A review of the week’s plant-based nutrition news 21st November 2021

This week I cover plant-based diets and prostate cancer, the impact of the Eat-Lancet diet on health outcomes, healthy diets for prevention and treatment of depression and a summary of findings from the EPIC-Oxford study.

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PLANT-BASED DIETS AND PROSTATE CANCER: I have discussed the plant-based diet index (PBDI) often. Developed by researchers at Harvard University, it is a way of scoring food frequency questionnaire data to assess the impact of a plant-based dietary pattern without necessarily including vegan or vegetarian participants. The overall PBDI gives a positive score to all plant foods and a negative score all animal foods. The healthy PBDI scores only healthy whole plant foods positively but unhealthy plant foods such as refined grains and sugar, negatively, thus being able to tease out the impact of a healthy or unhealthy plant-based diet. We now have quite a lot data demonstrating that adherence to a healthy plant-based diet can reduce the risk of a number of chronic conditions, with any incremental shift towards a plant based diet being beneficial. You can read my previous summary here.

This study, for the first time, assesses the impact of adherence to the PBDI on the risk of prostate cancer in 47,239 men participating in the the Health Professionals Follow-up Study. During the 20.7 years of follow up, 6,655 men were diagnosed with prostate cancer, including 515 with advanced stage at diagnosis, 956 with lethal disease (metastasis or death) and 806 prostate cancer deaths (fatal). Greater adherence to an overall plant-based diet was associated with a 19% significantly lower risk of fatal prostate cancer. In men age <65 years, greater adherence to a healthy plant-based diet was associated with a 16% lower risk of total prostate cancer and a 44% reduction in lethal prostate cancer (prostate cancer death or distant metastasis) and 38% reduction in fatal prostate cancer (prostate cancer death). However, there were no associations between overall or healthy PBDI and prostate cancer in men ≥65 years. Surprisingly, more unhealthful plant-based foods were associated with a lower risk of lethal and fatal prostate cancer in men over age 65 years. This suggests that so called unhealthy plant foods may not be detrimental as such in the context of prostate cancer, but this would need confirmation in future studies.

There are several proposed reasons for these finding, including the anti-inflammatory potential of plant-based diets, the avoidance/limitation of dietary carcinogens and lower levels of IGF-1 when avoiding meat and dairy.

Of course, this study can not be extrapolated to make conclusions about vegetarian or vegan diet as <1% of participants followed such a diet pattern with the rest still consuming animal foods. However, the data do support the international cancer prevention guidelines which state ‘Make wholegrains, vegetables, fruit and pulses (legumes) such as beans and lentils a major part of your usual daily diet’. We also know that a plant-based diet can be useful even after a diagnosis of prostate cancer. Read my article on diet and prostate cancer here.

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EAT-LANCET INDEX AND MORTALITY: The Eat-Lancet planetary health plate was published in 2019, but very few studies have assessed the adoption of such a diet pattern on health outcomes. The authors of the current study developed a dietary index by classifying food components as either ‘emphasised foods’ or ‘limited foods’ based on the EAT-Lancet diet. Emphasised food components were vegetables, fruits, unsaturated oils, legumes, whole grains, nuts, and fish. Food components classified as limited were beef and lamb, pork, poultry, eggs, dairy, potatoes and added sugar. Dietary intakes were evaluated based on reported amounts in grams per day in uncooked weight, which is in line with how the target intake levels are expressed in the EAT-Lancet diet. This index consisted of the 14 food components, with a possible range of 0–3 points for each component; 0 points indicating low adherence to the target for the food component in the EAT-Lancet diet, and 3 points indicates high adherence thus the highest score would be 42, reflecting the best adherence to the diet.

The study assessed the impact of the Eat-Lancet index on mortality in the Malmö Diet and Cancer cohort, which included 22,421 participants who were between 45–73 years at baseline. During the 20 years of follow-up there were 7,030 deaths. The participants scored between 5–35 points on the dietary index (median 17.9 points) with women having a higher score than men. Less than 1% of participants met the target for legumes (median 6g/d) and nut consumption (median <2g/d) and less than 5% met the target for whole grains, beef and lamb, and pork consumption. Those most adherent to the index had a lower energy intake and higher carbohydrate and fibre intake.

The results showed that those participants most adherent to the Eat-Lancet index (>22 points) had a 25% reduction in death compared to those least adherent (<14 points). Each incremental increase in points resulted in a 4% lower risk of all-cause mortality. Both cancer and cardiovascular mortality were significantly lower; 24% and 32% reduction respectively. The foods most associated with a lower risk of mortality were higher consumption of whole grains, vegetables, and fruits and there was a higher risk of mortality with egg consumption. Interestingly, higher potato intake was also associated with a higher risk of mortality. This latter finding is not consistent in the nutrition literature and continues to be a subject of debate.

Overall, this study provides us with reassuring evidence that a diet pattern designed to keep the food system within planetary boundaries will also benefit human health. The key will be to increase the acceptance and adoption of such a diet.

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DIET AND DEPRESSION: This is an area of growing interest. The impact of diet and lifestyle interventions for promoting mental health and well-being are often underestimated with clinicians mostly focusing on pharmaceutical and psychological therapies. This is in part due to the fact that the level of evidence supporting dietary interventions remains weak.

This study adds to the current body of evidence and brings together data from 28 meta-analyses that assess the impact of dietary patterns, foods and beverages and various nutrients for the prevention and treatment of depression. The analysis showed that a healthy diet pattern (high in fruits and vegetables, fish, legumes, nuts, and cereals, with a low intake of red and processed meat), including the Alternate Healthy Eating Index, were associated with a lower risk of depression but diets with higher inflammatory potential with a higher risk. Mediterranean, vegetarian and Western-style diets were not specifically associated with a higher or lower risk. For specific foods, ultra-processed foods, sugar-sweetened beverages, red and processed red meat were associated with a higher risk of depression. These foods are all associated with higher levels of inflammation. Fish, fruits, vegetables, tea, coffee and caffeine predicted a lower risk of depression. For nutrients, the strongest association was for zinc consumption with mixed results for omega-3 fatty acids, which had a benefit in prospective cohort studies but not in randomised controlled studies. Dietary magnesium did not have an impact on the risk of depression.

For treatment of depression the analysis showed a benefit for omega-3 fatty acids, acetyl L-carnitine (known for its function on peripheral lipid metabolism, has been reported to take part in brain lipids synthesis and improve neurofunction via increasing antioxidant activity and enhancing cholinergic neurotransmission), dietary zinc, low calorie diets, cocoa-rich foods and probiotics.

The main reasons for these findings include the potential to lower inflammation through healthful dietary choices, a key mechanism involved in the development of depression. Foods most associated with lower inflammation are healthy whole plant foods. In addition, healthy diets and foods promote the health of the gut microbiome where most of the brain active hormones are synthesised and there is direct communication between the gut and the brain via the vagus nerve.

Overall this is a useful study confirming our knowledge to date but also demonstrating that we need better quality data. In the meantime, this knowledge emphasises the importance of healthy diets, rich in plant foods, for maintaining mental health and well-being and should form part of our approach for both prevention and treatment of depression.

There are in fact some emerging data to suggest a healthy plant-based diet may be beneficial for depression. A recently published cross-sectional study from Australia, including 219 adults following a vegetarian or vegan diet, found that a a high-quality plant-based diet (as defined as the PBDI) may be protective against depressive symptoms in vegans and vegetarians.

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PLANT-BASED DIETS AND HEALTH OUTCOMES IN THE UK: This is a great overview of the knowledge amalgamated from various UK-based studies on the health of vegans, vegetarians and those that do not eat meat. The review is based largely on results from EPIC-Oxford study (the European Prospective Investigation into Cancer and Nutrition Oxford cohort). EPIC-Oxford is a cohort of 65,000 men and women living throughout the UK and was established in the 1990s with recruitment targeted to identify as many vegetarians as possible thus 50% of the participants do not eat meat. It should be noted that the main disadvantage within the EPIC-Oxford study are that the number of vegans in the study is too small (~2,500 vegans) to give accurate information for many health outcomes and thus data on vegetarian and vegan participants is often analysed together. I have summarised some of the headline findings focusing on the vegan participants.

Protein intake: mean protein intake in vegans was 13.1% of energy but 16.5% and 8.1% of male and female vegans respectively were considered to have inadequate intakes of protein compared to <5% in omnivores.

Saturated fat intake: was lower at 6.9% of energy compared to 10.4% in omnivores.

Fibre intake: 28.9g/d in vegans compared to 21.7g/d in omnivores, measured as non-starch polysaccharides, which would be ~30% higher if expressed as g/d of AOAC fibre (the more conventional way of expressing fibre intake).

Vitamin B12: 52% of vegans were deficient in B12 compared to <1% of omnivores.

Calcium intake: At recruitment calcium intake in vegans was 582 mg/d and 610 mg/d in women and men respectively but at follow up this was 848 mg/d, which may have been the result of more questions on the questionnaire, and greater availability in shops, for plant-based dairy replacements with calcium fortification.

Other nutrients: Vegans in EPIC-Oxford had lower plasma concentrations than meat-eaters of eicosapentaenoic acid, docosahexaenoic acid and vitamin D.

Cardiometabolic risk factors: Vegans had a significantly lower body mass index (BMI) and less weight gain over time compared to meat eaters. Vegans also had lower cholesterol and LDL-cholesterol levels and blood pressure.

Bone mineral density: heel bone mineral density has only been assessed in participants of the UK biobank study. The T-scores were lower for vegans but still considered normal.

Cardiovascular disease: Vegetarians in the EPIC-Oxford study had a 23% lower risk of ischaemic heart disease over an 18 years follow up compared to meat eaters. Vegans had an 18% reduced risk but because of the lower numbers this was not statistically significant. The risk of haemorrhagic stroke for vegans and vegetarians combined was higher than meat eaters (48% higher risk). Numbers of events in vegans alone were too small to draw conclusions. Possible reasons for this include lower LDL-cholesterol and lower B12 levels in vegans.

Type 2 diabetes: Risk of type 2 diabetes was 47% lower in vegans compared to meat eaters although but this advantage disappeared after adjusting for BMI, meaning that the lower risk of type 2 diabetes was due to the lower BMI in vegans. This is still a very valid result given that the risk of type 2 diabetes is primarily driven by carrying too much body weight.

Cancer risk: In the EPIC-Oxford study there was an 18% lower risk of total cancer in vegans compared to meat eaters but caution advised with interpretation because there were only a few associations with individual cancer sites, which are not consistent across different studies.

Fractures: Vegans had a 43% increased risk of total fracture and 131% increased risk of hip fracture. This was only in women with a BMI <22.5. Of note, these results were not correlated with vitamin D levels which we know from prior analyses were lower in the vegan participants.

Other conditions: Diverticular disease, 73% reduction in risk in vegans; Gallstones, vegetarians and vegans combined had an increased risk of gallstones compared to meat eaters after adjustment for BMI; Kidney stones, vegetarians and vegans combined had a 31% reduced risk of kidney stones compared to meat eaters; All-cause mortality, no advantage for longevity for vegans and vegetarians compared to meat eaters.

Overall conclusions from the EPIC-Oxford study are that vegans are mostly meeting nutrient requirements considered optimal for good health. The exceptions from this cohort were vitamin B12, calcium and vitamin D. In addition, the consequences of lower levels of DHA/EPA are unknown. Many of the benefits (and potentially the harms cf fractures/stroke) of a vegan diet relate to lower intakes of saturated fat and fibre and the lower BMI, blood pressure, cholesterol.

For further information and context, you can read my previous articles on stroke and fractures.

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