Effect of Soy and Soy Isoflavones on Obesity-Related Anthropometric Measures: A Systematic Review and Meta-analysis of Randomized Controlled Clinical Trials

Abstract

Soy may be a suitable food for anti-obesity efforts because of its high protein and isoflavone content. We conducted this meta-analysis to evaluate potential effects of soy and soy isoflavones on weight, waist circumference, and fat mass. PubMed, MEDLINE, Scopus, EMBASE, and Cochrane databases were searched. Twenty-four trials with soy and 17 trials with isoflavones passed the eligibility stage. According to the results, soy showed no overall statistically significant effect on weight, waist circumference, or fat mass, but a significant increasing effect on weight was observed in some circumstances: for instance, in obese subjects [mean difference (MD): 0.80 kg; 95% CI: 0.15, 1.45 kg; P = 0.02], with ingestions of ≥40 g soy protein/d (MD: 0.94 kg; 95% CI: 0.11, 1.77 kg; P = 0.03), with short-term applications (1-3 mo) (MD: 0.45 kg; 95% CI: 0.05, 0.86 kg; P = 0.03), and when soy was compared with meat (MD: 0.36 kg; 95% CI: 0.09, 0.64 kg; P = 0.03) and whey protein (MD: 1.53 kg; 95% CI: 0.10, 2.96 kg; P = 0.04). In contrast to the effects of soy on weight, soy significantly decreased waist circumference in older ages (MD: -0.36 cm; 95% CI: -0.71, -0.01 cm; P = 0.04), in women (MD: -0.32 cm; 95% CI: -0.57, -0.08 cm; P = 0.01), and at doses of <40 g soy protein/d (MD: -0.31 cm; 95% CI: -0.57, -0.05 cm; P = 0.02). Isoflavone studies, conducted only in women, showed that isoflavones may reduce body mass index (BMI; in kg/m²) (MD: -0.26; 95% CI: -0.55, 0.04; P = 0.085), especially in dosages <100 mg/d (MD: -0.48; 95% CI: -0.90, -0.06; P = 0.02) and in intervention periods of 2-6 mo (MD: -0.28; 95% CI: -0.56, 0.00; P = 0.053), but no effect was observed in higher doses or longer intervention periods. Also, a trend for reduced BMI after consumption of isoflavones was observed in Caucasians (MD: -0.35; 95% CI: -0.74, 0.04; P = 0.08). Overall, results showed that, although soy is the major source of isoflavones, soy and isoflavones may have different impacts on weight status.

Keywords: fat mass; isoflavones; soy; waist circumference; weight.

FIGURE 1

Summary of the screening and selection process of trials included in the meta-analysis of the effect of soy and soy isoflavones on obesity-related anthropometric measures.

FIGURE 2

Forest plot of clinical trials examining the effect of soy on body weight (kilograms) with subgroup analysis based on BMI status of participants. Data are expressed as MDs between treatment and control groups with 95% CIs. Estimates were pooled by using the random-effects, inverse-variance model. MD, mean difference.

FIGURE 3

Forest plot of clinical trials examining the effect of soy on body weight (kilograms) with subgroup analysis based on dosage. Data are presented as MDs between treatment and control groups with 95% CIs. MD, mean difference.

FIGURE 4

Forest plot of clinical trials examining the effect of soy on body weight (kilograms) with subgroup analysis based on intervention length. Data are presented as MDs between treatment and control groups with 95% CIs. MD, mean difference.

FIGURE 5

Forest plot of clinical trials examining the effect of isoflavones on BMI (in kg/m²) with subgroup analysis based on ethnicity. Data are presented as MDs with 95% CIs. MD, mean difference.

FIGURE 6

Forest plot of clinical trials examining the effect of isoflavones on BMI (in kg/m²) with subgroup analysis based on isoflavones dosage. Data are presented as MDs with 95% CIs. MD, mean difference.