Vitamin D is an essential nutrient that plays many important roles in the body. Unlike other vitamins, vitamin D functions as a hormone in the body and is primarily synthesized in the skin through exposure to sunlight. While sun exposure is the best natural source, there are also dietary sources that can help you meet your daily vitamin D needs.
What is Vitamin D?
Vitamin D is a fat-soluble vitamin that is found in two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the form that is synthesized in the skin from exposure to sunlight, while both D2 and D3 can be obtained from the diet.
Some of the main functions of vitamin D in the body include:
- Aiding in calcium absorption and promoting bone health
- Supporting immune system function
- Facilitating cell growth and neuromuscular function
- Reducing inflammation
Without adequate vitamin D levels, you have an increased risk of bone disorders like osteoporosis or osteomalacia. Vitamin D deficiency has also been associated with an increased susceptibility to infections, autoimmune diseases, cardiovascular disease, and some cancers.
How is Vitamin D Absorbed in the Body?
In order to be used by the body, vitamin D must first be absorbed through the digestive tract. There are a few key steps involved in the absorption process:
- Dietary vitamin D is incorporated into micelles – tiny droplets of fat and bile that allow fat-soluble vitamins like D to be absorbed.
- The micelles transport the vitamin D to the enterocytes lining the small intestine where it is taken up into the cells.
- Once inside the enterocytes, vitamin D binds to a protein called vitamin D-binding protein which facilitates its movement and metabolism.
- The vitamin D-binding protein transports vitamin D to the liver, which is the first site of hydroxylation. Vitamin D undergoes hydroxylation first in the liver and then the kidneys to form the active form of vitamin D known as calcitriol.
- Calcitriol circulates through the blood to tissues like the intestines, bones, and kidneys where it can carry out its vitamin D functions.
This complex absorption process relies on adequate fat intake, a healthy gastrointestinal tract, and proper liver and kidney function. Any issues with fat digestion or absorption, gut damage, or health conditions affecting the liver or kidneys can impair vitamin D absorption.
Factors that Impact Vitamin D Absorption
There are several key factors that can affect how efficiently your body is able to absorb and utilize vitamin D from your diet or sun exposure.
Fat intake
As a fat-soluble vitamin, vitamin D requires some dietary fat in order to be absorbed. Fat helps stimulate the production of bile and the formation of micelles that allow vitamin D to be taken up by intestinal cells. Low fat diets or health conditions that impair fat absorption like celiac disease, Crohn’s disease, cystic fibrosis, or pancreatic enzyme deficiency can all impact vitamin D absorption.
Gut health
The overall health of your gastrointestinal tract plays an important role in vitamin D absorption. Damage to the intestinal lining from conditions like celiac disease, IBS, ulcerative colitis, or infection can reduce the surface area available for absorption. Surgeries like gastric bypass that shorten the small intestine also impact nutrient absorption.
Liver and kidney function
Since vitamin D undergoes hydroxylation in both the liver and kidneys to form the active calcitriol, impaired function of these organs can affect vitamin D activation and utilization. Liver disease like cirrhosis, as well as chronic kidney disease, are both associated with vitamin D deficiency.
Magnesium status
Magnesium plays an important role in metabolizing and activating vitamin D. Low magnesium levels can impair the activity of enzymes responsible for vitamin D activation. Ensuring adequate magnesium status through diet or supplementation may help optimize vitamin D absorption.
Age
As we age, our skin becomes less efficient at synthesizing vitamin D from sun exposure. The elderly are also more likely to have reduced gastric acid secretion, gut permeability, and liver and kidney function – all factors that can impair vitamin D absorption. Older adults may need higher intakes from food or supplements to meet needs.
Obesity
Individuals who are obese tend to have lower blood levels of vitamin D. Excess body fat may sequester or trap vitamin D so that less is bioavailable and able to enter circulation. Obese individuals often require 2-3 times more vitamin D intake to achieve adequate blood levels.
Genetic factors
There are several genetic factors that can affect vitamin D absorption and activation. Polymorphisms in genes coding for proteins involved in vitamin D transport, liver hydroxylation, and vitamin D binding have all been shown to influence vitamin D status.
Top Food Sources of Vitamin D
While sun exposure is the ideal way to meet vitamin D needs, dietary sources are also important – especially during winter months or for those with limited sun exposure. Here are some of the top food sources of vitamin D:
| Food | Vitamin D (IU per serving) |
|---|---|
| Salmon, cooked (3.5 ounces) | 564 |
| Herring, cooked (3 ounces) | 384 |
| Sardines, canned (1.75 ounces) | 270 |
| Cod liver oil (1 tablespoon) | 227 |
| Tuna, canned (3 ounces) | 154 |
| Egg yolks | 41 |
| Mushrooms, raw (1 cup) | 21 |
| Fortified milk (1 cup) | 115-124 |
Fatty fish like salmon, mackerel, sardines, and tuna are among the best natural sources, along with fish liver oils. Smaller amounts can be found in egg yolks, beef liver, cheese, and mushrooms.
Many foods are also fortified with vitamin D like milk, yogurt, orange juice, cereals, and oatmeals. Check the nutrition label to see how much vitamin D has been added.
Supplements to Enhance Absorption
In some cases, vitamin D supplementation may be recommended by your healthcare provider to help optimize blood levels, especially in those with absorption issues or known deficiencies. Here are some supplements that can enhance vitamin D absorption:
Vitamin D
Vitamin D supplements are available in two main forms – vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 tends to be more effective at raising blood levels for most people. Typical doses range from 1,000 – 5,000 IU per day.
Vitamin K
Vitamin K is needed for the activation of osteocalcin, a protein involved in bone mineralization. Ensuring adequate vitamin K intake improves calcium and vitamin D utilization for bone health.
Magnesium
Magnesium supports several steps in vitamin D metabolism and activation. Supplementing with magnesium may help optimize vitamin D absorption and activity.
Calcium
Taking vitamin D with calcium supplements or calcium-rich foods can enhance absorption of both nutrients. This synergistic relationship helps promote bone health.
Fatty acids
Getting adequate healthy fats from supplements like fish oil, flaxseed oil or vitamin E can help support absorption of fat-soluble vitamin D.
Healthy Lifestyle Tips to Improve Absorption
Aside from diet and supplementation, there are several lifestyle factors that can optimize your body’s ability to absorb and utilize vitamin D from sun and food sources:
- Get moderate sun exposure for 10-30 minutes several times per week, avoiding excessive burning.
- Consume foods rich in vitamin D along with a source of fat for absorption.
- Maintain a healthy body weight – obesity is linked to lower vitamin D levels.
- Exercise regularly – physical activity may enhance vitamin D absorption.
- Manage chronic health conditions affecting liver, kidney or gut function.
- If deficient, take vitamin D supplements with your largest meal for optimal absorption.
- Check blood levels of vitamin D to determine if supplementation is needed.
Who is at Risk for Deficiency?
Certain populations are at a greater risk of vitamin D deficiency and may especially benefit from efforts to enhance vitamin D absorption from the diet or sun exposure. These risk groups include:
- Older adults – less efficient synthesis from sun, decreased absorption
- People with dark skin tones – increased melanin reduces vitamin D production
- People who are obese – vitamin D may be deposited in fat stores
- Exclusively breastfed infants – low vitamin D in breastmilk
- Those with gastrointestinal disorders – decreased absorption
- Individuals with chronic kidney or liver disease – impaired activation of vitamin D
- People with osteoporosis or other bone disorders
Ensuring adequate sun exposure, vitamin D intake from foods and supplements, and optimizing absorption factors can help reduce deficiency risk in these groups.
Signs and Symptoms of Deficiency
Vitamin D deficiency can occur when absorption and intake are insufficient over time. Here are some of the most common signs and symptoms:
- Bone loss, fractures, frequent bone/joint pain
- Impaired muscle function, weakness
- Feeling fatigued or tired frequently
- Weakened immune system, frequent colds/flu
- Hair loss
- Mood changes, like depression or anxiety
- Impaired wound healing
- Bone deformities like rickets in children
Testing blood levels of 25-hydroxy vitamin D is the best way to assess status and determine if you are deficient. Most experts consider levels below 20 ng/mL to be deficient.
Conclusion
Optimizing vitamin D absorption requires getting adequate sun exposure, consuming enough from dietary sources, and supporting proper micelle formation and gut health. Taking steps to improve absorption and address any deficiencies is crucial, since vitamin D is involved in so many vital functions and disease states are associated with low levels.
Discuss your individual vitamin D needs with your healthcare provider, especially if you have any absorption issues or risk factors for deficiency. With a combination of prudent sun exposure, a balanced diet rich in sources like fatty fish, and targeted supplementation if warranted, you can help your body effectively absorb and utilize the vitamin D it needs for optimal health.