Increasing
protein intake and soy consumption appear to be promising approaches to prevent
metabolic syndrome (MetS). However, the effect of soy consumption on
insulin resistance,
glucose homeostasis, and other characteristics of MetS is not frequently studied in humans. We aimed to investigate the effects of a 4-wk, strictly controlled, weight-maintaining, moderately
high-protein diet rich in soy on
insulin sensitivity and other cardiometabolic risk factors. We performed a randomized crossover trial of 2 4-wk 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 soy meat analogues and soy nuts containing 30 g/d
soy protein. For our primary outcome, a frequently sampled intravenous
glucose tolerance test (FSIGT) was performed at the end of both periods. Plasma total,
LDL, and
HDL cholesterol,
triglycerides,
glucose,
insulin, and
C-reactive protein were assessed, and blood pressure, arterial stiffness, and intrahepatic
lipid content were measured at the start and end of both periods. Compared with the mixed-
protein diet, the
soy-protein diet resulted in greater
insulin sensitivity [FSIGT:
insulin sensitivity, 34 ± 29 vs. 22 ± 17 (mU/L)(-1) · min(-1), P = 0.048; disposition index, 4974 ± 2543 vs. 2899 ± 1878, P = 0.038; n = 11]. Total
cholesterol was 4% lower after the
soy-protein diet than after the mixed-
protein diet (4.9 ± 0.7 vs. 5.1 ± 0.6 mmol/L, P = 0.001), and
LDL cholesterol was 9% lower (2.9 ± 0.7 vs. 3.2 ± 0.6 mmol/L, P = 0.004; n = 15). Thus, partly replacing meat with soy in a moderately
high-protein diet has clear advantages regarding
insulin sensitivity and total and
LDL cholesterol. Therefore, partly replacing meat products with soy products could be important in preventing MetS. This trial was registered at clinicaltrials.gov as NCT01694056.