Does the Paleo Diet Really Work?

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A critical review of the evidence

One of the most controversial diets in recent times is the Paleolithic (“Paleo”) diet, also known as the Stone Age diet. The Paleo diet seeks to address 21^st century ills by revisiting the way humans ate during the Paleolithic era, more than 2 million years ago. As a nutritionist, I’m intrigued by this diet and by claims that it may reduce many of the chronic diseases that afflict society today.

Paleo proponents believe that because our genetics and anatomy have changed very little since the Stone Age, we should eat foods similar to those available during that time to promote good health. Stone Age people hunted, fished, and gathered wild plants for food, and had a high level of physical activity.^[1] If they lived long enough, they were believed to experience less modern-day diseases like heart disease. Although living beyond the age of 40 was rare according to Paleolithic records,^[2],[3] studies of hunter-gatherer tribes show they are largely free of the cardiovascular diseases common in industrialized societies.^[4]

Foods generally permitted as a part of the Paleo diet include fresh lean meats, fish, shellfish, eggs, nuts, seeds, fruits, vegetables, olive oil, coconut oil, and small amounts of honey.^[1] Proponents emphasize choosing fruits and vegetables with a low glycemic index (i.e., foods that cause a lower and slower rise in blood glucose). Foods generally not allowed on the diet include dairy products, grains (whole or refined), legumes (peanuts, beans, lentils), sugar, salt, refined vegetable oils such as canola, and most processed foods in general, along with alcohol and coffee.

The Paleo diet has many advocates, but it has been criticized due to exaggerated claims made by some of its proponents. In this article we’ll review the evidence for and against the Paleo diet.

What are the benefits of a Paleo diet?

Some aspects of the Paleo diet are highly desirable, including the elimination of processed foods and sugars, and an increased consumption of vegetables and fruits. There is overwhelming evidence that consuming refined grains and added sugars (especially in drinks) increases the risk of obesity, diabetes, and cardiovascular disease.^{[5],[6],[7],[8]} A strong case can be made that refined sugar should be avoided altogether.^[9] In contrast, non-starchy vegetables, whole fruits, legumes, and whole kernel grains may help protect against chronic diseases.^[5]

Vegetable consumption is universally recommended across evidence-based healthy diets.^[10] Vegetables have a low glycemic index and are rich in fiber and a variety of important micronutrients, including vitamins, minerals, and antioxidants.^[11] Higher vegetable consumption is correlated with a lower risk of hypertension, type 2 diabetes mellitus, and cardiovascular disease.^{[9],[12],[13]} Whole fruits are similarly recommended across a variety of heart-healthy diets due to their antioxidant and anti-inflammatory effects, generally low glycemic index, and high satiety profile.^[14]

Both the Paleo and control groups lost weight, but there was no difference in weight loss between the groups at the two-year mark.

As of this writing, a limited number of controlled clinical trials have studied the effects of the Paleo diet. A 2019 meta-analysis of 11 trials showed an average weight loss of approximately 3.5 kg in individuals consuming Paleo diets.^[15] While 11 trials may sound impressive, one of the studies enrolled only seven subjects on the Paleo diet, and six subjects on the control diet, which is hardly a convincing sample size. Consistent with the small size of the 11 studies, not one of them was given an “A” grade for evidence quality: four studies received a “B”, and seven studies were graded “C” by the authors. Also, most of the studies had durations of six months or less. In a longer-term study that was not included in the meta-analysis, 35 obese postmenopausal women were placed on the Paleo diet for two years, while a control group consumed the Nordic Nutrition diet (a healthy diet that emphasizes fatty fish, berries, root vegetables, nuts, legumes, low-fat dairy, and whole grains).^[16] Both the Paleo and control groups lost weight, but there was no difference in weight loss between the groups at the two-year mark.

Some studies suggest that the Paleo diet may lower blood cholesterol, triglycerides, and other metabolic disease markers,^{[16],[17],[18]} while one study observed an increase in these parameters in healthy subjects.^[19] Meta-analyses, again involving very small numbers of subjects, suggest that a Paleo diet may improve some blood markers more effectively than control diets.^[20],[21] However, some of the control diets had excessively high levels of carbohydrates. Critics have pointed out that high-carb, low-fat diets should not be considered as controls,^[22] because they are not heart-healthy.^[23],[24] Larger clinical studies, using a more relevant control such as the Mediterranean diet, are needed.

Are there risks associated with a Paleo diet?

Omitting whole grains, dairy, and legumes can lead to suboptimal intake of important nutrients.

Advocates of the Paleo diet have not shown whether eliminating dairy, legumes, and whole grains is good for your health. Moreover, omitting whole grains, dairy, and legumes can lead to suboptimal intake of important nutrients. One study showed that the dietary intake of thiamine, folate, magnesium, calcium, iron, and iodine decreased significantly in individuals adopting Paleo or Atkins-style (high-protein) diets.^[25]

Dairy products are a major source of calcium and vitamin D, and much larger clinical studies and meta-analyses have shown that the inclusion of dairy in the diet is one factor that may decrease the risk of osteoporosis.^[26],[27] Dairy consumption was associated with lower risk of mortality and major cardiovascular disease events in a diverse multinational cohort.^[28] Although some plant foods contain calcium, it can be difficult for Paleo followers (or vegans) to meet recommended daily calcium intakes unless supplements are included in their diets.^{[29],[30],[31]} Bone broth is commonly advocated as part of the Paleo diet, but a single serving provides less than 5% of the daily recommended calcium intake for adults.^[32]

Modern lifestyles limit our exposure to sunlight, and the incidence of vitamin D deficiency has been surging.

Vitamin D is another nutrient of concern. In hunter-gatherers, vitamin D was mainly obtained via the body’s synthesis of this hormone from ultraviolet irradiation (sunlight exposure) of the skin.^[33] Modern lifestyles limit our exposure to sunlight, and the incidence of vitamin D deficiency has been surging.^[34],[35] Vitamin D3 (cholecalciferol) supplements seem to be the best way to ensure adequate vitamin D status for individuals who do not consume dairy or other vitamin D-fortified foods.^[36],[37]

The elimination of salt in the Paleo diet also increases one’s risk for iodine deficiency.^[25],[38] In a 2-year prospective randomized trial of healthy postmenopausal overweight or obese women who consumed a Paleo diet, there was a 50% decrease in the average urinary iodine concentration after 6 months.^[38] Thus, iodine supplementation may be advisable for Paleo followers.^[38]

Paleo diets exclude whole grains and legumes, even though epidemiological studies have shown that such foods can help protect against many chronic diseases.^[9] Legumes offer a sustainable source of protein and fiber, promote weight loss due to their low calorie and high nutrient density, and are associated with a reduction in cardiovascular disease and all-cause mortality.^[39] Whole wheat, brown rice, barley, and oats are important sources of dietary fiber, minerals (including iron and magnesium), and essential B vitamins.^[40],[41] Whole grain intake also has been associated with lower risks of cardiovascular disease, cancer, and all-cause mortality.^[42]

Long-term Paleo diet followers experienced a massive reduction in Bifidobacterium, a bacterial genus that plays a critical role in gut health and immune function.

Finally, although the Paleo diet is often promoted for gut health, the exclusion of whole grains and legumes reduces the intake of prebiotic fibers that promote a healthy microbiota.^{[43],[44],[45],[46]} A 2019 study raised red flags when it showed that long-term Paleo diet followers experienced a massive reduction in Bifidobacterium,^[47] a bacterial genus that plays a critical role in gut health and immune function.^[48] Owing to their beneficial effects, bifidobacterial species such as B. lactis are frequently included in probiotic formulations. Importantly, low levels of bifidobacteria are associated with inflammation.^[49]

The same 2019 study showed an increased abundance of Hungatella, a bacterial genus that spits out trimethylamine (TMA), resulting in higher serum levels of trimethylamine-N-oxide (TMAO) in Paleo followers.^[47] TMAO is associated with an increased risk of cardiovascular disease,^[50] although recent studies suggest that TMA itself may be the culprit.^[51] Higher grain consumption was associated with lower TMAO levels.^[47] Although scientists are still sorting out the various dietary influences on TMAO, preliminary evidence suggests that whole grains and other dietary fiber sources may be required to reduce levels of this compound and maintain gut and cardiovascular health.^[52] The authors state, “Taken together with the greater observed serum TMAO concentrations, it cannot be concluded that the Paleoithic diet is associated with improved gut health and a reduction in risk of cardiovascular disease as promoted.”

Benefits of a hunter-gatherer lifestyle

Many Paleo proponents are focused on the diet, but hunter-gatherer lifestyles also feature a high level of physical activity. Archaeological evidence shows that our skeletons have become lighter and more fragile since Paleo times, a result of our increasingly sedentary lifestyles.^[53],[54] Further evidence suggests that 80% of U.S. adults and adolescents are not sufficiently active.^[55] Sedentary lifestyles are associated with an increased risk of cardiovascular disease, diabetes, and many cancers.^{[56],[57],[58],[59]} “Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations,” said Dr. Colin Shaw from the University of Cambridge.^[60] “There’s seven million years of hominid evolution geared towards action and physical activity for survival, but it’s only in the last say 50 to 100 years that we’ve been so sedentary — dangerously so.”

Summing up

At this time, it cannot be concluded that the Paleo diet per se is associated with a reduction in cardiovascular disease risk as is often promoted. Preliminary studies suggest that Paleo diets may help reduce body weight and improve blood cholesterol levels, at least over the short term, but may have harmful effects on other cardiovascular risk markers such as TMAO over the long term. The high level of physical activity associated with Paleolithic life inevitably is a factor, oft neglected, that contributes to weight management and overall health. Because the exclusion of whole grains, legumes, and dairy can reduce valuable bifidobacteria populations in the gut and increase the risk for micronutrient deficiencies, a probiotic supplement that includes Bifidobacterium spp. and a high-quality multivitamin and mineral supplement that includes calcium, iodine, and vitamin D3 may be advisable to protect Paleo diet followers from these potential adverse effects.

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