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Low-carb vs. high-carb: who's less hungry?
The fad diet wars continue, with more and more variations of a low carbohydrate diet being invented every few months, it seems. There's Atkins, South Beach, keto, paleo, and a host of other diets that essentially direct you to stop eating grains in favor of protein (in the form of meats, most often) and fat.

When the low-carb hype doesn't add up
The polyphenols (antioxidants) in green tea have been shown in animal and in vitro studies to have protective benefits against cardiovascular disease and cancer. Human studies, however, have been small and their results varied.

Low-carb diets do not burn fat
Proponents of low-carb diets will tell you that a high-fat, high-protein, low-carbohydrate diet will induce your body to burn fat. The theory is that a higher intake of carbohydrates in the diet causes greater insulin secretion, and that results in the body directing its circulating fat into storage.


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Which diet has the best evidence?

elements of a Mediterranean diet, including fruits, vegetables, fish, lean meats, legumes, and whole grains, on a white background

It is notoriously difficult to do high-quality nutrition research. The gold standard of research, of course, is the randomized, controlled trial, in which a particular treatment (say, a particular diet) is compared to another treatment - ideally a placebo (in dietary terms, usually the participants' usual diet). This goes on for as long as possible while the researchers observe the differences in outcomes.

As I'm sure you've guessed, there are a few barriers to performing randomized controlled trials with people's diets. First, what if the participants don't follow their assigned diet very well? The ideal solution is to provide them with all of their meals, and that not only gets expensive, it can lead to people dropping out of the study if they don't like the food.

Simply asking people to stick to a diet is fraught with uncertainty, as keeping a food diary to report everything they eat is notoriously unreliable: people like to report that they did what they are supposed to do, so any cheating may be minimized.

This isn't to say that randomized, controlled trials are impossible, but they are comparatively few and far between and tend to be much smaller than the more common prospective studies like the Nurses' Health Study, the Health Professionals Follow-up Study, or the PREDIMED study out of Spain, all of which include thousands upon thousands of people.

A common means of grouping together randomized controlled trials, for even greater quality of evidence, is the meta-analysis. For a meta-analysis the authors identify specific types of research (in this case, a randomized, controlled trial) and group the results of these trials together in an effort to magnify the trials' effects, whether positive or negative.

A group of researchers from multiple regions of Italy came together to perform a meta-analysis of meta-analyses, or as it is described in academic circles, an umbrella review of meta-analyses of randomized, controlled trials.

The authors searched published research articles and identified those meta-analyses of randomized, controlled trials that included adults of at least 18 years of age; identified particular dietary patterns; measured outcomes such as Body Mass Index (BMI); cholesterol scores; glucose, insulin, or HgA1C scores; or blood pressures.

The 80 studies included low carbohydrate diets, high-protein diets, the paleolithic diet, vegetarian diets, low-fat diets, Mediterranean diet, DASH (Dietary Approaches to Stop Hypertension) diet, Nordic diet, and low-glycemic-index or low-glycemic-load diets.

For each meta-analysis the authors also analyzed the quality of the included research. The authors state that the included articles made it difficult for them to analyze some research, as the definitions of some diets varied: for example, some of the studied "low-carbohydrate diets" allowed up to 45% of total calories from carbohydrates, while other studies of "low carbohydrate diets" limited carbohydrate intake to 25% of total calories.

Similarly, a "high-protein diet" might be defined by one study as including at least 20% of total calories, while another defined a "high-protein diet" as including at least 25% of total calories.

"Vegetarian" diets had similar problems, with some "vegetarian" diets being what I would consider lacto-ovo vegetarian and others vegan. (It's difficult to evaluate evidence when the evidence isn't actually standardized.)

Worse yet, most (not all) of the meta-analyses compared their focus diet with "any other dietary invervention, without specific indication" (meaning the comparison could be to literally any other way of eating, from all fast food, all the time, to bread and water).

Other research into these popular diets had serious flaws as well: Paleolithic diets, the authors note, were subject to "extensive publication bias, selective outcome reporting, and potential conflict of interests," and for high-protein diets, "the quality of published meta-analyses ... is critically low".

For all the diets included, however, "the Mediterranean diet was the only diet that demonstrated significant and beneficial effects for all the parameters analyzed [from body weight to blood pressures], without evidence of potential adverse effects."

More randomized, controlled trials and larger sample sizes were found for the Mediterranean and low-carbohydrate diets, while the number of trials and sample sizes were under 500 people for paleolithic, Nordic, and portfolio dietary patterns. The overall quality of the studies was moderate-to-high for studies including low-carbohydrate, low-fat, Mediterranean diet, Nordic diet, and low-glycemic-index/low-glycemic-load diets, and no studies with moderate or high methodological quality for high-protein diets, paleolithic diets, or DASH diets.

The authors performed detailed analyses of the quality of the evidence for the various diets' effects on loss of body weight, cholesterol scores (including HDL, LDL, total cholesterol, and triglycerides), blood pressures, and glycemic profile, finding that "only the Mediterranean diet showed significant beneficial effects... for all the parameters analyzed, without evidence of detrimental effects."

What this means for you

The current state of nutrition research is not perfect, and this article demonstrates that - although it is improving all the time. Still, this article concludes that the best research we have right now shows that a Mediterranean-style diet is more likely than other popular diet plans to help people improve cholesterol scores, blood pressures, and glycemic scores.

Granted, more and better quality research is needed on the other diets, but a Mediterranean-style diet is still one of the least restrictive healthy diet patterns.

First posted: August 5, 2020