Fat-soluble vitamins and why some supplements go with food

Two families of vitamins

Vitamins are grouped by whether they are fat-soluble or water-soluble. The four fat-soluble vitamins are A, D, E and K. The body stores them in the liver, fatty tissue and muscles¹.

The nine water-soluble vitamins are vitamin C and the B vitamins. These are not stored, and leftover amounts leave the body in urine¹. Vitamin B12 is the exception: it can be stored in the liver for many years¹.

The grouping matters for absorption. In the gut, vitamins A and K are packed into micelles before they are absorbed^{2, 3}. For vitamin K, this happens through the action of bile and pancreatic enzymes³.

What a meal with fat changes

Fat in a meal sets off several digestive steps. The pancreas releases fat-digesting enzymes and the gallbladder releases bile. Bile salts emulsify fats and activate pancreatic lipase, an enzyme that breaks fats down⁴.

Vitamin E depends on this process. The US Office of Dietary Supplements (ODS) states that the digestive tract requires fat to absorb vitamin E⁵.

Vitamin D is less dependent on fat. ODS notes that fat in the gut enhances vitamin D absorption, though some is absorbed even without dietary fat⁶.

A 2015 study tested this directly⁷. Fifty healthy older adults took a single 50,000 IU dose of vitamin D3 with breakfast. One group ate a fat-free meal. Two groups ate meals with 30% of calories from fat, each with a different mix of fats. Peak blood vitamin D3 was 32% higher with the fat-containing meals than with the fat-free meal⁷. The mix of fats made no significant difference⁷.

The study measured blood vitamin D3 in the hours after one dose. It did not look at long-term vitamin D levels.

The link also shows up with medicines. ODS notes that orlistat, a weight-loss medicine, can reduce absorption of vitamin A and other fat-soluble vitamins².

Omega-3 fats and meals

Omega-3s are a family of fats. The main ones are ALA, EPA and DHA⁸. The body can convert some ALA into EPA and DHA, but reported conversion rates are below 15%⁸. EPA and DHA are found in fatty fish such as salmon, mackerel, tuna, herring and sardines⁸.

A 2018 review in the journal Nutrients notes that many researchers stress the role of fat in food for omega-3 absorption. Some suggest taking omega-3 products with high-fat food⁴.

A 2017 trial gives one example⁹. Forty-two healthy Japanese men took a prescription omega-3 preparation on three occasions: while fasting, before a meal and after a meal. Blood levels of EPA and DHA were lower in the fasting and before-meal conditions than after a meal⁹. Diarrhoea was reported by five men when fasting and six before a meal, but by none after a meal⁹.

These results come from specific preparations and groups of people. They point to a pattern rather than a rule for every product.

Water-soluble vitamins work differently

For water-soluble vitamins, a larger dose is not always absorbed in proportion.

• Vitamin C: about 70 to 90% is absorbed at intakes of 30 to 180 mg a day. Above 1 g a day, absorption falls below 50%, and the excess is excreted in urine¹⁰.
• Vitamin B12: stomach acid frees it from food. It then binds intrinsic factor, a protein made in the stomach, before it is absorbed¹¹. About 50% is absorbed from doses below 1 to 2 µg, but only about 2% of a 500 µg dose¹¹.

Food can matter for some minerals too. ODS notes that calcium supplements are generally absorbed better when taken with food. Calcium citrate depends less on stomach acid, so it can be taken without food¹².

Timing basics

• Read the label first. If it says to take a product with food, follow that. Given the studies above, a regular meal that contains some fat suits fat-soluble vitamins and omega-3.
• Make it routine. Taking a supplement with the same meal each day can make it easier to remember.
• Check your medicines. Vitamin K can interact seriously with blood thinners such as warfarin. Sudden changes in vitamin K intake can change their effect³. High-dose vitamin E can raise bleeding risk with these medicines⁵.
• Know that limits exist. ODS lists daily upper limits for adults. They are 3,000 µg for preformed vitamin A², 100 µg (4,000 IU) for vitamin D⁶ and 1,000 mg for vitamin E⁵. No upper limit was set for vitamin K, because of its low potential for toxicity³.

Your doctor or pharmacist can check what fits with your diet, your medicines and any blood tests you have had.

This article is general information, not medical advice. Talk to your doctor or pharmacist before starting a supplement, especially if you are pregnant, breastfeeding, taking medicines or managing a health condition.