Just because you eat something doesn’t mean you’re getting all its nutrients. In fact, you’re never really getting it all; the process of digestion always results in some lost nutrition.
Other factors can impact absorption. Age is one.
The older a person gets, the less they get from the food they eat. Some nutrients become more difficult to absorb and metabolize, which is why supplements are often suggested for this segment of the population.
The bioavailability of a certain food can also influence what you get out of it (bioavailability is a measure of how quickly and how much nutrition/medicine is absorbed from a given source). Iron from plant foods, for example, has very low bioavailability, yet it is highly bioavailable in meats.
Combinations of food can also impact what you get out of them. Vitamin C can help with iron absorption. Coffee can inhibit it.
The same can apply to medications. If you’re taking certain medications with coffee or juice instead of water, you likely are not getting the full benefit. This can be particularly concerning if you’re taking meds for pain relief, supplements to fix deficiency, or more.
Although there are a host of health benefits associated with drinking coffee and it is recognized as safe (within limits), it likely is not the best thing to drink with medication.
Coffee can limit the effectiveness of many drugs and reduce their bioavailability, thereby limiting the benefits you may be expecting. In some cases, like Aspirin, coffee can actually boost absorption.
But the best move is to avoid taking supplements and medication with coffee. Take them with a glass of water roughly 30-60 minutes before or following a cup of coffee.