Although soy has nutritional benefits, excessive intake should be avoided, as we shall see below.
Soy phyto-oestrogens
The general population recommendation not to exceed 1 mg/kg/day of phyto-oestrogens per day should be particularly observed in pregnant women (and during breastfeeding) [1].
Phyto-oestrogens (particularly isoflavones) are substances naturally present in certain plants, particularly soy. They are considered endocrine disruptors. Indeed, they share a structural similarity with oestradiol (an oestrogen) and are capable of binding to oestrogen receptors.
Studies increasingly show that soy isoflavones not only affect fertility but also impact the reproductive organs of the foetus and pregnancy outcomes (including miscarriage) [2].
According to animal studies, this could also have an impact on breast or testicular cancer in children later in life [3]. However, this has not been proven in humans.
Soy contains anti-nutrients
Anti-nutrients are compounds that interfere with the absorption of nutrients.
Soy is particularly rich in phytic acid. This is a substance that binds to certain minerals and thereby prevents their intestinal absorption. This applies to calcium, magnesium, copper, iron and zinc, for example [4].
However, some soy-based foods may contain less phytic acid. This is the case for fermented foods. Indeed, phytic acid decreases by 30.7% during fermentation [5]. Soy sauce, tempeh, miso and natto are fermented products and therefore contain lower levels of phytic acid.
Soy also contains an inhibitor of an enzyme that enables protein absorption [6].
Soy can affect the thyroid
Maternal and foetal thyroid hormones regulate key processes in the development of the foetus's brain and nervous system, including nerve cell growth and neuron formation [7].
Iodine deficiency significantly increases the anti-thyroid effects of soy, whereas iodine supplementation is protective. Therefore, its effects on the thyroid will partly depend on iodine status [8].
Soy contains pesticides
Even in the case of non-genetically modified soy, the majority of it is grown on large farms that use glyphosate. This is a pesticide used to control weeds. It is also used to dry the plant before it does so naturally, in order to achieve a faster harvest. Consuming soy containing pesticides is particularly harmful during pregnancy. Indeed, exposure to glyphosate during the first trimester has been linked to a higher risk of gestational diabetes [9].
Furthermore, glyphosate has been linked to congenital abnormalities and other reproductive problems, including hormonal and placental abnormalities [10]. It also impacts embryonic development [11].
Of course, if you consume organic soy, the risks are much lower. First, it will not be genetically modified. Secondly, its cultivation does not involve the use of pesticides. There may be residues that could result from contamination by neighbouring non-organic crops, but the levels will be considerably lower.
Soy contains aluminium
To manufacture soy protein isolate — the high-protein derivative of soy used in snacks, infant formula, protein bars, breakfast cereals, baked goods, ice cream and yoghurt — soy beans undergo washing in aluminium tanks, among other processes. This results in high levels of aluminium in the final product. Consequently, soy-based formula may contain around 1,000% more aluminium than standard milk-based formula [12].
A major example of this contamination issue is soy-based infant formula. It is notably considered to contain too much aluminium [13].
Aluminium preferentially accumulates in the brain and has been linked to neurological problems [14]. Aluminium also crosses the placenta with ease. In mouse studies, it has shown toxicity to placental and uterine cells [15].
Based on an average intake of two servings of soy drink per day (250 ml per serving), this results in a significant daily aluminium intake, though one that remains below the recommended maximum amount. However, higher consumption could exceed the recommended maximum value for aluminium [16].
