Review of the week’s plant-based nutrition news 16th May 2021

This week I cover the benefits of nitrate-rich vegetables, vegan diets and bone health, the importance of vitamin D, negative impacts of sugary drinks and air pollution from animal agriculture.

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DIETARY NITRATE INTAKE, BLOOD PRESSURE (BP) AND CARDIOVASCULAR DISEASE (CVD): Some of the healthiest vegetables to consume are those high in inorganic nitrates, particularly leafy greens and beetroot. This is because nitrate is converted to nitric oxide in the body, crucial for optimal vascular health. Many lines of evidence support the consumption of nitrate-rich plant foods for reducing BP and risk of CVD.

This study examined the impact of long-term habitual nitrate intake in participants of the Danish Diet, Cancer, and Health Study cohort on BP and CVD. Nitrate consumption was calculated using the published reference database. 53,150 participants were included in the study with a median age of 56 years at entry and a median follow up of 21 years. The median nitrate intake from vegetables was 59mg/day and non-vegetables 15mg/day. The vegetables contributing the most to nitrate intake were lettuce (41%), potato (22%), celery (10%), carrot (5%), and spinach (3%). Consumption of nitrate-rich foods was associated with other beneficial health behaviours and better educational attainment.

The results showed that those in the highest quintile of vegetable nitrate consumption (141mg/day) compared to the lowest quintile (23mg/day) had significantly lower systolic and diastolic BP, 2.58mmHg and 1.38mmHg respectively. This included participants taking and not taking BP lowering medications. There was no association with non-vegetable nitrate consumption and BP.

There was a non-linear association between vegetable nitrate intake and risk of CVD. The maximum impact was seen in participants in quintile 3 of intake (59mg/day) or above with a 12% reduction in CVD, 17% reduction in ischaemic heart disease, 17% reduction in ischaemic stroke, 15% reduction in heart failure and 26% reduction in peripheral artery disease, compared to quintile 1. The only condition in which there was further reduction in risk with higher intake was with peripheral artery disease. Those in quintile 5 of consumption had a 35% reduction in risk. There was no association with risk of haemorrhagic stroke or atrial fibrillation. Non-vegatable nitrate intake resulted in lower risk of CVD hospitalisation (6%), haemorraghic stroke (19%) and peripheral artery disease (15%). The association between CVD and nitrate intake was stronger in those consuming higher amounts of alcohol, suggesting a partial mitigation of the adverse impact of alcohol on cardiovascular health.

These results support the findings of previous intervention studies that have demonstrated lowering of blood pressure with consumption of vegetable nitrate such as beetroot juice. Even a 2mmHg lowering of blood pressure is associated with a significant lowering in future risk of CVD. Other prospective cohort studies have also shown a similar reduction in CVD risk with higher consumption of nitrates. The maximum effect in this current study is seen around 60mg/day which is equivalent to 1 cup of leafy greens. Of course, this type of study can not prove cause and effect, but there is no down side to consuming a portion of leafy greens every day.

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VEGAN DIET AND BONE FRACTURES: There remain concerns about adequacy of a vegan diet for bone health. A meta-analysis of studies in 2019 showed a signal for increased fracture risk in vegans compared to omnivores, although diet quality was only taken into account in one of the studies included. In Nov 2020, data from the EPIC-Oxford cohort here in the UK raised concerns for increased fracture risk in vegans and also those not consuming meat.

This new study examined fracture rates in the well characterised Adventist Health Study 2. This analysis included 34,542 participants, including 2832 vegans followed since early 2000’s. Compared to omnivores there was an increased risk of hip fracture, a surrogate for osteoporosis, in vegan women. There was no increased risk in men. This was after controlling for usual confounders such as body weight, physical activity, use of HRT etc. The 55% increased risk sounds alarming but in absolute terms this is an extra 1.5 hip fractures amongst 1000 vegans per year.

Of note, the increased risk in women disappeared in those who were supplementing with calcium and vitamin D. Among vegans, 32% supplemented with both vitamin D and calcium compared with ∼50% in the other dietary groups. The mean dosage of each supplement differed little by dietary pattern: ∼660 mg Ca/d and ∼13.5 μg vitamin D/d. The impact of each supplement individually was assessed although it was not possible for vitamin D alone because only 373 women were taking vitamin D supplements alone and among these there were only 2 hip fractures. In participants who only supplemented with calcium, there was a nonsignificant increased risk for all vegetarian dietary patterns, with pesco-vegetarians and vegans having the highest. The total calcium intake in vegans who were supplementing was around 1000mg a day.

Although there remain a number of open and unanswered questions, a clear signal for increased risk of bone fracture in vegan, Causasian women has now been seen in 2 large and well conducted studies. In both the EPIC-Oxford nad AHS-2, vegans had lower dietary intakes of vitamin D and calcium. Thus, optimising calcium and vitamin D intakes seems to be essential, whilst making sure a vegan diet is healthy and composed of whole plant foods. We certainly need more data on non-Caucasian populations.

All diet choices have potential benefits and risks. This paper acknowledges the significant reduction in other comorbidites in vegans including lower risk of heart disease, overweight, type 2 diabetes and certain cancers. These advantages are pretty huge in my book. We however need to understand, acknowledge, mitigate and be honest about any potential downsides. For more information about vegan diets and bone health, read my article here. We have a useful factsheet for ensuring optimal nutrient intake for bone health which you can find here.

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VITAMIN D AND COVID-19: Vitamin D has a number of beneficial effects in addition to promoting bone health. This includes immune-regulatory effects, impacting virtually all immune cells in some way. Prior to COVID-19, observational studies had consistently shown an association between low blood levels of vitamin D and the risk of acute respiratory infection. In addition, supplementation with vitamin D in intervention studies are associated with a lowering of this risk. Thus, there has been a great deal of interest in the role of vitamin D supplementation for prevention of COVID-19 infection.

This study analysed observational data on 8297 subjects in the UK Biobank who had COVID-19 test results and vitamin D levels measured between March and June 2020. Researchers also assessed genetically predicted vitamin D concentrations and the impact of vitamin D supplementation. Of note, the measurements and information were from 2006–2010 rather than being current.The results did not find an association between vitamin D levels or genetically predicted vitamin D levels and risk of COVID-19. However, there was an association between vitamin D supplementation and a 34% reduction in risk of COVID-19 infection. The association between the use of vitamin D supplements and the risk of COVID-19 infection did not vary according to the different levels of circulating or genetically predicted vitamin D levels. The study also assessed the impact of habitual use of other supplements including, vitamin A, B, C, E, multivitamin, calcium, folic acid, zinc, iron, selenium, glucosamine or fish oil. The analysis did not find an association between these supplements and risk of COVID-19.

These results add to the large number of studies on the topic with data so far unable to prove a causal association between higher vitamin D levels and prevention or reduction in severity of COVID-19 infection. However, no one doubts the importance of adequate vitamin D for maintaining a healthy immune system. The trouble is the blood levels are pretty arbitary and what is considered optimal is still controversial. A level of 50nmol/l is thought to be adequate for bone health but a concentration of 75nmol/l seems more appropriate for optimal immune function. All immune cells have the ability to convert vitmain D to its active form and blood levels may not reflect the actual cellular or tissue level of vitamin D. The current study used >50nmol/l as the highest level and thus may not have been able to detect an impact of higher levels of the immune system. In addition, the study examined vitamin D levels and supplementation a median of 10 years prior to the COVID-19 test.

There are inherent limitations to this study, but the those on a plant-based diet, vitamin D supplementation and achieving good circulating levels seems essential. More information on vitamin D can be found in our factsheet here.

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SUGAR-SWEETENED BEVERAGES (SSB) AND RISK OF COLORECTAL CANCER (CRC) IN WOMEN: There has been a worrying increase in rates of colorectal cancer in people less than the age of 50 years in high income countries. This has occurred in a timeframe too short to be related to genetic changes thus diet, lifestyle and environmental factors are top of the list of potential causes. This study adds to our knowledge on this topic.

This study for the first time examines the association between SSB consumption and risk of early-onset CRC (less than age of 50 years) in 95,464 participants of the Nurses’ Health Study followed for a median of 24 years. SSB consumption of 2 or more servings a day compared to less than once per week was associated with a 2.2 times increased risk of developing CRC. In the 41,272 participants in which data was available, each additional serving per day at age 13–18 years was associated with a 32% increased risk of CRC. Replacing one serving per day of SSB with artificially-sweetened beverages, coffee and milk, reduced the risk of CRC by 17–36%. Of note, substituing SSB for fruit juice did not reduce the risk of CRC.

These data add to a large body of data demonstrating the adverse health consequences of consuming SSBs, including overweight and obesity, cardiovascular disease and type 2 diabetes. There has been at least one study to suggest an increased risk of cancer in general in those consuming the most SSBs. There are a number of plausible reasons for this. SSB suppress satiety and promote the overconsumption of calories thus are associated with weight gain, a major risk factor for CRC. The high glycaemic index of these drinks promotes inflammation and insulin resistance. Fructose in SSBs has also been shown to disrupt the gut lining, increase gut permeability and adversely affect the gut microbiome.

Of note, we know that milk and dairy consumption in general is associated with a decreased risk of CRC, predominantly because of the calcium content. Calcium supplements also have a similar affect. This does not mean dairy is necessary, only that it is better than consuming SSB. Thus, sticking to a fibre-rich plant-diet and optimising calcium intake should be prioritised. After all, dairy has adverse effects too, increasing the risk of prostate cancer and possibly breast and endometrial cancer.

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IMPACT OF ANIMAL AGRICULTURE ON AIR POLLUTION: Our food system is a major contributor to a number of inter-related crises including climate, biodiverity and nutrition. Animal agriculture is increasingly recognised as a major factor. However, we don’t often consider the impact of animal agriculture on air pollution.

This first of a kind study from the US measured how the production of various foods impacted air quality, particularly fine particulate matter. The study quantifed the air quality–related health damages attributable to 95 agricultural commodities and 67 final food products, which encompasses >99% of agricultural production in the US. The pollutants considered included amnonia, nitrogen oxides, sulfur dioxide, and nonmethane volatile organic compounds. Animal food production has a greater impact on air pollution because of pollution released from the manure of animals themselves and from fertilizer use and tillage of land when growing the crops that they eat. We know that chronic exposure to particulate matter increases the risk of a number of chronic diseases, including heart disease, stroke and cancer.

The study showed that 17,900 deaths are atrributable to air polluation caused by agriculture. This is mainly driven by ammonia emissions, from livestock waster and fertiliser application, which account for 69% of these deaths. 80% of deaths (12,700) are attributable to animal-based foods (when impacts of animal feed production are included) and 20% (3,200) to plant-based foods. The study shows that per serving, the average air quality-related harm of red meat to human health is two times greater than that of eggs, three times greater than those of dairy products, seven times greater than those of poultry, 10 times greater than those of nuts and seeds, and at least 15 times greater than the average of other plant-based foods. The authors conclude ‘Substituting poultry for red meat could prevent 6,300 annual deaths (40% of total deaths from food production). Even greater benefits of 10,700 to 13,100 deaths prevented per year (68 to 83%) could be achieved from more ambitious shifts to vegetarian, vegan, or flexitarian diets such as the planetary health diet of the EAT-Lancet Commission.’

There is now overwhelming evidence supporting a global shift to a healthy plant-based diet. This study adds just one more reason. The powerful video below highlights just how urgently we need to take action to halt the damage we are causing to the planet, which is ultimately impacting human health.

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