If you don't get out in the sun enough then you might want to supplement the Vitamin D that you skin makes with some Vitamin D3 capsules.
If you can't seem to eat enough leafy green vegetables then you might go for some supplemental Vitamin K.
If you buy supermarket produce that is likely stored in warehouses for a period of time then you may wish to get some Vitamin C (as vitamin C is used up by the fruit or vegetable over storage time).
And if you cook most of your fruits and vegetables you may want more Vitamin C (as Vit. C in food is degraded by the heat of cooking).
If the soil your vegetables are grown in isn't enriched with a full range of 72 trace minerals then you probably need trace & major mineral... as a dietary supplement.
If you eat mostly cooked food you may need supplemental digestive enzymes to help digest your protein, fats and carbohydrates. The older we get, the greater our need for these enzymes, and other digestive aids, as well.
Missing enzymes can lead to another problem, ... the inability to transform molecules such as glucosamine into the form our bodies can actually use. When this loss of enzymes occurs we may experience symptoms of "aging", such as painful joints, slow wound healing, and abnormal cell growth.
the general rule seems to be, "if you can't get enough of a good thing, then supplement it!" This sounds simple and it actually can work as long as you stick to whole food supplements. For instance, supplementing with stabilized rice bran makes sense because the body can digest and absorb nutrients from rice bran. If you would take supplements of the same exact nutrients that are found in rice bran, but of a synthetic or denatured variety, you probably would have poor absorption of them. The body recognizes whole foods and readily accepts this form of nutrient.
VitaminMonkey thinks, "Supplements are a good thing!"