MedicalResearch.com Interview with:
Isao Saito, MD, PhD
Department of Basic Nursing and Health Science,
Ehime University Graduate School of Medicine
Medical Research: What is the background for this study? What are the main findings?
Response: Type 2 diabetes is a major lifestyle-related disease with a rapid increasing prevalence in Japan. One meta-analysis of six cohort studies showed that an increase in daily food intake of 1.15 servings of green leafy vegetables was associated with a 14% reduction in the incidence of type 2 diabetes. Therefore, it is evident to think that green and yellow vegetables have beneficial effects against type 2 diabetes. Nonetheless, the relationship of their nutritive content with insulin resistance is poorly understood.
We conducted the Toon Health Study initiated in 2009, which was a prospective cohort study of the Japanese general population. The cohort study was intended to characterize environmental risk factors related to incident diabetes and cardiovascular disease. Participants were recruited from the general population aged 30–79 years who were living in Toon City, Ehime Prefecture, Japan.
Of them, we investigated 951 Japanese men and women aged 30–79 years who were not undergoing treatment for diabetes and measured their serum β-carotene and retinol concentrations. A 75-g oral glucose tolerance test was performed and the Homeostasis Model Assessment for Insulin Resistance (HOMA-IR) and the Matsuda Index were calculated as measures of insulin resistance.
Multivariable-adjusted odds ratios of the highest quartile of serum β-carotene compared with the lowest quartile for HOMA-IR >1.6 and Matsuda Index <4.9 were 0.56 (95% confidence interval, 0.34–0.94) and 0.62 (0.37–1.02), respectively. When stratified by sex and overweight status, these associations were observed for women and non-overweight individuals. Serum retinol concentration was not associated with either index. Furthermore, according to the nutritional survey, serum β-carotene concentration was associated with green and yellow vegetable intake (p = 0.01).
Medical Research: What should clinicians and patients take away from your report?
Response: This study provided that a higher serum β-carotene concentration was associated with protection against insulin resistance, while serum retinol concentrations were not associated either way in middle-aged Japanese. The findings thus suggest that serum β-carotene, due to higher intake of green and yellow vegetables, confers beneficial effects against insulin resistance.
The effect of β-carotene on insulin sensitivity was apparent among women or non-overweight individuals in present study. Although we did not deny the association for men or overweight individuals, health professionals should encourage them primarily to take more green and yellow vegetables for diabetes prevention.
Medical Research: What recommendations do you have for future research as a result of this study?
Response: Since the present report was based on a cross-sectional design, the causality remains unclear. Although we consider that it seems reasonable to conclude that serum β-carotene concentration depends on green and yellow vegetable intake, which may prevent development of insulin resistance, a future prospective study is needed to confirm the association.
We consider that high serum β-carotene concentration was due to high intake of green and yellow vegetables and found the moderate association between them in present study. However, we could not suggest the recommended amount of green and yellow vegetable intake to keep adequate serum β-carotene concentration, especially for Japanese.
Furthermore, there may be residual confounding factors related to socioeconomic status (SES) that influences the intake of vegetables. According to the National Health and Nutrition Survey of Japan, higher income was related to higher vegetable intake. We hypothesize that inequity of health is derived largely from low SES, resulting in low intake of green and yellow vegetable and increasing diabetes in Japan.
Kana Higuchi, M.S. Isao Saito, M.D., Ph.D. Koutatsu Maruyama, Ph.D. Eri Eguchi, M.P.H., Ph.D. Hiromi Mori, Ph.D. Sakurako Tanno, M.D., Ph.D. Susumu Sakurai, Ph.D. Taro Kishida, Ph.D. Wataru Nishida, M.D., Ph.D. Haruhiko Osawa, M.D., Ph.D. Takeshi Tanigawa, M.D., Ph.D
Nutrition Available online 24 March 2015
MedicalResearch.com Interview with: Isao Saito, MD, PhD (2015). β-carotene From Green and Yellow Vegetables May Improve Insulin Sensitivity