More Proof That Fruit, Vegetables, Whole Grains May Stop Diabetes

Marlene Busko

July 09, 2020

Two studies published this week in BMJ provide support for eating more fruits, vegetables, and whole grain foods to lower the risk of developing diabetes.

In a pooled analysis of three large prospective American cohorts, people with the highest versus lowest total consumption of whole grain foods had a significantly lower risk of type 2 diabetes.

"These findings provide further support for the current recommendations of increasing whole grain consumption as part of a healthy diet for the prevention of type 2 diabetes," write the authors led by Yang Hu, a doctoral student at Harvard T.H. Chan School of Public Health, Boston, Massachusetts.

Similarly, in a large European case-cohort study, people with higher values for plasma vitamin C and carotenoids (fruit and vegetable intake) had a lower incidence of type 2 diabetes.

"This study suggests that even a modest increase in fruit and vegetable intake could help to prevent type 2 diabetes...regardless of whether the increase is among people with initially low or high intake," write Ju-Sheng Zheng, PhD, University of Cambridge, UK, and colleagues.

Individual Whole Grain Foods

Previous studies have shown that high consumption of whole grains is associated with a lower risk of developing chronic diseases, including type 2 diabetes, cardiovascular disease, obesity, and some types of cancer, Hu and colleagues say.

Although research has shown that whole grain breakfast cereal and brown rice are linked with a lower risk of type 2 diabetes, the effect of other commonly consumed whole grain foods — which contain different amounts of dietary fiber, antioxidants, magnesium, and phytochemicals — has not been established.

Hu and colleagues analyzed pooled data from 158,259 US women who participated in the Nurses' Health Study (1984-2014) or the Nurses' Health Study II (1991-2017) and 36,525 US men who took part in the Health Professionals Follow-Up Study (1986-2016), who were free of diabetes, cardiovascular disease, and cancer.

Participants' baseline consumption of seven types of whole grain foods — whole grain breakfast cereal, oatmeal, dark bread, brown rice, added bran, wheat germ, and popcorn — was based on self-replies to food frequency questionnaires.

During an average 24-year follow-up, 18,629 participants developed type 2 diabetes.

After adjusting for body mass index, lifestyle, and dietary risk factors, participants in the highest quintile of total whole grain consumption had a 29% lower risk of incident type 2 diabetes than those in the lowest quintile.  

The most commonly consumed whole grain foods were whole grain cold breakfast cereal, dark bread, and popcorn.  

Compared with eating less than one serving a month of whole grain cold breakfast cereal or dark bread, eating one or more servings a day was associated with a 19% and 21% lower risk of developing diabetes, respectively.

For popcorn, a J-shaped association was found for intake, where the risk of type 2 diabetes was not significantly raised until consumption exceeded about one serving a day, which led to about an 8% increased risk of developing diabetes — likely related to fat and sugar added to the popcorn, say the researchers.

For the less frequently consumed whole grain foods, compared with eating less than one serving a month of oatmeal, brown rice, added bran, or wheat germ, participants who ate two or more servings a week had a 21%, 12%, 15%, and 12% lower risk of developing type 2 diabetes, respectively.

Lean or overweight individuals had a greater decreased risk of diabetes with increased consumption of whole grain foods; however, because individuals with obesity have a higher risk of diabetes, even a small decrease in risk is still meaningful.

Limitations include the study was observational and may have had unknown confounders, and the results may not be generalizable to other populations, the authors note. 

"Five a Day" Fruits and Vegetables

Only one previous small published study from the United Kingdom has examined how blood levels of vitamin C and carotenoids are associated with incident type 2 diabetes, Zheng and colleagues write.

They investigated the relationship in 9754 adults who developed new-onset type 2 diabetes and a comparison group of 13,662 adults who remained diabetes-free during an average 9.7-year follow-up, from 340,234 participants in the European Prospective Investigation Into Cancer and Nutrition (EPIC)-InterAct study.

Participants were from Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, and the UK, and incident type 2 diabetes occurred between 1991 and 2007.

The researchers used high-performance liquid chromatography-ultraviolet methods to determine participants' plasma levels of vitamin C and six carotenoids (α-carotene, β-carotene, lycopene, lutein, zeaxanthin, β-cryptoxanthin), which they used to calculate a composite biomarker score.  

The recommendation to eat at least five fruits and vegetables a day corresponds to eating ≥ 400 g/day, according to Zheng and colleagues.

The self-reported median fruit and vegetable intake in the current study was 274, 357, 396, 452, and 508 g/day from lowest to highest quintile.   

After multivariable adjustment, higher levels of plasma vitamin C and carotenoids were associated with an 18% and 25% lower risk of incident type 2 diabetes per standard deviation, respectively.  

Compared with patients whose vitamin C and carotenoid composite biomarker scores were in the lowest 20%, those with scores in the top 20% had half the risk of incident diabetes.

Increasing fruit and vegetable consumption by 66 g/day was associated with a 25% lower risk of developing diabetes.

"These findings provide strong evidence from objectively measured biomarkers for the recommendation that fruit and vegetable intake should be increased to prevent type 2 diabetes," according to the researchers.

However, consumption of fruits and vegetables remains far below guideline recommendations, they observe.

"Although five portions a day of fruit and vegetables have been recommended for decades, in 2014-2015, 69% of UK adults ate fewer than this number, and this proportion is even higher in European (EU) adults (86%)."

Zheng and colleagues acknowledge that study limitations include those that are inherent with observational studies.

Although they could not distinguish between juice, fortified products, or whole foods, the analyses "were adjusted for vitamin supplement use, and suggest that as biomarkers of fruit and vegetable intake these findings endorse the consumption of fruit and vegetables, not that of supplements," they maintain.

The study by Hu and colleagues was funded by the National Institutes of Health. The InterAct project was funded by the EU FP6 program. Biomarker measurements for vitamin C and carotenoids were funded by the InterAct project, EPIC-CVD project, MRC Cambridge Initiative, European Commission Framework Program 7, European Research Council, and National Institute for Health Research. Zheng has reported receiving funding from Westlake University and the EU Horizon 2020 program. Disclosures for the other authors are listed with the articles.  

BMJ. Published online July 8, 2020. Study 1, Study 2

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