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J Endocr Soc. 2017 Apr 3;1(5):423-430. doi: 10.1210/js.2016-1068. eCollection 2017 May 1.

Soy Protein Improves Cardiovascular Risk in Subclinical Hypothyroidism: A Randomized Double-Blinded Crossover Study.

Author information

1
Department of Academic Endocrinology, Diabetes, and Metabolism, Hull York Medical School, Hull HU3 2JZ, United Kingdom.
2
Department of Academic Cardiology, University of Hull, Hull HU3 2JZ, United Kingdom.
3
Sidra Medical Research Centre, 26999, Doha, Qatar.
4
Weill Cornell Medicine Qatar, 24144, Doha, Qatar.

Abstract

Background:

Soy protein with isoflavones appears to have an adverse effect on thyroid function, but it is not known whether it is the protein or isoflavone component that is deleterious. The effect of isoflavone-free soy on thyroid function was determined in patients with subclinical hypothyroidism, with a secondary aim of assessing its effect on cardiovascular risk indices.

Methods:

This was a randomized, double-blind, crossover study involving 80 patients with subclinical (compensated) hypothyroidism. Patients were randomly assigned to either isolated soy (isoflavone-free) protein (SP) or casein protein (CP) supplementation for 8 weeks, washed out for 8 weeks, and then crossed over for a further 8-week period.

Results:

Thyroid function was unaffected by either a SP or CP. There were significant decreases in fasting glucose (4.7 ± 0.6 vs 5.5 ± 1.4, P < 0.01), insulin resistance (3.3 ± 3.0 vs 3.8 ± 3.4, P = 0.05), total cholesterol (4.4 ± 0.9 vs 5.3 ± 1.2, P < 0.01), triglycerides (0.9 ± 0.5 vs 1.7 ± 0.9, P < 0.1), and highly sensitive C-reactive protein (hsCRP; 0.8 ± 0.7 vs 2.6 ± 2.8, P < 0.01) in the SP group compared with the CP group. Blood pressure, low-density lipoprotein, and high-density lipoprotein remained unchanged in both groups.

Conclusion:

SP alone had no effect on thyroid function in patients with subclinical hypothyroidism and resulted in a significant reduction in fasting glucose, insulin resistance, total cholesterol, triglycerides, and hsCRP compared with CP.

KEYWORDS:

cardiovascular risk; isoflavone; soy protein; subclinical hypothyroidism

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