When it comes to combining vitamins, it pays to know which may block each other from doing their good work and which may amplify their effects. 

Fat-soluble vitamins A, D, E, and K:

• Absorption is improved if you take fat-soluble vitamins with a meal that contains healthy fats. One study found that taking vitamin D with your largest meal of the day increases your blood level by 50%.

• Vitamin A may be better absorbed when taken with vitamin E, but Johns Hopkins University research shows doses of vitamin E over 400 IU a day are associated with a higher risk of all causes of death — maybe because most vitamin E supplements contain just one of the eight forms of vitamin alpha-tocopherol.  Make sure you buy vitamin E containing mixed tocopherols, or at least the one most experts think is beneficial: gamma topherol. 

Water-soluble vitamins C and the Bs (B6, B12, biotin, folate, niacin, pantothenic acid, riboflavin, and thiamine):

• Take water-soluble vitamins on an empty stomach with a glass of water. 

• Excessive doses of vitamin B6 can produce peripheral neuropathy, especially if your levels of B12, B2, and B9 are low.

• Vitamin C increases absorption of iron from supplements and food.

Increase your vitamin intake with fresh fruits and veggies, oily fish, nuts, and seeds. Twice daily take half a multivitamin/multimineral that delivers the recommended levels of nutrients, not mega-doses. A balanced intake is what you're aiming for.

Get a blood test to check for vitamin deficiencies, then follow your doctor's advice.