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Quality of vegetarian diets is important when reducing diabetes risk

Plant based diets have been found to lessen the risk of diabetes in previous studies, but a new study published in PLOS Medicine has found that the quality of the diet is important too.

Prior studies on plant based diets and type 2 diabetes (T2D) have defined plant based diets as ‘vegetarian’ and not further differentiated those who eat plant foods that are less nutrient dense such as refined grains, potatoes and sugar sweetened foods from those who eat a healthier plant based diet, to see what effect this has on T2D risk.

Study details

Scientists from Harvard T.H. Chan School of Public Health, Brigham and Women’s Hospital and Harvard Medical School in Massachusetts analysed data from three studies that followed more than 200,000 male and female health professionals across the US for more than 20 years, regularly collecting information on their diet, lifestyle, medical history, and new disease diagnoses.

They found that a diet focused on plant foods and low in animal foods was associated with a reduction of about 20% in the risk of diabetes. Diets higher in healthy plant foods such as whole grains, fruits, vegetables, nuts, legumes, vegetable oils and tea and coffee were associated with a larger decrease in diabetes risk - 34%.

Diets high in less healthy plant foods such as refined grains, sugar sweetened drinks and fruit juices, potatoes and sweet desserts was associated with a 16% decreased risk in diabetes.

Animal foods were defined as animal fats, dairy, eggs, fish/seafood, meat (poultry and red meat), and miscellaneous animal-based foods. The study also found that a modest reduction in animal foods from 5-6 servings a day to 4 servings a day significantly reduced T2D risk.

Participants who scored higher on plant based and healthy plant based indices were found to be older, more active, leaner, and less likely to smoke at the study baseline than participants with lower scores, but these variables were adjusted for in the statistical analysis.

The authors also controlled for other dietary patterns that have been associated with reduced diabetes risk such as the Mediterranean diet, the Alternate Healthy Eating Index (aHEI), and Dietary Approaches to Stop Hypertension (DASH) and found the results largely unchanged indicating that the association of a healthy plant based diet with reduced diabetes risk has an independent association.

What are the protective mechanisms?

The authors discuss several mechanisms through which a healthy plant based diet could lower the risk of type 2 diabetes, based on existing research:

  • High fibre improves glucose control and inflammation

  • Antioxidants reduce oxidative stress and have beneficial effects on glucose metabolism

  • Low saturated fats shown to be anti-inflammatory

  • Specific micronutrients such as magnesium (which may be high in plant based diets)

play a key role in glucose metabolism

  • High fibre and low calorie plant based diets may promote weight loss and maintenance (although BMI -body mass index- was adjusted for in the study, the risk reduction was stronger in non-obese participants)

  • Healthy plant based diets may improve the gut microbiome and moderate diabetes risk

When looking at a less healthy plant based diet, the effect of sugar in particular should be taken into account. High sugar intake can lead to inflammation, weight gain and increased diabetes risk.

Public health messages

The results of this study may inform future public health recommendations regarding plant based diets and the types of plant foods which should be most often consumed.

The beneficial results seen with a reduction in animal foods could also be applied to those people who are transitioning to, or including more, plant based foods. When the authors analysed how the results would be affected by scoring positively for fish and yoghurt (which have shown health benefits in previous research), the positive associations between healthy plant based diet and diabetes risk only changed slightly.

Certain animal foods such as red and processed meat have previously been shown to be associated with increased risk of diabetes and other diseases whereas dairy, lean poultry and fish have shown null or inverse associations. So for health risk an initial reduction in low quality animal foods as part of an overall strategy to move towards a more plant based diet may be beneficial, which reflects current public health messages.

Study limitations

The participants’ diet was self-reported so measurement errors are inevitable, but the use of cumulative measures of diet over time may reduce errors and also reflect long term dietary habits. The authors also made assumptions about the healthiness of plant foods, even though prior evidence was used, so there was an element of subjectivity. Their findings need to be replicated in future studies and with other populations other than health professionals.

“Our study supports current recommendations to shift to diets rich in healthy plant foods, with lower intake of less healthy plant and animal foods” concluded the authors.

Source: PLOS Medicine

Published online http://dx.doi.org/10.1371/journal.pmed.1002039

“Plant-Based Dietary Patterns and Incidence of Type 2 Diabetes in US Men and Women: Results from Three Prospective Cohort Studies”

Authors: A. Satija et al.

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