Animal and Plant Proteins Impact Risk of Type 2 Diabetes Mellitus in The Framingham Study
Authors: Castor Laura Lara , Bradlee M. Loring , Singer Martha R. , Moore Lynn L. 
Published: November 2023

An estimated 375 million people worldwide suffer from type 2 diabetes mellitus (T2DM). Diet and exercise are considered key prevention strategies for this disorder. Few longitudinal studies have examined the separate effects of animal and plant proteins although those that have often found an adverse effect of animal as well as total protein on diabetes risk. Other studies of the macronutrient composition of the diet have found that lower carbohydrate/higher protein diets are variably associated with T2DM risk. Given that higher protein diets are associated with greater maintenance of skeletal muscle mass in older adults, it is logical that protein might have beneficial effects on glucose metabolism. We used data from 1,423 subjects enrolled in the longitudinal Framingham Offspring Study with approximately 20 years of follow‐up to estimate the effects of animal and plant proteins on risk of T2DM in middle‐aged men and women. Diet was assessed using three‐day dietary records from exam visits 3 and 5 and protein intake was estimated after adjusting for body weight using residuals from a linear regression model. Glucose was measured after an overnight fast and impaired fasting glucose (IFG) was defined as a FG level >100; diabetes was diagnosed using a standardized and validated protocol. To estimate the relative risk (hazards ratio, HR) of IFG or T2DM, we used Cox proportional hazards modeling. After adjusting for age, sex, education level, physical activity, cigarette smoking, height, and energy intake, we found that subjects with the highest total protein intakes (≥100 g for men, ≥90 g for women) had a 31% reduced risk of IFG/T2DM (HR: 0.69; 95% CI: 0.54, 0.89). The highest (vs. lowest) intakes of animal protein (≥65 g for men; ≥55 g for women) led to a 27% reduced risk of IFG/T2DM (95% CI: 0.60, 0.89) while the highest intakes of plant protein (≥25 g vs. <20 g) led to a borderline statistically significant 19% lower risk. We then cross‐classified subjects according to the combined intakes of animal or plant protein and whole grains. Here we found that the combined effects of higher animal protein and higher intakes of whole grains led to a 25% lower risk (95% CI: 0.60, 0.93) of IFG/T2DM. The effects of plant proteins were similar in that higher intakes of plant proteins and whole grains led to a 19% lower risk of IFG/T2DM (95% CI: 0.66, 1.00). Finally, we examined the long‐term risk of IFG/T2DM associated with the combined exposures to higher intakes of animal or plant proteins and higher levels of physical activity. In this analysis, there was effect modification of protein intake by physical activity with respect to the IFG/T2DM outcome. Higher intakes of animal protein alone (with low physical activity) led to a 27% lower risk; higher activity alone (without higher intakes of animal protein) led to an 18% reduction in risk. Together higher animal protein and higher activity levels led to a statistically significant 40% reduction in risk of IFG/T2DM. There was evidence for positive synergism as well between plant protein intake and physical activity but these effects were slightly weaker. This prospective study of middle‐aged men and women provides important evidence to suggest that both animal and plant protein consumption, especially when combined with an active lifestyle, may be a strategy for lowering long‐term risk of diabetes.