Various sources say that soaking alone is not enough to reduce phytates in oatmeal. The Nourishing Home recommends adding some other grains with higher phytase content.
The one exception to the above soaking rule is oats. Oats contain a large amount of hard-to-digest phytates and other anti-nutrients. Unfortunately oats are so low in phytase (the enzyme that helps to break down phytates), that soaking them in warm water mixed with an acid medium is not enough to adequately break down the large amount of anti-nutrients that naturally occur.
However, with the help of some additional phytase added to the soak (in the form of rolled rye flakes, or if you’re GF use ground buckwheat groats – both are high in phytase) – along with a full 24-hour soak time – a fairly decent amount of the anti-nutrients can be removed, making the oats more digestible and nutritionally sound.
Here is her recipe for oatmeal: