Those chocolate pots are essentially made from soya milk. Soya milk is basically ground up soy beans in water (that is what rice and oat milk is as well really).
So giving those is giving soya milk - so a full soy challenge
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Hidden soy is things like soy flour in a loaf of white bread, or a muffin, or textured vegetable protein in processed goods. Or a small amount of soy sauce in a recipe.
To do a challenge you really do need to do the 'real thing'. If you go for a teeny hidden thing it can take up to 2 weeks for the tolerance level to be reached, and then you end up never being sure whether they reacted to it or not (as usually by then something else will have happened as well, like a new tooth coming, another new food, a cold etc). So you are best to give a small amount of the genuine article and see. If you were retrying something they were sensitive to at a younger age you would do it differently - this is to try something that may or may not be an issue.
If you give a normal portion of a new food (a baby portion not an adults) then the reaction shoukd show within 4-5 days.
But on a one off it is SO hard to say, and you really do want to be sure with this. So I would heal his bum and get his gut back to normal, then retry, but half a pot is too much. They are for adults, not babies. I would just give 1 tsp and see, if nothing give 2 the next day, and then if nothing 3 the next. That would mean you got to a normal child portion within 5 days and would be sure of whether your LO is reacting or not. You could use a soy yoghurt, or even soy milk. Whatever really.
To my knowledge they will either tolerate soy or they won't. There a degrees of dairy tolerability due to the proteins being denatured by the cooking or fermentation process (that is why yoghurts and cheese are often tolerated first). But these soy products are not manufactured in the same was as soy milk does not behave in the same way as cows milk.
Does that help ladies
. You even had me on then alpro website to check my soy knowledge
. DD was intolerant via BM, but the last time I tested myself was months ago. I am thinking of doing a formal test soon as well, but I think she is a bit young yet.