulin decreased. All of which suggests that the best thing you can do for your health if you’re overweight is to lose weight, by whichever method works best for you.
We should also ask the question of what constituted low-fat or low-carb in the study. For the low-carb group, daily carbohydrate ranged from 97g at the beginning to 132g at the end (subjects were instructed to slowly titrate their carbs up during the 12 months). For the low-fat group, daily carbs ranged from 205g to 212g. Both approaches are fairly moderate in my eyes. You might see that as a strength or a weakness of the study, depending on your viewpoint.
But what about individual differences? Surely certain people will respond better to low-carb and others to low-fat approaches? That’s probably the case on some level but the study did in fact looks at two potential hypotheses related to inter-individual differences. Certain gene polymorphisms have previously been associated with better responses to low-carb or low-fat. But when the researchers analysed the data, they found that these genes had no effect on how well subjects responded to respective diets. Another theory proposes that those with a greater insulin response to carbohydrate, indicating potential insulin resistance, would do better with low-carb approaches. Yet the study concluded that insulin response did not predict outcomes.
This is just one study in the large and complex space of weight-loss. And there are certainly limitations to the experimental design and statistical power. As with all nutritional studies, I wonder about the accuracy of self-reported dietary intake. Intake was recorded using surprise visits and multiple-pass interviews to determine average intake. But participants were well aware of what was expected of them through educational seminars and so there is an inherent risk of performance bias (in which subjects tell researchers what they want to hear). I wonder how likely it is that the standard deviation of carbohydrate intake for the LC groups was only around 4g per day!
Certainly, I don’t think we can conclude from this study alone that there are no differences between low-carb and low-fat approaches to weight-loss. But we can say that the differences between moderate low-carb and moderate low-fat approaches are probably fairly small, if they do exist. Moreover, delineating nutritional approaches strictly in terms of macronutrients may be missing the point. Both groups, on average, achieved a meaningful weight-loss and my conclusion is that this happened mostly as a result of an increase in food quality and a resulting reduction in caloric intake. What we really need to get to the bottom of is why, with the same level of education and guidance, some people lose 30kg while others gain 10kg.
The paper is strong ammunition against zealots on both sides of the fence: if anyone claims that low-carb or low-fat is the only way to lose weight, you know they haven’t considered the evidence. Nutrient density is primary and educating people on this piece is what matters most. Equally, although the study found no evidence for a mechanism underlying individual responses to diets, that doesn’t mean there is no such mechanism. I believe there most likely is, even if it’s as simple as the fact that some people prefer to eat carbs and others prefer to eat fat (there may well be genomic features that define this). I’m generally in favour of a moderate low-carb approach. I think the data in this paper indicates a subtle but potentially real advantage of limiting carbohydrate to a moderately low level and that has always been my preferred method for fat-loss. What do you think?