While many of us can benefit from supplementing with vitamins, more isn’t always better—because not all vitamins are created equal!
You see, there are two types of vitamins out there—‘fat-soluble’ vitamins and ‘water-soluble’ vitamins—and our bodies process them differently. While we’re able to handle higher doses of water-soluble vitamins, taking too much of fat-soluble vitamins can actually backfire on our health.
Here’s what you need to know about vitamin dosage, and how to supplement safely.
The Basics
First things first: ‘Water-soluble’ describes a substance that dissolves in water, while ‘fat-soluble’ describes a substance that dissolves in fat.
The average cell in our body contains a membrane that’s made of fats and an interior full of water, and these different ‘fatty’ and ‘watery’ structures use and store different vitamins, explains dietitian and researcher Suzanne Dixon, M.P.H., M.S., R.D.N. Our ‘fatty’ cell membranes depend on fat-soluble vitamins to function, while the metabolic reactions that occur inside our cells (like turning food into energy) rely on water-soluble vitamins.
Fat-Soluble Vitamins
The fat-soluble vitamins include vitamin A, vitamin D, vitamin E, and vitamin K. What’s unique about these vitamins: They can be stored in our liver and fat cells for long periods of time, until our body needs to use them. “Say your intake of fat-soluble vitamins goes up and down over a period of days, weeks, or even months,” explains Dixon. “Your body will store excess when your intake is high and draw from your stores when your intake is low.”
Cool, right? There’s a catch, though: Over time, too much buildup of fat-soluble vitamins in our body can potentially cause health issues, so it’s important we pay special attention to their RDAs (recommended dietary values) when supplementing. (The RDA is the average level of daily intake the government believes meets the needs of most healthy people.)
Vitamin A, which is critical for immune function, vision, reproduction, cellular communication, and cell growth, comes in two forms: ‘preformed’ vitamin A, which we get from animal foods like organ meat, and beta-carotene, which we get from plant foods and convert into vitamin A. The RDA for preformed vitamins is 900 micrograms a day for men and 700 micrograms a day for women. According to Dixon, any intake above 3,000 micrograms per day, though, is considered dangerous. (This only applies to preformed vitamin A, though excess beta carotene can turn your skin slightly orange.)
“Taking excess vitamin A has been linked with serious health problems, including osteoporosis, liver damage and jaundice, hair loss, blurry vision, bone pain, poor appetite, dizziness and confusion, nausea and vomiting, sensitivity to sunlight, dry and peeling skin, mouth sores, and infections,” warns Dixon, who advises only those under the care of a doctor for a deficiency take more than the RDA of preformed vitamin A. Excess A can also cause birth defects, so this warning is especially important for pregnant women. It also explains why medications used for skin conditions that are vitamin A derivatives, such as retinoids, are so carefully prescribed.
On the other end of the spectrum is vitamin D, which plays important roles in calcium absorption and bone health, cell growth, immune function, neuromuscular activity, and inflammation regulation. Unlike vitamin A, vitamin D can be tolerated at doses higher than the RDA for prolonged periods of time. The RDA for vitamin D 600 IU per day for adults, but the ‘tolerable upper intake level’ or ‘UL’ (the highest level of long-term daily intake likely not to pose any risk for the general population) is more than six times that number: 4,000 IU per day.
Since vitamin D deficiency, which is linked to rickets in children and osteoporosis (a condition marked by weak, brittle bones) in adults, is pretty common, some people may need to supplement with much more than the RDA. In fact, doctors often prescribe as much as 10,000 IU a day for deficient patients, says Dixon, who recommends having your levels tested at your annual physical.
Last but not least of the fat-soluble vitamins are vitamin E and vitamin K. Vitamin E is an antioxidant that protects cells from free radical damage, supports immune function, enhances cell signaling, and moreintegral to blood clotting, bone metabolism, and regulating blood calcium levels. Neither E or K runs the risk of toxicity like vitamin A, and neither is considered a nutrient of concern for common deficiency like vitamin D.
Water-Soluble Vitamins
Unlike fat-soluble vitamins, water-soluble vitamins—vitamin C and the B vitamins—are not stored in our body for a long time. When water-soluble vitamins enter our body, they are utilized quickly or excreted. “We generally have a limited capacity to store water-soluble vitamins, so when we take extra, we just excrete the excess in our urine,” Dixon explains. That’s why you’ll notice your urine changes colors when you take high-dose B vitamin or multi; that neon hue is just the excess riboflavin (B2) leaving your system.
Water-soluble vitamins’ rapid exit from our body significantly decreases their risk of toxicity, explains Dixon. But that doesn’t mean you should go willy-nilly 24/7: “Any vitamin, if taken in very high amounts—like many thousands of times the RDA—can cause health problems,” she says. The threshold for water-soluble vitamins is just generally higher than that for fat-soluble vitamins.
Taking a little bit of extra vitamin C (an antioxidant that helps us form and maintain tissues like bones, blood vessels, and skin) is okay, and is even recommended for populations like smokers, whose bodies use the vitamin more rapidly to combat the oxidative stress caused by smoking, says Dixon. Just keep in mind that the RDA for vitamin C is 90 milligrams a day for men and 75 milligrams a day for women, and smokers are advised to take just an extra 35 milligrams, a very small amount of ‘extra.’
Taking even higher doses of C seems to have short-term benefits, too: For example, while taking extra vitamin C doesn’t seem to ward off getting a cold, taking a few hundred milligrams can boost your immune system, which is helpful during cold season, says Dixon.
Related: Get Your B Vitamins Straight: A Guide To What’s What
The rest of the water-soluble vitamins out there are the B vitamins, which play key roles in our body’s conversion of food into energy. The Bs include thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), vitamin B6 (pyridoxine), folate (vitamin B9), vitamin B12, biotin (vitamin B7) and pantothenic acid (vitamin B5). While the B’s are generally tolerated in doses higher than their varying RDAs, there is one to be wary of: vitamin B6. Research suggests that supplementing with more than 100 milligrams of B6 (its RDA is 1.3 milligrams) for a year or longer can lead to nerve damage, painful and unsightly skin patches, extreme sensitivity to sunlight, nausea, and heartburn.
Keep your vitamin dosages straight with this infographic: