A food with no weight
A capsule or a tablet doesn't have a weight you'd ever measure, so it doesn't need one. Create it once as a custom food: name the serving after the dose — capsule, tablet, softgel, gummy, scoop, drop, or packet — and leave its weight empty. The form confirms it with No weight: logged per capsule, not in grams, and from then on you log it by the unit: "2 capsules", never grams.
Calories can be zero, and vitamin D can be entered in IU, exactly as the bottle prints it.
From a label or a photo
Scanning a nutrition label also reads Supplement Facts: vitamin D printed in IU is converted to micrograms for you.
With photo logging, water, black coffee, zero-calorie sodas, and capsules are recognized as close to zero calories, without taking the calories of a similar food from the catalog.
Days with only a supplement
A day where you logged under 200 kcal — a coffee and a vitamin, say — still counts as a day you logged, but it doesn't enter your averages, your expenditure, or your check-in: a single capsule shouldn't read as a day you barely ate.
From the next day, the journal marks it Not in your averages with a Review action, where you can mark it as a fast or enter the day's total if you tracked it somewhere else. See Fasting Days, Incomplete Days, and Manual Totals.
Taking them every day
If you take the same supplements daily, a routine logs them in one tap on the days you choose.