Introduction
Adiponectin is the most abundant circulating adipokine secreted from adipose tissue in the blood. The fundamental role of adiponectin in fatty acid oxidation and glucose homeostasis, suppressing fat accumulation and promoting antioxidation reported in previous studies1-3. Lower adiponectin level reported in type 2 diabetes, metabolic syndrome, and cardiovascular disease (CVD)4; therfore in these patients lower adiponectin level has the etiological importance of forming insulin resistance5. Higher adiponectin concentration associated with lower risk of CVDs6. Leptin, other well-known adipokine also reported to be involved in glucose uptake and glycolysis exactly effect on glycolytic enzymes; it‘s role in production of inflammatory cytokines also has been reported7,8. Leptin mostly known as a hormone which regulate food intake and energy expenditure 9.
Soy is a valuable plant protein source; its use is common in several parts of the world particularly southeastern parts of asia10. Soy and soy products because of its isoflavones known as one of the rich sources of micronutrients, phytochemicals and powerful antioxidants. Biologically active heterocyclic phenols of soy after ingestion could act sevral function in body 11. Genistein and daidzein, the main isoflavones of soy implicated in preventing metabolic diseases such as cancer, osteoporosis, infertility, diabetes and CVDs 12. These isoflavomnes are potent anti-inflammatory agents; Equol, the metabolite of diadzein following function of gut microbiome, reported to possess antioxidant effects and also it has a tendency for binding to estrogen receptors 13. Apart from different varient of soy and their possible different effect, some polymorphisms alter the individual’s response to isoflavones, worsening inflammatory status and causing mutations, but in some polymorphisms, soy intake decreases tumor growth by reducing inflammation 14.
However, some experimental studies have reported beneficial anti-inflammatory effects of soya protein or isoflavone, shown by increasing adiponectin15-17. Previous studies reported inconsistent results regarding effect of soy and soy products on adiponectin and leptin level; several studies showed no statistically significant change in the adiponectin18-26. Of interest, Llanos et al. 27reported soy protein supplement (40 g) resulted in significant adiponectin reduction. Furthermore, there was a trend toward decreased circulating adiponectin with the soy supplement compared to placebo in Maskarinec et al. study20, consistent with the report by Llanos et al.27. Most studies did not report any change in leptin as well; except but in two studies,26,27intervention of soy protein supplement resulted in lower leptin reduction compared with control group. Due to lack of comprehensive systematic review and meta-analysis on one hand, and inconsistent results on the other hand, we aimed to systematically review the effect of soy and soy products on circulating adiponectin and leptin. Findings of present study enlighten our knowledge in this context for recommendation.