Tuesday, October 6, 2015

The Truth Behind the Paleo Diet

Unless you have been living in a cave you would have heard about the Palaeolithic (Paleo) diet – a diet intended to reflect the eating habits of our hunter-gatherer ancestors who lived between 2.5 million and 10,000 years ago. While the science demonstrates that Paleo diets varied widely based on geographic location and food availability, the key principles of ‘modern day' Paleo diets are to enjoy an abundance of meat, other animal proteins, most fruit, most vegetables (but starchy vegetables like potatoes are restricted by some Paleo dieters), nuts and seeds, along with the strict avoidance of grains, legumes and dairy. Here we review the science presented in a recent meta-analysis on the short term effect of Paleo diet and also, while we are at it explore whether the assumptions that underpin these principles stack up.  

A look at the latest science:
Given the ongoing recent public interest in the Paleo diet, it is not surprising researchers are increasingly studying the healthfulness of this diet. One such study was a recent meta-analysis of randomised control trials (RCT) in which the authors concluded that based on data from the four included studies, there was moderate quality evidence to suggest that the Paleo diet may improve short term (2 weeks to 6 months) markers of metabolic syndrome (i.e. glucose tolerance, triglycerides, blood pressure) to a greater extent than other evidence based dietary interventions. While the headlines in the media covering this paper didn’t hold back and the findings made a splash, the authors cautioned that more evidence is needed before any changes to current guidelines can be made. Here is a snapshot of each study:

The first study was a 12-week RCT of 38 men with heart disease randomly assigned to the Paleo diet. Of the 29 who completed the study the researchers found greater improvements in glucose tolerance compared to those on a Mediterranean-styled diet(1). In the second study, a 6 month RCT, 17 adults with type 2 diabetes patients were randomly assigned to eat a Paleo diet or a standard ‘diabetes diet’. Of the 13 people whose data was included in the analysis, greater improvements in HbA1c (a measure of glucose control), cardiovascular disease risk factors and weight were seen in individuals consuming the Paleo diet(2). Whilst the findings of these two RCTs are favourable for the Paleo diet, a limitation of any comparisons that can be drawn is that participants were not limited in the amount of food and energy they consumed. As a consequence individuals on the Paleo diet in the first study actually consumed 25% less kilojoules than the control (Mediterranean) diet over 3 months (Lindberg et al.) and 16% less (Jonsson et al.) than the control diet over 6 months in the second study. As energy intake is a predictor of weight gain or loss and also influences risk factors for disease it is not surprising the diet where people consumed fewer kilojoules led to greater improvements in weight and markers of disease.

In the third two-week RCT, 34 individuals with metabolic syndrome were randomised to the Paleo diet group or an energy matched reference diet based on the Dutch Health Council guidelines. Interestingly, despite this being a randomised controlled trial the baseline characteristics of people assigned to each group differed significantly (a risk with small sample size) and people in the Paleo diet group had significantly higher body weights, BMI’s, ‘characteristics of metabolic syndrome’, TG:HDL-c, lower HDL-c levels (good cholesterol), higher CRP (marker of inflammation) as well as borderline significantly higher waist circumferences. Essentially the Paleo group appears to have been less healthy at baseline. After an energy matched two week intervention the Paleo diet group experienced greater benefits including lower blood pressure, total cholesterol, triglycerides and higher HDL cholesterol(3). Given the differences being the groups at baseline, the direct comparison of diets is limited as the improvement in health may have been confounded by the baseline health status of participants.

The final study was a two year RCT including 70 obese post-menopausal women assigned to either a Paleo diet or diet based on Nordic Nutrition Recommendations. Despite losing twenty-one women at follow-up, the authors reported a significant decrease for both groups in total fat mass and waist circumference at 6 months and 24 months. Whilst there was a significantly more pronounced fat loss in the Paleo group at 6 months, this was not significant at 24 months. Triglyceride levels decreased significantly more at 6 and 24 months in the Paleo group versus the control. Changes in all other markers including blood glucose, blood pressure, heart rate, CRP and blood cholesterol were not significantly different between diet groups.

Closer inspection of this study finds that despite regular information and food preparation sessions, participants in the Paleo group struggled to adhere to the assigned protein target (30% of energy intake) assessed via Nitrogen excretion in the urine. This raises the question of the practicality and sustainability of a Paleo-styled diet in the long term. Furthermore in the control group, while they were encouraged to enjoy a diet based on Nordic Nutrition Recommendations the average fibre intakes did not meet the adequate intake of 25g for women, and actually dropped slightly from baseline. This raises the question whether the food choices in the control group reflected a balanced diet with higher fibre choices as encouraged by the Nordic Nutrition Recommendations(4).

Given the few studies and limitations outlined above, it is not surprising the authors of the recent meta-analysis concluded that further studies are needed to evaluate the short term effect of the Paleo diet. Additionally, this meta-analysis did not evaluate the long term effects of Paleo diets, which are largely unknown. On the other hand, the totality of the scientific research, including evidence from some of the healthiest populations around the world (blue zones), supports the consumption of higher intakes of whole grains, high fibre grain foods and legumes for health and longevity across the lifespan(5-7). In addition to this there is actually observational evidence in humans to suggest long term lower carbohydrate intake (which may be achieved following a Paleo Diet) may actually increase your risk of early death(8).

The science behind the paleo story
So despite the lack of scientific evidence to support changes to dietary recommendations (in the short term and certainly not in the long term) in favour of a lower carb Paleo lifestyle, why are there more people than ever suggesting we should all “gopaleo” or “#primal”? The answer to this question is the Paleo story which has thrust this diet into the public’s attention, through an appealing and compelling narrative that tells the nostalgic story of returning to health through eating like our hunter-gatherer ancestors. The strict avoidance of grains and legumes is weaved into this story using three central assumptions. These are three assumption which are disputed by the science.

The first assumption is simply that Palaeolithic people did not consume grains and legumes. This assumption has been contradicted by an Italian team of archaeologists and researchers who recently unearthed stone tools used by humans over 30,000 years ago to grind wild oats into flour(9). Even more tellingly, studies of human fossils have found remains of grains and legumes on the teeth of Paleo humans(10).

The second assumption of the Paleo diet is that the human body is not genetically adapted to eat plant foods rich in starch like grains and legumes. Once again, a recent review has discredited this assumption. The preparation and cooking of ancient carbohydrate/starch rich foods is thought to have prompted the origin of genes in our mouths which secrete enzymes involved in the breakdown of dietary carbohydrate into glucose. Since glucose is the preferred source of fuel for the brain, experts have suggested that the consumption of carbohydrate by our ancestors would have been critical in the development of the human brain(11). Therefore, contrary to the Paleo story’s assumption, the evidence suggests that humans have in fact evolved to consume foods carbohydrate foods such as grains, legumes and other starchy fibre and nutrient rich plant foods.

The third assumption is that because of assumptions one and two, grains and legumes must be unhealthy for humans and contribute to modern day chronic diseases. This is contradicted by the review of the evidence which underpins the Australian Dietary Guidelines that supports the nutritional and health of enjoying a variety of grain foods, mostly whole grain or high fibre grain foods, and legumes(6). The scientific evidence indicates that people who eat more whole grain or high fibre grain foods and legumes are less likely to be overweight and have a lower risk of developing chronic diseases, including heart disease, type 2 diabetes and certain cancers(12-16). In fact a recent comprehensive review of 304 meta-analyses and systematic reviews has even shown that whole grain or high fibre grain foods offer the greatest protection against diet related disease, over any other core food group(17).

The Bottom Line
When all the current evidence is considered the story behind the ‘Paleo diet’ just doesn’t stack up.
There are a very small number of studies, all with limitations, that indicate a potential short-term benefit of the paleo diet. Watch this space, as more studies may emerge. In the meantime we need to remember that there are a large number of studies that show the benefits of including quality grains and legumes in the diet. So at this point it appears the positives of keeping grains in far outweigh those of eliminating them.

Like any energy restricted weight loss diet, a Paleo diet which restricts energy intake is likely to result in weight loss and as a result, favourable health measures in the short term. There is however concern that individuals following the Paleo diet may be at risk of falling short of obtaining important nutrients and fibres from whole grains, high fibre grain foods and legumes which may have a negative impact on chronic disease risk over the lifespan(6, 18-23).

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2.            Jonsson T, Granfeldt Y, Ahren B, Branell UC, Palsson G, Hansson A, et al. Beneficial effects of a Paleolithic diet on cardiovascular risk factors in type 2 diabetes: a randomized cross-over pilot study. Cardiovasc Diabetol. 2009;8:35.
3.            Boers I, Muskiet FA, Berkelaar E, Schut E, Penders R, Hoenderdos K, et al. Favourable effects of consuming a Palaeolithic-type diet on characteristics of the metabolic syndrome: a randomized controlled pilot-study. Lipids in health and disease. 2014;13:160.
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6.            NHMRC. Australian Dietary Guidelines Providing the scientific evidence for healthier Australian diets. 2013 Accessed online January 2014.
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