Soy Myths: Is Tofu Actually Bad for You?

The root of almost every soy fear is a single word: phytoestrogens. Soy contains compounds called isoflavones, which are classed as phytoestrogens because they can weakly interact with the body's oestrogen receptors. The word sounds alarming, and it launched a thousand warnings that eating soy floods you with a female hormone.

But isoflavones are not human oestrogen. They are plant compounds with a very different, much weaker action, and they favour a specific type of receptor that does not behave like the hormone people picture. The scary headlines came largely from studies that fed rodents massive doses of isolated isoflavones, animals that process these compounds differently from humans, at intakes far beyond anything you would get from tofu. Extrapolating that to your dinner plate is where the science broke down.

This is the most repeated claim, and the human evidence against it is about as clear as nutrition science gets. A meta-analysis that pooled 38 clinical studies found that neither soy foods nor soy isoflavones changed men's total testosterone, free testosterone, or oestrogen levels. The intakes covered ranged well above typical consumption, and still nothing moved.

The myth is kept alive by a couple of case reports of individuals eating truly extreme amounts of soy, far beyond normal, whose hormones shifted and then returned to normal when they stopped. Anecdotes at the fringe are not the same as an effect in ordinary eaters. For anyone eating tofu, tempeh, edamame or soy milk as part of a normal diet, there is no measurable hit to testosterone.

Because isoflavones can touch oestrogen receptors, and because some breast cancers are driven by oestrogen, the theory that soy might fuel breast cancer sounds plausible. In humans it has not held up, and the evidence actually points the other way.

Reviews of the clinical and population research find that soy intake does not increase breast cancer risk. More striking, a meta-analysis of women already diagnosed with breast cancer found that higher soy intake after diagnosis was associated with a reduction in recurrence and in mortality. The likely reason is that in human breast tissue, isoflavones appear to act more like a mild block than an accelerant on the oestrogen signal, though the mechanism is still being worked out.

The thyroid worry has a kernel of truth wrapped in overstatement. A review of 14 clinical trials concluded that in people with a healthy thyroid and adequate iodine, soy foods do not disrupt thyroid function. For the general population, this is simply not a concern at normal intakes.

There are two real, narrow caveats. First, soy may matter more for people who are iodine-deficient, which is uncommon in places where salt is iodised. Second, and more practically, soy can reduce the absorption of the thyroid medication levothyroxine if taken at the same time. That is a timing issue, not a reason to avoid soy.

• If your thyroid is healthy, eat soy freely. The evidence shows no meaningful effect on thyroid function.
• If you take levothyroxine, separate it from soy by a few hours, the same advice that applies to coffee, calcium and iron.
• If you have a diagnosed thyroid condition, raise your soy intake with your doctor rather than guessing.

Lost in the fear is how good a food soy is. It is one of the very few plant proteins that is complete, meaning it contains all the essential amino acids in useful amounts, so it stands on its own without needing to be combined with other foods. That makes it the standout choice for anyone building muscle on plants.

Beyond protein, reviews link soy foods to a lower risk of heart disease, helped by replacing saturated-fat-heavy foods and by soy's own effect on cholesterol. It is cheap, it keeps well, and it takes on whatever flavour you cook it with. Judged as a food rather than as a rumour, soy is one of the strongest additions you can make to a diet, plant-based or not.