i suspect low stomach acid interfering with uptake. some vit C or ACV or fresh lemon. dont forget the salt but you can skip the tequila. lol
but seriously, if you have inadequate stomach acid, which naturally goes down with age, then all nutrients are hampered getting into the system. proper levels of potassium and low enough ph (high acidity) are crucial for digestion that facilitates nutrition.
heartburn is a sign of not enough acid. if the stomach does not have low enough pH then the flap doesnt close properly and what acid is in there leaks up unto the tube and burns. people take tums or something and neutralize the acid even further, relieving symptoms and guaranteeing the same problem again with progression to get worse. the cure is simple vinegar.