I've heard that soaking dried beans for 24 hours reduces the phytate level, allowing for greater access to nutrients. Is this true?

Answer:

Beans and other legumes contain "antinutrient" compounds, such as phytate, that can limit the body's absorption of nutrients from these foods, such as iron, zinc, and calcium. Soaking can help reduce the amount of phytates, but research suggests that 24 hours is too long. (Be aware that simply cooking beans, without soaking them first, will not decrease phytate levels).

A review of 33 studies that tested the effect of soaking beans or chickpeas found the greatest and most consistent decreases in phytate levels occurred when the beans or chickpeas were soaked for 12 hours in distilled water at room temperature, but studies that used tap water showed similar results. This reduced phytate levels by up to 66% (Haileslassie, Int J Food Sci Technol 2016). Although some websites claim that adding acid (such as citric acid) to the soaking water further decreases phytate levels, studies conducted this way have not shown additional benefit. 

Although soaking may also slightly decrease nutrients in the legumes, most studies maintain that soaking beans and chickpeas for a moderate amount of time, such as 12 hours, increases their overall nutritional value. Soaking legumes for longer may result in a greater loss of nutrients. [Note: An added benefit of soaking is that it can also reduce the complex sugars in beans which can cause gas or bloating (USDA; Messina, Am J Clin Nutr 2014)].

After soaking, it is important that you drain the water and use new water to cook the beans, as this has been shown to further reduce antinutrient levels in the beans (Fernandes, Int J Food Sci Technol 2010).

Interestingly, one study showed canned beans had lower phytate levels than dried unsoaked beans, indicating that the canning process (which may include soaking, or blanching or pressure cooking at high heat for a short period of time, depending on which process is used) is also effective in reducing antinutrients. Phytate levels in canned beans were reduced by 92% in black-eyed beans, 70% in red kidney beans, 68% in mung beans, and 75% in pink beans (Tabekhia, J. Food Sci. 1980).

A note on grains: Soaking rice and other grains can also reduce their phytate levels, but it reduces nutrient levels so much that it is not worth soaking (Lestienne Food Chem 2005; LWT Food Sci Tech 2013).

Although soaking grains and legumes to reduce phytates may help to improve mineral absorption, it's also worth noting that phytic acid is sold as a supplement (as inositol hexaphosphate or IP6), based on preliminary evidence of anti-cancer and other beneficial effects. However, these benefits have yet to be established in clinical trials.

Be aware that many plant-based meat alternative products also contain a high amount of phytate, which may reduce the absorption of non-heme iron, zinc and calcium found in these products.