Fat-Soluble Vitamins A and D

The Science of Supplementation: Fat-Soluble Vitamins, Part 1

July 16, 2018

Previously in this article series, I have discussed digestion before and after WLS, and the nine water-soluble vitamins. Now, I will introduce you to the four fat-soluble vitamins: vitamin A, D, E, and K. In this article, I'll explain the Fat-Soluble vitamins A and D.

Fat-soluble vitamins are like oil and therefore do not dissolve in water like the water-soluble vitamins. They require small fat globules in order to be absorbed via the small intestines and then into the blood to be used within the body. Because of this, they are much better absorbed when consumed with sources of fat.

Fat-Soluble Vitamins A and D

These vitamins can also be stored and recycled in our body when not in use, which makes toxicity a concern if we supplement too much. The primary storage locations are the liver and fat tissues. Because our body is able to store fat-soluble vitamins, they are not as readily excreted as water-soluble vitamins, as we need to be aware of the possibility of toxicity if we take too much. In this article, I will discuss vitamins A and D.

Unless otherwise specified, it is important to take fat-soluble supplements with a small amount of fat to help aid in absorption.

Some supplements, such as most vitamin D supplements, are sold as oil capsules so that you don’t need to worry about eating food when you take the supplement as there is a little bit of fat in the capsule itself. For biliopancreatic diversion with duodenal switch (BPD/DS) patients, with the increased risk of fat malabsorption, they are often recommended to take dry forms of fat-soluble vitamins. However, these should still be taken with food as there is no oil in the dry forms.

Vitamin A

Vitamin A is made up of a group of compounds, including retinol, retinal, retinoic acid and retinyl esters. All forms of vitamin A must be converted into the active form of retinal or retinoic acid.

Vitamin A is essential for your vision. It helps maintain the function of cells in your eyes, and it also helps form tear fluid. Vitamin A helps support your immune system. It also helps with growth in children, and is important for reproductive health, including fertility and fetal development.

Retinol is the most common form found in foods. Preformed vitamin A is only found in animal sources of food, and can be found in fatty foods such as fish liver oil, beef liver, butter, milk and egg yolks.

However, our body can make vitamin A out specific carotenoid antioxidants found in fruits and vegetables. One such micronutrient is beta-carotene, which is found in carrots and other leafy green vegetables such as spinach and kale. These nutrients that can be converted to vitamin A are often referred to as provitamin A. Of note, beta-carotene is also water-soluble, which is why we can eat carrots on their own and still absorb beta-carotene and convert it to vitamin A. Other sources of vitamin A include sweet potatoes, cantaloupe, broccoli, squash, apricots, and tomatoes.

Vitamin A deficiency is rare in developed countries. A strict vegan diet may put you at risk of vitamin A deficiency, as only animal products contain pre-formed vitamin A. Prolonged diarrhea can also lead to vitamin A deficiency.

While plenty of fruits and vegetables contain provitamin A, it needs to be converted to the active form, and this conversion process may not be efficient, depending on your genetics. Vitamin A deficiency can lead to dry eyes, skin problems, poor wound healing, alopecia (hair loss), an increased risk of infection, night blindness, and in severe cases, total blindness. A physical sign to look for are Bitot’s spots or white spots on the sclera (white part) of the eye. Another side effect of deficiency is a loss of taste, as vitamin A and zinc metabolism are interrelated, and zinc deficiency has a large effect on taste.

Approximately 14% of pre-op patients are deficient in vitamin A. Fat malabsorption increases your risk of vitamin A deficiency after WLS. Vitamin A deficiency is considered common (50%) after BPD/DS within a year of surgery, and can increase to up to 70% by four years post-op. It is also known to occur after RYGB and AGB, but less information is available on post-op deficiency for SG patients.

The UL has been established at 10,000IU/day. Vitamin A toxicity, also known as hypervitaminosis A, is rare but can be very serious. Symptoms of toxicity include hair loss, decreased appetite, blurred vision, headache, fatigue, stomach pain, vomiting, skin problems, and at extremely high doses, it can be fatal. Excess vitamin A can poorly affect the liver and bones. It is also important during pregnancy to avoid high amounts of vitamin A, due to the risk of birth defects. Increased intake of provitamin A (recall this is found in fruits and vegetables) does not lead to hypervitaminosis A, as our body can more readily excrete excess provitamin A.

The current RDA is 3000IU (900mcg) for men and 2333IU (700mcg) for women. Due to the different way each WLS affects absorption of fat, there are different recommendations from ASMBS for each surgery. See the table below for specific recommendations.

3000IU (men)
2333IU (women)
5000IU 5000-10,000IU 10,000IU 10,000IU

Vitamin D

Vitamin D was the fourth vitamin discovered after vitamins A, B and C. However, the B vitamins were soon discovered to be more than one molecule, but the name for vitamin D had already stuck. Since then, it has been nicknamed the sunshine vitamin.

Vitamin D works closely in tandem with phosphorus and calcium for bone health. It is vital for absorbing calcium and phosphorus from our diet, and it helps regulate levels of these minerals in the blood, as they are important minerals involved in bone growth and maintaining bone health. Vitamin D also plays a role in reducing inflammation, modulating cell growth, and supporting neuromuscular and immune function. Vitamin D and calcium are especially important in pre- and postmenopausal women.

There are many forms of vitamin D, but only two types available via our diet: ergocalciferol or vitamin D2, which is found primarily in mushrooms; and cholecalciferol or vitamin D3, which is found in egg yolks and fatty fish, and is added to fortified dairy products, such as milk, yogurt, and margarine. Vitamin D is not typically added to cheese and ice cream, but can be found in fortified orange juice and some breakfast cereals. The body also produces vitamin D3 from a form of cholesterol after sun exposure, which is part of why your body needs cholesterol (it’s not all bad!) and hence the nickname of the sunshine vitamin.

Vitamin D3 is considered more effective in the body, so it is recommended to consume sources/supplements containing D3 rather than D2.

Up to 90% of people with obesity have vitamin D deficiency, and it is an ongoing concern in today’s population. We are more at risk for deficiency as we age, as our skin loses its ability to synthesize vitamin D in response to sun exposure as well as it once could. Those with dark skin are also at increased risk of deficiency, due to increased levels of melanin in the skin decreasing the skin’s ability to synthesize vitamin D in response to sunlight. Deficiency can occur after RYGB due to fat malabsorption and bypassing the site of absorption in the small intestines, but the prevalence after WLS is unknown. Vitamin D has been shown to spontaneously increase shortly after surgery, believed to be caused by the rapid weight loss immediately after surgery, where the stores of vitamin D in adipose (fat) tissue are released to circulate in the blood.

Physical symptoms of deficiency can include tingling or cramping, as well as muscle weakness and bone pain, fatigue, and hair loss. Vitamin D deficiency presents as rickets in children, or osteomalacia in adults, which is characterized by misshapen or brittle bones. Low vitamin D can lead to hypocalcemia (low calcium levels), which can lead to tetany. Low levels of vitamin D have also been linked to increased risk of heart attacks as well as death from cancer.

The recommended daily allowance (RDA) of 600IU is not sufficient to prevent deficiency in most adults despite being sufficient to prevent rickets in children, so 800IU is typically recommended for adults. A vitamin D level, specifically D,25(OH) level, between 20-50ng/mL is considered sufficient for most healthy adults, but for post-op WLS patients, it is recommended to maintain a vitamin D level greater than 30ng/mL. To achieve this, a minimum of 3,000IU is recommended, but some people may require more to maintain higher levels of vitamin D.

RDA All WLS Types
600IU ≥3000IU

Because of its close working relationship with calcium, vitamin D toxicity can be mediated in the body by changing calcium metabolism, which can be extremely dangerous as hypercalcemia (elevated calcium levels) it can cause heart and kidney damage. Symptoms of toxicity include weight loss from nausea or lack of appetite, headaches, and fatigue. Up to 10,000IU has been shown to be tolerable by most patients in research, rather than the established UL of 4,000IU, but it is important not to exceed 10,000IU daily unless instructed by your doctor.

Fat Soluble Vitamins Summary

To recap, vitamin D is important for bone health, while vitamin A plays a role in the health of your eyes and skin. Fat soluble vitamins do not dissolve in water and are best absorbed when consumed with fat. Because our body can store and recycle these fat-soluble vitamins, we are not at as high of a risk of deficiency for vitamins A, E, and K as we are at risk for some of the water-soluble vitamins. However, vitamin D remains a nutrient of concern both before and after WLS. And because our bodies can store these nutrients, we should be aware of the symptoms of toxicity.

These are just two of the fat-soluble vitamins. Please stay tuned for my next article, which will be looking at the other fat-soluble vitamins, vitamins E and K!


  • UL: tolerable upper intake level
  • DRI: dietary reference intake
  • AGB: adjustable gastric band
  • SG: sleeve gastrectomy
  • RYGB: Roux-en-Y gastric bypass
  • BPD/DS: biliopancreatic diversion with duodenal switch



Fat-Soluble Vitamins A and D



Bec McDorman, MS, RDN discovered her passion for health and wellness after undergoing Roux-en-Y Gastric Bypass surgery in 2010 to lose more than 100lbs. Bec has received her masters from Cal Poly Pomona and completed her dietetic internship at Johns Hopkins Bayview Medical Center. She has reached her goal of being a registered dietitian so she can help pre- and post-op bariatric patients with their journey.

Read more articles by Bec!