A review of the week’s plant-based nutrition news 17th October 2021
This week I cover papers on nut consumption and weight gain, vegan versus Mediterranean diet, omega-3 fatty acids and health outcomes and meat consumption in the UK.
NUTS AND BODY WEIGHT: There has always been an assumption that the consumption of nuts contributes to weight grain given the high fat (40–70% fat) content and hence higher energy density. However, clinical studies do not support this assumption. The current systematic review and meta-analysis strengthens the evidence for a lack of association.
This analysis included data from 6 prospective cohort studies and 86 randomised controlled studies (RCTs) and assessed the association of nut consumption with global and abdominal measures of obesity. The prospective cohort studies were from USA and Europe, included participants with a median age of 48 years, with a median follow up of 18 years. The median nut intake in the highest quintile of intake was 7g/day ranging from 3 to 28 or more g/day. The RCTs were mainly from USA and included predominantly participants who were overweight or obese. These RCTs examined a wide range of different nuts with a median dose of 45.5g per day (range 5–100g) and a median follow up of 8 weeks.
The results showed that nut consumption was associated with a decrease in the risk of overweight or obesity (7% reduction) and higher nut consumption was associated with weight loss and reduced risk of weight gain. Nut consumption was also associated with a lower risk of elevated waist circumference. Measures of adiposity were lower with doses of >45.5g per day with a dose-response relationship between nut consumption and lower body weight and body fat. For body mass index, waist circumference, and waist-to-hip ratio, the maximum protective dose appeared to be around 50 g/day.
These results are in line with those from prior meta-analyses. There are a number of reasons why despite the high energy density, nut consumption is not associated with weight gain. These included the high mono- and polyunsaturated fat content which may have a greater thermic effect compared to saturated fat, the satiating effect of protein and fibre and the physical structure of nuts such that not all the fat is absorbed. Based on these data, a daily 40–45g protion of nuts per day would be a good addition to global diets, which are in general quite low in nut consumption, at around 16g per day, mostly from peanuts. The authors conclude ‘Current evidence demonstrates the concern that nut consumption contributes to increased adiposity appears unwarranted’.
There is often criticism that studies on nut consumption are usually funded by the nut industry. This is true, but we need to find a way of assessing these studies, addressing any potential bias and then using the information to inform clinical practice. Without going into detail, the data on nut consumption is very consistent, be it observational or interventional studies. Inclusion in the diet, especially in place of animal-based foods, is beneficial to health with improvements in cardiovascular biomarkers, reduction in cardiovascular events and possible benefits for cancer prevention.
VEGAN VERSUS MEDITERRANEAN DIET: This study is a further analysis from the vegan versus mediterranean (Med) diet trial conducted by the PRCM. Sixty-two overweight participants were put on a vegan or Med diet for 16 weeks. After a 4-week washout period, participants swapped to the alternate diet. The Med diet followed the diet prescribed in the PREDIMED study and included 50g of olive oil daily. The vegan diet was intended to contain approximately 75% carbohydrates, 15% protein and 10% fat from whole plant sources, no added oil and a B12 supplement. Body weight, blood pressure, body composition, insulin resistance and glucose tolerance were measured before and after each phase. In the first analysis from this study, the results showed that the actual fat intake was 43% of calories in the Med group and 17% in the vegan group. Although participants could eat as much as they wanted, the vegan group consumed around 500 less calories per day and had a higher intake of fibre and lower intakes of saturated fat and cholesterol. The vegan group lost an average of 6kg, but the Med diet group lost no weight. The vegan group also had significant reductions in body fat and visceral fat and significant improvements in blood total and LDL-cholesterol levels and insulin sensitivity. These parameters did not change in the Med diet group. Both groups had reductions in blood pressure but there was a greater effect in the Med diet group.
The current secondary analysis examines weight loss and changes in the gut microbiome in the study participants. The participants on the vegan diet lost on average 6kg, with this weight loss being mainly from fat, compared to no significant weight loss in the Med diet group. Distinct changes were observed in the gut microbiome in participants on the vegan and Med diet group. Some of these changes were correlated with weight loss. However, the changes observed were not necessarily predictable from prior literature. For example, a reduced ratio of Bacteroidetes to Firmicutes bacterial species is often observed in people who are overweight, with an increase in Bacteroidetes species being associated with a more healthy weight. However, in both diet groups, Bacteroidetes significantly decreased and the ratio of Firmicutes to Bacteroidetes to increased. Prevotella increased in both diet groups, but not significantly. This bacteria is associated with high fibre diets. There was also a non significant increase in in the abundance of Faecalibacterium prausnitzii on the vegan diet, reflecting a higher intake of resistant starch. Proteobacteria decreased on the vegan diet, in line with prior data that correlates this decrease with weight loss.
Overall, the vegan diet resulted in greater weight loss, in part due to lower calorie intake despite no instruction to limit calories or portion size and also due to a greater loss of fat mass. This weight loss was associated with changes in the gut microbiome. Not all the changes were predictable from prior literature, but overall, reflected a change to a more fibre-rich plant-based diet with bacteria associated with the production of short chain fatty acids, which are beneficial for a number of health outcomes. Longer term studies would of course be useful.
MARINE OMEGA-3 FATTY ACID SUPPLEMENTS: The role of long chain omega-3 fatty acid supplements, DHA and EPA, mainly from fish, has been studied for decades. The use of these supplements are widespread amongst the general population despite the health benefits being questionable. It appears the main benefit, if any, is for secondary prevention of cardiovascular events (for people with known cardiovascular disease), where EPA supplements, rather than DHA, may have a beneficial role (although not proven is every study), particularly in people with high triglyceride levels.
What is now emerging is that higher doses of omega-3 fatty acid supplements are associated with a higher risk of atrial fibrillation. This was highlighted in a paper and summary in JAMA in March 2021. Now a further meta-analysis including 81,210 participants from 7 trials, followed for a median of 5 years shows that supplements of >1g/day resulted in a significantly increased risk of atrial fibrillation, with each 1g/day associated with an 11% increased risk.
Studies with omega-3 supplementation are difficult to interpret because often baseline dietary consumption of omega-3 fats are not taken into account and a variety of doses and formulations have been used. The most recent Cochrane review on increasing the consumption of long-chain and short chain omega-3 fatty acids from supplements, plants or fish, did not support this strategy for primary and secondary prevention of cardiovascular disease. The impact is minimal at best, with the exception of the high dose supplements for reducing triglyceride levels.
Given more than 80% of CVD could be prevented by addressing lifestyle factors such as overweight/obesity, diet, smoking, alcohol and physical inactivity, it seems our efforts are better directed at public health measures where there is little risk of side effects yet huge benefits for health and well-being. A plant-based or vegan diet is a very effective way of reducing cardiovascular risk factors and the incidence heart disease.
ALPHA LINOLENIC ACID (ALA) AND HEALTH OUTCOMES: This is a very useful paper as it helps answer the question about the impact of consuming short chain omega-3 fatty acids, ALA (alpha linolenic acid), on health outcomes. ALA is an essential fatty acid that has to acquired from the diet and is found from plant sources including flaxseeds, walnuts, chia seeds, soya beans and canola oil. ALA is converted in the body to the long chain omega-3 fatty acids, predominantly eicosapentaenoic acid (EPA) and, to a lesser extent, docosahexaenoic acid (DHA). This study examines the association of dietary intake and tissue biomarkers of intake on risk of mortality from all causes, cardiovascular disease (CVD) and cancer.
The systematic review and meta-analysis included 41 papers. 26 papers assessed the association between ALA and all cause mortality (11 on dietary intake, 13 on tissue levels of ALA, and two on both), 26 articles described the relation between ALA and CVD mortality (13 on dietary ALA intake, nine on tissue levels of ALA, and four on both), and 14 papers investigated the link between ALA and cancer mortality (10 on dietary ALA intake and four on tissue levels of ALA).
There is a lot of data in this study! The headline message is that higher dietary intake of ALA was associated with lower risk of mortality from coronary heart disease (11% reduction), cardiovascular disease (8% reduction), and all-causes (10% reduction), but was not clearly associated with cancer risk in dose-response analysis. Higher tissue levels/blood concentrations of ALA were also associated with lower all-cause mortality and coronary heart disease mortality. A linear dose response association was found between dietary ALA intake and CVD mortality, such that a 1g/day increase in ALA intake was associated with a 5% lower risk of CVD mortality.
Of note, there was a 6% increase in cancer mortality when comparing those with an intake of 1.35g per day to those with an intake of 0.7g/day, in the order of 6% increase. However, the robustness of this association is questionable given that for the non-linear dose-response analysis, there was no significant positive association between ALA and cancer mortality with dietary intakes from 0.27 to 3 g/day.
High levels of intake in this analysis were around 1.6g to 2.3g/day depending on the study. Adequate intakes of ALA are considered to be 1.6g per day for men and 1.1g for women. Some vegan authorities suggest that those on a 100% or plant-based diet, should aim for double the recommended intake given that a plant-based diet is low in preformed DHA and EPA, unless consuming algae supplements. This would amount to at least 2.2g per day.
Of course the current study has the same weaknesses of any observational study, in that dietary data collection may not be accurate, there may be residual confounding and of course causality can not be proven. However, given the essential nature of ALA in the diet, it is best to ensure a daily, reliable source and there may be a potential for benefits for at least cardiovascular health. Here is a nice summary on omega-3 fatty acids and food sources are summarised towards the end of the article.
MEAT CONSUMPTION IN THE UK: Over a ten year period, Britons are eating around 17% less meat, according to this new study from the UK National Diet and Nutrition Survey. Meat consumption has reduced from 103·7g to 86·3g per day with a 13.7g and 7g reduction in red and processed meat respectively. This reduction was mainly in the consumption of beef, lamb and sausages. However, there has been an increase in consumption of white meat by 3.2g per day. People born in 1960–79 and 1980–99 and White individuals were the highest meat consumers. 66% of people met the SACN (Scientific Advisory Committee on Nutrition) target of less than 70g per day of red and processed meat. Meat intake increased over time among people born after 1999, was unchanged and lowest among Asian and Asian British populations, and decreased in all other population subgroups. There was no difference in intake with gender or household income. There was no change in fish consumption. There was also an increase in people identifying as vegetarian or vegan during this time people with 4.5% recorded in 2018/2019.
Although this trend in meat reduction, with its co-benefits for planetary health, are encouraging, it is not enough. When it comes to meeting climate targets, we need to move rapidly to a more plant-based diet, particularly in high income countries such has the UK. This recent analysis in the BMJ suggested that meat consumption needs to fall by 79% and 68% in USA and Europe, respectively, to meet recommendations of the Eat-Lancet commission and the adoption of the planetary health plate. In addition, there needs to be better public health messaging. White meat, such as chicken, is not healthy in itself. It may be better than consuming red meat but still has a similar adverse impact on blood cholesterol levels, is associated with weight gain, hypertension, gastrointestinal disorders and diabetes. In substitution analysis, swapping poultry for plant sources of protein, such as nuts and beans (and also fish), significantly reduced the risk of cardiovascular disease and premature death from all causes.
With this in mind, it is great to see this new report from the Academy of Medical Sciences and the Royal Society calling on Government to implement strategies that are not only necessary to combat climate change, but will also improve human health and save lives. The report summarises the actions necessary in five main areas of our society; use of fossil fuels, travel, housing, healthcare and food production and diet. With regards food and diet, the authors recommend policy change to encourage the consumption of more fruits, vegetables and legumes, whilst reducing the consumption of meat and ultraprocessed foods. The report states that even modest diet changes ‘could results in an estimated reduction of 17% in diet-related GHG emissions, avoidance of almost 7 million years of life lost prematurely in the UK over the following 30 years (with reductions in coronary heart disease responsible for 70% of this impact), and an increased average life expectancy by over 8 months.’ Frustratingly, an overemphasis is placed on potential nutrient deficiencies when consuming a vegetarian and vegan diet, without acknowledging that all diet patterns have the potential for deficiencies if not well planned.
Listen to this excellent interview with PETA campaigner about the new study documenting UK meat consumption. Despite the negative framing of the questions, Dr Cary Bennett does a great job at bringing back the conversation to the important points.