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PubMed This is a summary of 27 peer-reviewed journal articles Updated
Pediatrics

Why Vitamin D Levels Drop: Understanding Your Risk Factors

At a Glance

Vitamin D levels can fall when the skin gets too little sunlight, the body makes less with age or darker skin, or vitamin D is trapped in body fat, poorly absorbed, or broken down by medicines. Breastfed infants need routine supplementation.

While many people think of Vitamin D as something we get only from food, it is unique because your body is designed to make it yourself through skin exposure to sunlight. However, this process is easily interrupted by environmental factors, lifestyle choices, and underlying health conditions. Understanding why you might be deficient is the first step toward maintaining healthy levels.

The Sun and Your Skin

The most common cause of low Vitamin D is a lack of cutaneous synthesis—the process where your skin uses ultraviolet B (UVB) radiation from the sun to create Vitamin D3 [1][2]. Several factors can prevent this process from working effectively:

  • Geography and Season: If you live at a high latitude (far north or south of the equator), the sun’s angle during winter months is too low for UVB rays to reach the earth effectively. In these regions, it is very difficult to make enough Vitamin D from sunlight for several months of the year [3][4].
  • Melanin and Skin Tone: Melanin is the pigment in your skin that provides natural protection against sun damage. Because melanin absorbs and filters UVB radiation, people with darker skin tones generally produce Vitamin D more slowly than those with lighter skin. This means more time in the sun may be required to achieve the same levels [5][6].
  • Sun Protection: While essential for preventing skin cancer, sunscreen with a high SPF can significantly reduce Vitamin D production in the skin [7][8]. Similarly, clothing that covers most of the body or spending the majority of time indoors—common for office workers and those in assisted living—reduces your body’s ability to manufacture the vitamin [9][10]. (Note: Do not stop using sunscreen, deliberately sunbathe, or use tanning beds to treat deficiency. Diet and clinician-directed supplementation are safer and more reliable.)

Why Age Matters

As you get older, your skin’s ability to produce Vitamin D decreases significantly; research suggests it can be up to four times lower in older adults compared to younger people [11].

Older adults, particularly those in nursing homes or long-term care facilities, face a “perfect storm” of risk factors: reduced skin efficiency, less time spent outdoors, and diets that may be lower in Vitamin D-rich foods like fatty fish [12][13]. For this reason, supplementation is often more practical and effective than relying on sun exposure alone in these populations [14].

Weight and Digestion

Sometimes the problem isn’t how much Vitamin D you make or take, but how your body handles it.

  • Obesity: Vitamin D is fat-soluble, meaning it is stored in body fat. In people with a high Body Mass Index (BMI), Vitamin D can become “sequestered” or trapped in fat cells, leaving less of it circulating in the bloodstream where it is needed [15][16]. Because of this, individuals with obesity often require higher doses of supplements to reach healthy blood levels [17].
  • Malabsorption: Because Vitamin D is absorbed in the gut along with dietary fats, any condition that interferes with fat digestion can lead to deficiency. This includes Inflammatory Bowel Disease (Crohn’s or Ulcerative Colitis), Celiac disease, and Cystic Fibrosis [18][19][20].
  • Bariatric Surgery: Procedures like gastric bypass or sleeve gastrectomy change how the digestive system absorbs nutrients. People who have had these surgeries are at a very high risk for long-term deficiency and may need specialized types of Vitamin D that are easier to absorb [21][22].

Medication and Infant Health

Certain medications can speed up the breakdown of Vitamin D in your body. In particular, some antiseizure medications (like carbamazepine or phenytoin) and long-term use of glucocorticoids (steroids) can lower your levels and may require active monitoring by your doctor [23][18][24].

Finally, Vitamin D is a critical concern for the youngest patients. Human breast milk is naturally low in Vitamin D. To prevent rickets (a disease that causes soft, weak bones and skeletal deformities), pediatric experts recommend that all exclusively breastfed infants receive a daily, age-appropriate pediatric supplement of 400 IU of Vitamin D starting shortly after birth, regardless of maternal status [25][26]. Formula-fed infants also need 400 IU daily if they do not consume approximately 1 liter (32 ounces) of fortified formula per day [27].

Common questions in this guide

Why can I have low vitamin D even when I eat foods that contain it?
Vitamin D comes from skin production as well as food, and sunlight-related production can fall with high-latitude winters, indoor living, covered skin, sunscreen, darker skin, or older age. Obesity, poor absorption, bariatric surgery, and certain medicines can also lower the amount available in the bloodstream. A clinician can help identify which factors apply and whether supplementation is appropriate.
Does having darker skin increase the risk of vitamin D deficiency?
Yes. Melanin absorbs some of the sun’s ultraviolet B rays, so darker skin generally makes vitamin D more slowly than lighter skin. This is not a reason to stop sunscreen or deliberately sunbathe; diet and clinician-guided supplements are safer ways to address low levels.
How do obesity and weight-loss surgery affect vitamin D?
Vitamin D is stored in body fat, so in obesity less may remain in the bloodstream. Gastric bypass and sleeve gastrectomy can also change nutrient absorption and increase the long-term risk of deficiency. Some people may need a different dose or a more readily absorbed form, but the choice should be guided by a clinician.
Which digestive conditions and medicines can lower vitamin D?
Crohn’s disease, ulcerative colitis, celiac disease, and cystic fibrosis can interfere with absorption of vitamin D and dietary fat. Carbamazepine and phenytoin, along with long-term glucocorticoid use, can lower vitamin D levels by affecting how the body handles the vitamin. Ask a clinician whether your condition or medications warrant monitoring.
How much vitamin D does a breastfed baby need each day?
Exclusively breastfed infants should receive 400 IU of vitamin D daily starting shortly after birth because human breast milk is naturally low in vitamin D. Formula-fed infants also need 400 IU daily if they do not drink about 1 liter, or 32 ounces, of fortified formula each day. A pediatrician can recommend an age-appropriate product and dosing method.
Can winter or staying indoors cause vitamin D deficiency?
Yes. At high latitudes, winter sunlight may not provide enough ultraviolet B radiation for the skin to make adequate vitamin D, and indoor living or clothing that covers most of the skin further limits production. Do not use tanning beds or stop sun protection to treat deficiency; discuss food and supplements with a clinician.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my skin tone or the latitude where I live mean I need a higher daily dose of Vitamin D than the standard recommendation?
  2. 2.How does my current BMI affect the way my body uses Vitamin D supplements, and do I need a different dose because of it?
  3. 3.Can you check if any of my current medications, like my antiseizure drugs or steroids, are causing my Vitamin D levels to drop?
  4. 4.Since I have a digestive condition (like IBD or a past bariatric surgery), should I be taking a specific type of Vitamin D, like calcifediol, that is easier for me to absorb?
  5. 5.My baby is exclusively breastfed; what is the safest way to ensure they are getting the 400 IU of Vitamin D they need every day?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (27)
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    The action spectrum for vitamin D3: initial skin reaction and prolonged exposure.

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This page explains common reasons vitamin D levels may be low for informational purposes only and does not replace medical advice. Ask your clinician or your child’s pediatrician before changing supplements or medication.

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