What metrics can scientist get from the human body to judge what has been recently eaten

So, AI can do a lot of things. But it needs input data. So I was thinking, do we currently have any technology that could generate metrics that an AI might be able to train on such that it could estimate calories gotten from a meal. Not what was in the food, but what the body actually absorbed.

Obviously it could be used to make a killer diet tracking app. Cause tracking what you eat is the worst part.

And collecting the metric doesn't have to be "practical" today. This is just more of a thought experiment. So if currently it would require multiple blood tests per day or something, that would still be interesting to me.