This new trial has some major problems, but I believe it nevertheless adds to the weight of the evidence on "paleolithic"-type diets. The first problem is the lack of a control group. Participants were compared to themselves, before eating a paleolithic diet and after having eaten it for 10 days. Ideally, the paleolithic group would be compared to another group eating their typical diet during the same time period. This would control for effects due to getting poked and prodded in the hospital, weather, etc. The second major problem is the small sample size, only 9 participants. I suspect the investigators had a hard time finding enough funding to conduct a larger study, since the paleolithic approach is still on the fringe of nutrition science.

I think this study is best viewed as something intermediate between a clinical trial and 9 individual anecdotes.

Here's the study design: they recruited 9 sedentary, non-obese people with no known health problems. They were 6 males and 3 females, and they represented people of African, European and Asian descent. Participants ate their typical diets for three days while investigators collected baseline data. Then, they were put on a seven-day "ramp-up" diet higher in potassium and fiber, to prepare their digestive systems for the final phase. In the "paleolithic" phase, participants ate a diet of:

Meat, fish, poultry, eggs, fruits, vegetables, tree nuts, canola oil, mayonnaise, and honey... We excluded dairy products, legumes, cereals, grains, potatoes and products containing potassium chloride...

Mmm yes, canola oil and mayo were universally relished by hunter-gatherers. They liked to feed their animal fat and organs to the vultures, and slather mayo onto their lean muscle meats. Anyway, the paleo diet was higher in calories, protein and polyunsaturated fat (I assume with a better n-6 : n-3 ratio) than the participants' normal diet. It contained about the same amount of carbohydrate and less saturated fat.

There are a couple of twists to this study that make it more interesting. One is that the diets were completely controlled. The only food participants ate came from the experimental kitchen, so investigators knew the exact calorie intake and nutrient composition of what everyone was eating.

The other twist is that the investigators wanted to take weight loss out of the picture. They wanted to know if a paleolithic-style diet is capable of improving health independent of weight loss. So they adjusted participants' calorie intake to make sure they didn't lose weight. This is an interesting point. Investigators had to increase the participants' calorie intake by an average of 329 calories a day just to get them to maintain their weight on the paleo diet (bolding mine). Their bodies naturally wanted to shed fat on the new diet, so they had to be overfed to maintain weight.

On to the results. Participants, on average, saw large improvements in nearly every meaningful measure of health in just 10 days on the "paleolithic" diet. Remember, these people were supposedly healthy to begin with. Total cholesterol and LDL dropped, if you care about that. Triglycerides decreased by 35%. Fasting insulin plummeted by 68%. HOMA-IR, a measure of insulin resistance, decreased by 72%. Blood pressure decreased and blood vessel distensibility (a measure of vessel elasticity) increased. It's interesting to note that measures of glucose metabolism improved dramatically despite no change in carbohydrate intake. Some of these results were statistically significant, but not all of them. However, the authors note that:

In all these measured variables, either eight or all nine participants had identical directional responses when switched to paleolithic type diet, that is, near consistently improved status of circulatory, carbohydrate and lipid metabolism/physiology.

Translation: everyone improved. That's a very meaningful point, because even if the average improves, in many studies a certain percentage of people get worse. This study adds to the evidence that no matter what your gender or genetic background, a diet roughly consistent with our evolutionary past can bring major health benefits. Here's another way to say it: ditching certain modern foods can be immensely beneficial to health, even in people who already appear healthy. This is true regardless of whether or not one loses weight.

There's one last critical point I'll make about this study. In figure 2, the investigators graphed baseline insulin resistance vs. the change in insulin resistance during the course of the study for each participant. Participants who started with the most insulin resistance saw the largest improvements, while those with little insulin resistance to begin with changed less. There was a linear relationship between baseline IR and the change in IR, with a correlation of R=0.98, p less than 0.0001. In other words, to a highly significant degree, participants who needed the most improvement, saw the most improvement. Every participant with insulin resistance at the beginning of the study ended up with basically normal insulin sensitivity after 10 days. At the end of the study, all participants had a similar degree of insulin sensitivity. This is best illustrated by the standard deviation of the fasting insulin measurement, which decreased 9-fold over the course of the experiment.

Here's what this suggests: different people have different degrees of susceptibility to the damaging effects of the modern Western diet. This depends on genetic background, age, activity level and many other factors. When you remove damaging foods, peoples' metabolisms normalize, and most of the differences in health that were apparent under adverse conditions disappear. I believe our genetic differences apply more to how we react to adverse conditions than how we function optimally. The fundamental workings of our metabolisms are very similar, having been forged mostly in hunter-gatherer times. We're all the same species after all.