The research suggests that sunscreen use and chronic diseases - such as diabetes, celiac disease, and other conditions that affect the body's ability to absorb nutrients from food - contribute to nearly 1 million cases of vitamin D deficiency across the globe.

Study co-author Dr. Kim Pfotenhauer, from the College of Osteopathic Medicine at Touro University California, and colleagues recently reported their findings in The Journal of the American Osteopathic Association.

Vitamin D is important for bone health, as it helps the gut to absorb calcium. The vitamin also aids muscle and nerve function, and it helps the immune system to stave off infection.

Vitamin D deficiency - generally defined as having a serum 25(OH)D concentration lower than 20 nanograms per milliliter - may lead to loss of bone density, which can increase the risk of bone fractures and osteoporosis.

According to recommendations from the Food and Nutrition Board, adults should aim to get between 600 and 800 International Units of vitamin D every day.

While some foods contain vitamin D - including fatty fish, beef liver, and fortified breakfast cereals - these are usually in low amounts. Exposure to sunlight is considered one of the best sources of vitamin D; sunlight penetrates the skin and converts a vitamin D precursor, called 7-dehydrocholesterol, to the active form of vitamin D-3.

However, the risks that come with sunlight exposure - such as sunburn and skin cancer - cannot be ignored, and sunscreen is considered one of the best ways to protect against such harms.

In their new review, however, Dr. Pfotenhauer and team suggest that individuals should avoid sunscreen use when exposed to midday sun for up to 30 minutes twice weekly, in order to increase and maintain normal vitamin D levels.

Sunscreen may reduce vitamin D-3 production by 99 percent

The researchers came to their conclusions after conducting a review of clinical studies investigating vitamin D deficiency.

The team used this information to determine the scope of vitamin D deficiency worldwide, as well as risk factors for the condition and what can be done to boost vitamin D levels.

From their review, the researchers conclude that sunscreen use and diseases involving malabsorption of vitamin D - including Crohn's disease, diabetes, and chronic kidney disease - play a part in almost 1 million cases of vitamin D deficiency worldwide.

According to the team, using sunscreen with an SPF of 15 or higher can reduce the body's vitamin D-3 production by 99 percent.

To boost and maintain optimal vitamin D levels, the researchers recommend spending around 5 to 30 minutes in midday sun twice each week, without the protection of sunscreen.

"People are spending less time outside and, when they do go out, they're typically wearing sunscreen, which essentially nullifies the body's ability to produce vitamin D.

While we want people to protect themselves against skin cancer, there are healthy, moderate levels of unprotected sun exposure that can be very helpful in boosting vitamin D."

Dr. Kim Pfotenhauer

The researchers add that vitamin D supplementation is also a good way to boost vitamin D levels, as it does not pose the risks associated with sunlight exposure. However, the authors recommend consulting a physician before taking vitamin D supplements.