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.