A review of the week’s plant-based nutrition news 2nd October 2022
This week I focus on type 2 diabetes. Plant-based diets and lifestyle interventions are hugely beneficial for preventing and treating this condition. I summarise some new studies, including one on low-carb vegan diets!
Type 2 diabetes is at pandemic proportions globally. In the UK, 1 in 10 people over the age of 40 years have a diagnosis of type 2 diabetes. Yet, the condition is preventable and reversible in most cases by addressing diet and lifestyle factors.
One of the major benefits of a plant-based or vegan diet is the ability to significantly reduce the risk of type 2 diabetes by up to 50%, in part due to the association with a healthier body weight. To learn more about the power of a plant-based diet for prevention and treatment of type 2 diabetes, read my updated article here.
VEGETABLE CONSUMPTION AND TYPE 2 DIABETES: Our consumption of fruit and vegetables is woefully low in the UK. Only 28% of adults consume 5 portions a day. The vegetable category usually includes beans and legumes, which are consumed at very low levels in the UK. Recent statistics show that the average weekly consumption of canned beans is 24 grams and fresh beans is 16 grams. Yet fruit and vegetable consumption, including legumes, is consistently associated with better health outcomes and a lower risk of type 2 diabetes.
This paper investigated the association of vegetable consumption on the risk of type 2 diabetes over a 10 period. The study included 1,485 healthy adults living in Athens, Greece. Five categories of vegetable intake were assessed; allium vegetables, cruciferous vegetables, leafy vegetables, red/orange/yellow vegetables (i.e., tomato, carrot, pepper, pumpkin) and legumes, including soya. Potatoes were excluded. 100g was considered a portion.
The results showed that over the 10-year period, participants consuming at least 4 servings of vegetables per day had a 58% reduction in the risk of developing type 2 diabetes, with the effect greater in women than men. In women the strongest association was with consumption of allium vegetables, although all vegetable consumption was beneficial. In men, significant inverse associations with diabetes were found only with red/orange/yellow vegetables’ and legume consumption. A dose-response relationship was found with an increase in the daily consumption of vegetables by 1 serving associated with a 9% lower risk of developing diabetes. For women this resulted in a 14% lower risk and men a 5% lower risk.
A reminder not to fear fruit. People often worry about consuming ‘too much fruit’ when it comes to the prevention of type 2 diabetes. The confusion arises with the fact that fruit contains fructose and fructose consumption is associated with an increased risk of chronic disease. When fructose is present in whole fruit, combined with fibre and micronutrients, there is no detrimental effect on health. Quite the opposite. A paper from the EPIC study analysed fruit and vegetable consumption in 9754 participants with and 13, 662 participants without type 2 diabetes. The novel aspect of this study was that plasma biomarkers of fruit and vegetable consumption were measured, such as vitamin C and carotenoids, to a give a more accurate indication of consumption. The results showed that fruit and vegetable consumption was associated with a significantly reduced risk of type 2 diabetes, in the order of 20–30%, and that even small to moderate increases in consumption were beneficial.
LENTIL CONSUMPTION AND INSULIN RESISTANCE: While considering vegetables consumption, it’s worth noting this randomised study which specifically investigated the role of 8 weeks of lentil consumption on insulin resistance (the underlying mechanism by which type 2 diabetes arises).
The study included participants with increased waist circumference (male>40”, female>35”) in an 8-week dietary intervention that included 5 prepared mid-day meals each week that were isocaloric but varied in cooked green lentil dosage: 0g (CON), 300g (MOD), or 600g (HI). The CON group had a meat-based meal, the MOD group a mixture of meat and lentils and the HI group just lentils as the main protein source in the meal. A glucose tolerance test was done before and after the intervention and participants completed questionnaires on satiety and gastrointestinal symptoms. 30 participants completed the study.
The results showed that lentil only meal group (HI) had a reduction in insulin resistance with no or mild gastrointestinal symptoms such as bloating, cramping and abdominal discomfort. In contrast the meat-based (CON) group and meat/lentil (MOD) group had an increase in insulin resistance, albeit less so in the MOD group. However, satiety did not differ compared to meat-based meals.
These results should not come as a surprise. A systematic review and meta analysis of randomised studies showed that consumption of pulses (beans, chickpeas, peas, lentils) improved glucose regulation and lowered insulin resistance in people with and without type 2 diabetes. Some people do develop gastrointestinal symptoms when they first start to increase their consumption of legumes. Here is a great factsheet to help you address this issue if it arises.
SOYA AND CHRONIC DISEASE: Soya is a unique legume with well-established health benefits. It stands out from other legumes for its high isoflavone content. It is a ‘complete’ source of protein with a biological value equivalent to animal protein, whilst it’s production has a significantly lower impact on the environment. This scoping review identified and evaluated twenty-eight meta-analysis reports, published between 2000 and 2021, on the associations of soya intakes with cardiovascular, cancer and type 2 diabetes.
The results showed that soya consumption was associated with a significantly lower risk of cardiovascular disease, certain cancers (lung, colorectal, gastric, endometrial, ovarian, breast, prostate) and type 2 diabetes. The only positive association was an increased risk of gastric cancer with the consumption of fermented soya, likely due to the high salt content of products such as miso. With regards to type 2 diabetes, the results show that soya consumption is associated with a 23% reduction in the risk, with both soya protein and isoflavones contributing to this reduction.
When considering increasing soya foods in the diet and as part of a plant-based diet, it is ideal to aim for around 2 portions a day (80g), including soya, tempeh, edamame, natto. Fortified soya milk is also a great addition to the diet.
AMINO ACIDS AND RISK OF DIABETES: We have all been led to believe that animal protein is superior in quality to plant sources. However, when you consider human health outcomes, the opposite is true. Higher consumption of plant sources of protein, such as legumes and nuts, have consistently been associated with a lower risk of common chronic conditions and early death. One of the reasons is that animal and plant protein differ in the amounts of the different amino acids. As summarised in my article, animal protein has higher quantities of branched chain amino acids, which have been associated with insulin resistance and an increased risk of diabetes.
This paper examines the impact of sulfur amino acids (SAA — methionine and cysteine) found in higher quantities in animal protein, on the risk of type 2 diabetes. The analysis is from two different cohorts of the Framingham Heart Study and included 6427 participants. During the 23 years of follow-up, there were 472 cases of type 2 diabetes. The results showed that a higher intake of SAA was associated with a 40–80% increase in risk of type 2 diabetes. However, these associations were attenuated when adjusting for animal protein, suggesting animal protein (most likely red and processed meat) accounts for much of the association. Other studies have shown associations between SAA and overweight and obesity, fasting glucose and insulin resistance.
The authors conclude ‘Our findings show that excess intake of SAA is associated with higher risk of type 2 diabetes. Dietary patterns that are low in SAA diet may help in preventing type 2 diabetes.’ Turns out a plant-based diet is a great way to lower the consumption of SAA. The paper states that ‘it is estimated that SAA levels in legumes, which are considered high in SAA among plant protein sources, are only approximately 25% of the SAA content found in most animal-derived foods. This value drops to about 10% for most other plant protein sources’.
We have known for a long while that a healthy plant-based diet as defined by the plant-based diet index, rich in fruit, vegetables, whole grains, beans, nuts and seeds, is associated with around a 34% reduction in the risk of type 2 diabetes. This was shown by researchers at Harvard University from their analysis of 3 prospective cohorts (Nurses’ Health Study 1 and 2 and the Health Professionals Follow-up Study). The same researchers have now studied several plasma metabolites in 10,684 of these participants and were able to show that people consuming a healthy plant-based diet have a unique metabolite profile that is associated with this reduced risk of type 2 diabetes. This provides a more objective way of assessing adherence to a plant-based diet.
GESTATIONAL DIABETES AND IMPACT OF LIFESTYLE FACTORS: This is a hugely important study. Diabetes that develops in pregnancy, gestational diabetes (GD), heralds a significant increase (up to 10-fold) in the future risk of type 2 diabetes. Therefore, supporting women to reduce this risk is hugely important. We now have some strong evidence-based recommendations that can be made with the potential to make a huge different.
This study prospectively evaluated the associations of adherence to optimal levels of five modifiable risk factors, including healthy body mass index, high quality diet, regular physical activity, moderate alcohol consumption (5.0–14.9 g/day), and not smoking, with the risk of progression to type 2 diabetes among women with a history of GD from the longitudinal Nurses’ Health Study II with 28 years of follow-up. A healthy diet was defined by adherence to the Alternate Healthy Eating Index (AHEI) which gives positive markers to high intakes of plant foods and omega-3 rich foods, such as fish, and high marks for low intakes of red and processed meat and processed foods in general.
During the follow up period there were 924 cases of type 2 diabetes. The results showed that the most important factor predicting future risk was body weight. Participants who had optimal levels of all five modifiable factors after the index pregnancy had a more than 90% lower risk for developing type 2 diabetes compared with those who did not have any. The risk reduction with one, two, three, four, and five optimal levels of modifiable factors compared with none was 6%, 39%, 68%, 85% and 92% respectively.
Importantly, the lower risk of type 2 diabetes was evident even among women with higher genetic susceptibility and for those carrying excess body weight. There was a dose response with those adhering the most having a greater benefit, but any level of healthy habits was beneficial.
Of note, with alcohol consumption there was the usual U-shaped curve with moderate consumption associated with benefit but excess associated with harm. There is limited evidence to support the necessity of alcohol in the diet and most healthcare organisation now recommend avoiding or limiting it’s consumption given that it is a known cause of cancer and the data supporting its role in health promotion is very limited. The authors of this study also agree ‘Emerging studies suggest that even moderate intake of alcohol might be associated with higher risk of other diseases such as liver diseases, certain cancers, and possibly cardiovascular disease. The current dietary guidelines state that women who abstained from alcohol consumption should not start drinking for any reason.’
Overall, this study is pretty remarkable as it confirms that in 90% of women at high risk of type 2 diabetes, a healthy diet and lifestyle can prevent the disease.
LOW-CARB VEGAN DIET: Low-carbohydrate or low-carb diets have gained popularity for the treatment of type 2 diabetes. In the short term (<12 months) they show a benefit for weight loss, better glucose control and support people to reduce or eliminate their medications. However, there are concerns about the long term impacts of a low-carb animal based diet with observational studies suggesting an increased risk of cardiovascular disease, cancer and premature death. In addition, recommending an increased consumption of animal foods when we are in a climate and ecological emergency should be considered irresponsible.
There may indeed be a benefit to lowering carbohydrate consumption in people living with type 2 diabetes. This can be achieved with a plant-based eating plan. This was first shown by Dr David Jenkins in 2009 and then in 2014 with his Eco-Atkins diet. This is a healthy vegan/plant-based diet composed of 26% of energy from carbohydrates and the rest from plant sources of protein and fat. When compared to a high carbohydrate (58% of energy from carbs) lacto-ovo vegetarian diet, the low-carb vegan diet showed an improved ability to lower blood lipid levels.
This latest study from the same team of researchers is in people with type 2 diabetes. One hundred and sixty-four male and female participants with type 2 diabetes were randomly assigned to advice on either a low-carbohydrate vegan diet, high in canola oil and plant proteins, or a vegetarian therapeutic diet, for 3 months, with both diets recommended at 60% of calorie requirements. On the low-carbohydrate (32% of energy) vegan diet, participants were provided with a canola oil–enriched bread and high-protein simulated meat products (Loblaw Companies; Gardein Protein foods). The vegetarian diet emphasised fruit, vegetables, and low-fat dairy products, with avoidance of meat and snack foods and carbohydrates were 56% of energy.. Participants were also provided with wholewheat bread and with liquid egg whites to reduce dietary cholesterol intake (Blue Menu, Loblaw Companies). The vegetarian diet was similar to the DASH diet. Both diets were prescribed at 60% of energy requirement.
The results showed that both diets were similarly effective for weight loss, reduction in waist circumference, reduction in blood pressure and LDL-cholesterol, including small particle LDL-cholesterol, and improvements in diabetes control. Both diets demonstrated a reduction in absorption of cholesterol due to the increased consumption of plant sterols.
The major difference between the two dietary approaches was a significant reduction of more the 50% from baseline of greenhouse gas emissions with the vegan diet. This was a significantly greater reduction than the vegetarian diet, which did of course still resulted in a reduction in food-related greenhouse gas emissions. The authors conclude ‘our data indicate that diets higher in vegetable oils and plant proteins appear suitable as weight-reducing diets in type 2 diabetes. More plant-based diets can also have the advantage of being more environmentally sustainable while reducing risk factors for cardiovascular disease and diabetes complications’.
This study is a useful addition to the literature on plant-based diets. However, it would have been useful to have a animal-based, low-carb diet and a low-fat/higher carb vegan diet (like the dietary approach of Dr Neal Barnard and his research team) as comparator groups. It is also not possible to confirm whether the benefits are merely due to the reduction in calorie consumption or truly a reflection of the benefits of a plant-based diet.
For me, this study confirms that a low-carb vegan diet is feasible with benefits for cardiometabolic health, whilst being significantly more environmentally sustainable than a vegetarian diet or a meat-based low-carb diet.