Impact of plant protein (in particular soya) on aspects of the metabolic syndrome

Alpro Foundation Supported Research

Dr. Marco Mensink, Assistant Professor, Division of Human Nutrition, Wageningen University

Metabolic syndrome (MetS) is a cluster of metabolic features associated with an increased risk for type 2 diabetes and cardiovascular disease which include abnormal glucose homeostasis and insulin resistance. In recent years, high-protein diets have become increasingly popular as a means to reduce obesity and improve insulin resistance and glucose homeostasis. As well as the total protein content of the diet, the dietary protein source could be of relevance. The main objective of this Alpro funded research was to understand the role of total and plant (in particular soya) protein consumption on aspects of the MetS.

Dietary protein intake and incidence of type 2 diabetes

The long-term association between total, plant and animal protein intake and incidence of type 2 diabetes was investigated, using data from the EPIC InterAct case-cohort study. In total, 12,403 cases of type 2 diabetes and a stratified sub-cohort of 16,154 individuals from 8 European countries, with an average follow-up time of 12 years were included. After adjustment for diabetes risk and dietary factors the incidence of type 2 diabetes was greater in those with higher intakes of total protein (per 10g:HR 1.06; 95%CI 1.02-1.09, P-trend<0.001) and animal protein (per 10g:HR 1.05;95%CI 1.02-1.08,P-trend=0.001). This association was stronger in obese women (BMI>30 kg/m2). There was no association between consumption of plant protein and type 2 diabetes (per 10g:HR 1.04;95%CI 0.93-1.16,P-trend=0.098).

Conclusion: In this large cohort of European adults, a high total and animal protein intake was associated with a modest elevated risk of type 2 diabetes whilst plant protein intakes had no effect.

Exchanging soya protein for meat and other protein sources

Taking into consideration earlier findings that the type as well as the total quantity of protein consumed could be relevant for MetS outcomes, a strictly controlled intervention study was undertaken by the same team. The aim of the study was to investigate the effects a weight-maintaining moderately high-protein diet, rich in soya, on insulin sensitivity and other cardio-metabolic risk factors.

Study design

This was a randomized crossover trial of two 4-week diet periods in 15 postmenopausal women with abdominal obesity to test diets with 22 energy percent (En%) protein, 27 En% fat, and 50 En% carbohydrate. One diet contained protein of mixed origin (mainly meat, dairy, and bread), and the other diet partly replaced meat with soya meat analogues and soya nuts providing 30g soya protein daily.

At the end of each 4-week period, an intravenous glucose tolerance test was performed (FSIGT). Plasma total, LDL, and HDL cholesterol, triglycerides, glucose, insulin, and C-reactive protein, and blood pressure, arterial stiffness, and intrahepatic lipid content were measured at the start and end of both periods.

Results

Compared with the mixed-protein diet, the soya-protein diet resulted in greater insulin sensitivity (FSIGT-SI: P=0.048). In addition, the soya-protein diet resulted in significantly lower total and LDL cholesterol compared to the mixed-protein diet (total 4.9 +/- 0.7 vs. 5.1 +/- 0.6mmol/L, P=0.001 and LDL 2.9 +/- 0.7 vs. 3.2 +/- 0.6mmol/L, P=0.004).

Conclusion: Partly replacing meat with soya in a moderate high-protein diet has a clear advantages regarding insulin sensitivity and total and LDL cholesterol.

Both studies in summary

In view of the rapidly increasing prevalence of type 2 diabetes and MetS, it is advisable to take care when promoting iso-energetic diets high in dietary proteins. Moderately high protein intakes as apart of balanced healthy diet which does not exclude or reduce other foods groups could be promoted. Plant proteins and in particular soya food sources of protein, should be encouraged as they appear to offer favourable MetS outcomes when they are consumed in place of animal proteins.

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