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).
References
1. Lindeberg S, Jonsson T, Granfeldt Y,
Borgstrand E, Soffman J, Sjostrom K, et al. A Palaeolithic diet improves
glucose tolerance more than a Mediterranean-like diet in individuals with
ischaemic heart disease. Diabetologia. 2007;50(9):1795-807.
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.
4. Mellberg C,
Sandberg S, Ryberg M, Eriksson M, Brage S, Larsson C, et al. Long-term effects
of a Palaeolithic-type diet in obese postmenopausal women: a 2-year randomized
trial. European journal of clinical nutrition. 2014;68(3):350-7.
5. Kim Y, Je Y.
Dietary Fiber Intake and Total Mortality: A Meta-Analysis of Prospective Cohort
Studies. American journal of epidemiology. 2014;180(6):565-73.
6. NHMRC. Australian
Dietary Guidelines Providing the scientific evidence for healthier Australian
diets. 2013 Accessed online January 2014.
7. Blue Zones [cited 2015 March ]. Available from: http://www.bluezones.com/.
8. Noto H, Goto A,
Tsujimoto T, Noda M. Low-Carbohydrate Diets and All-Cause Mortality: A
Systematic Review and Meta-Analysis of Observational Studies. PloS one.
2013;8(1):e55030.
9. Mariotti Lippi M,
Foggi B, Aranguren B, Ronchitelli A, Revedin A. Multistep food plant processing
at Grotta Paglicci (Southern Italy) around 32,600 cal B.P. Proceedings of the
National Academy of Sciences. 2015.
10. Henry AG, Brooks
AS, Piperno DR. Microfossils in calculus demonstrate consumption of plants and
cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium).
Proceedings of the National Academy of Sciences. 2011;108(2):486-91.
11. Hardy K,
Brand-Miller J, Brown KD, Thomas MG, Copeland L. The Importance of Dietary
Carbohydrate in Human Evolution. The Quarterly Review of Biology. 2015;90(3):251-68.
12. Williams PG,
Grafenauer SJ, O'Shea JE. Cereal grains, legumes, and weight management: a
comprehensive review of the scientific evidence. Nutrition reviews.
2008;66(4):171-82.
13. Du H, van der A
DL, Boshuizen HC, Forouhi NG, Wareham NJ, Halkjær J, et al. Dietary fiber and
subsequent changes in body weight and waist circumference in European men and
women. The American journal of clinical nutrition. 2010;91(2):329-36.
14. Priebe MG, van
Binsbergen JJ, de Vos R, Vonk RJ. Whole grain foods for the prevention of type
2 diabetes mellitus. The Cochrane database of systematic reviews.
2008(1):Cd006061.
15. Jacobs DR, Jr.,
Marquart L, Slavin J, Kushi LH. Whole-grain intake and cancer: an expanded
review and meta-analysis. Nutrition and cancer. 1998;30(2):85-96.
16. Kushi LH, Meyer
KA, Jacobs DR, Jr. Cereals, legumes, and chronic disease risk reduction:
evidence from epidemiologic studies. The American journal of clinical
nutrition. 1999;70(3 Suppl):451s-8s.
17. Fardet A, Boirie
Y. Associations between food and beverage groups and major diet-related chronic
diseases: an exhaustive review of pooled/meta-analyses and systematic reviews.
Nutrition reviews. 2014:n/a-n/a.
18. Mellen PB, Walsh
TF, Herrington DM. Whole grain intake and cardiovascular disease: a
meta-analysis. Nutrition, metabolism, and cardiovascular diseases : NMCD.
2008;18(4):283-90.
19. Pereira MA,
O'Reilly E, Augustsson K, Fraser GE, Goldbourt U, Heitmann BL, et al. Dietary
fiber and risk of coronary heart disease: a pooled analysis of cohort studies.
Arch Intern Med. 2004;164(4):370-6.
20. Bazzano LA,
Thompson AM, Tees MT, Nguyen CH, Winham DM. Non-soy legume consumption lowers
cholesterol levels: A meta-analysis of randomized controlled trials. Nutrition,
metabolism, and cardiovascular diseases : NMCD. 2011;21(2):94-103.
21. Jayalath VH, de
Souza RJ, Sievenpiper JL, Ha V, Chiavaroli L, Mirrahimi A, et al. Effect of
Dietary Pulses on Blood Pressure: A Systematic Review and Meta-analysis of
Controlled Feeding Trials. American Journal of Hypertension. 2014;27(1):56-64.
22. Ha V, Sievenpiper
JL, de Souza RJ, Jayalath VH, Mirrahimi A, Agarwal A, et al. Effect of dietary
pulse intake on established therapeutic lipid targets for cardiovascular risk
reduction: a systematic review and meta-analysis of randomized controlled
trials. Canadian Medical Association journal. 2014.
23. Aune D, Norat T, Romundstad P, Vatten LJ. Whole grain and
refined grain consumption and the risk of type 2 diabetes: a systematic review
and dose-response meta-analysis of cohort studies. European journal of
epidemiology. 2013;28(11):845-58.
