Whether multivitamins and other dietary supplements are necessary for the general population is a source of debate. Supplements remain recommended for certain populations with specific conditions - such as pregnant women who should take folic acid to reduce the risk of neural tube defects, or children in developing countries whose diets do not provide enough vitamin A and iron. But recent studies have found there is insufficient evidence to recommend multivitamin supplements to the average healthy American, and that in fact, taking too much of certain vitamins can cause harm.

These studies seem to have little effect on the global supplement industry, which is worth an estimated $128 billion, according to 2017 data from the Nutrition Business Journal, or on the American public. Fifty-two percent of respondents to the 2011-2012 U.S. National Health and Nutrition Examination Survey reported using dietary supplements - unchanged from the 1999-2000 survey.

As a registered dietitian, I believe a nutritious diet is the best way to achieve a healthy foundation. Supplements (as the name suggests) can be used as a complement to help a person with certain deficiencies meet their nutrient needs. If you're taking a supplement because of such a deficiency, you should try to take it in a way that could promote optimal absorption. Supplement timing can seem complicated, so let's simplify when to take some of the most common dietary supplements and why.

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When to take supplements

There is debate about whether taking your vitamins in the morning or at night is best. The theory goes that because you're getting nutrients throughout the day from food, having your nutrition supplements at night helps your body get some nutrition as you sleep.

But Jeffrey Blumberg, a professor of nutrition science and policy at Tufts University in Boston, says, "Digestion slows down during sleep, so taking your nutrient supplement late at night would not be associated with an efficient absorption."

Neil Levin, a clinical nutritionist at NOW Foods, agrees that morning is best for multivitamins and any B vitamins. "Multivitamins tend to do best when taken earlier in the day, as the B vitamins in them might stimulate metabolism and brain function too much for a relaxing evening or before bed," Levin says.

Although morning is probably ideal, the best time of day is the time you'll remember. Put the supplement bottles on your kitchen counter next to your coffee maker, so they jog your memory when you reach for your morning cup. Or keep them in your lunch bag or briefcase so you'll remember them.

With food or without?

Most supplements should be taken with food to reduce the chances they'll upset your stomach and to stimulate digestion and improve absorption. For a select few, it really doesn't matter if you take them on an empty stomach. So which ones should you pay attention to?

Iron, magnesium and fish oil supplements are the most common culprits for digestive upset when taken on an empty stomach, so take extra care to have these with a meal or snack.

Fat-soluble vitamins A, D, E and K are better absorbed when you have them with a meal or snack that contains at least a teaspoon of fat, about 5 grams. The same goes for your multivitamin, which contains these vitamins. For example, if you're taking your multivitamin with your breakfast, make sure you're having some almond butter with your oatmeal or avocado with your eggs and toast.

For probiotics, preliminary research suggests taking them with a meal or 30 minutes before a meal could be better than taking them after eating.

Hydration is also important, Blumberg says. "Fluid intake is especially important for the disintegration of the supplement tablet or capsule and for dissolution of water-soluble nutrients such as vitamin C and B vitamins," he says. So be sure to wash down all supplements with a tall glass of water.

The main exception to the "take with food" rule for dietary supplements is with certain types of minerals. Only chelated mineral supplements can be taken without food, Levin says. Chelation occurs when a mineral has been bound to an acid, so it doesn't rely on your stomach acid to break it down. Calcium citrate and magnesium glycinate are the main examples. (If this level of detail is overwhelming, take your supplements with food to cover your bases.)

Better together

Some nutrient dynamic duos include vitamin D to boost calcium absorption and vitamin C to boost iron absorption. That's why taking in these nutrients simultaneously via supplements or boosting with food sources is ideal. A classic example is having your iron supplements with a glass of orange juice to get the absorption-boosting effects of the vitamin C.

Better apart

Calcium can affect your body's absorption of iron, zinc and magnesium. I recommend taking any calcium supplements at a different meal than any iron supplements or your multivitamin. Also, your body absorbs calcium more effectively when you take 600 milligrams or less at a time. If you're taking more than that per day, you'll want to split up the dosage into morning and evening doses.

Fiber is another nutrient you'll want to take apart from other supplements and medications because it interferes with absorption. I recommend doing so before bed if you aren't taking anything else at that time.

Here's a sample schedule for optimal absorption of the supplements named.

With breakfast

Multivitamin or prenatal multivitamin/folic acid

B vitamins

Omega-3s

Probiotics

With lunch

Calcium

Vitamin D

With dinner

Iron

Vitamin C

Before bed

Fiber supplement (with a large glass of water)

If it isn't practical for you to remember to take supplements at lunch or other points during the day, don't worry. Have your multivitamin and any fat-soluble vitamins (A, D, E and K) with food that contains some fat, keep your calcium and iron separate, and you'll be fine. You'll be even better off if you focus on eating nutritious whole foods, because science suggests that this, rather than supplements, is the optimal way to get your nutrients.Morning or night? With food or without? Answers to your questions about taking supplements