Has Switching to Standard Time Decreased Our Vitamin D Levels?

Vitamin D supplements may be beneficial in the winter .

Posted Nov 12, 2019

A day after most of the country switched from daylight saving to standard time, a local television commentator lamented the now early sunsets. “How can anyone get any sunshine?” he said. “By the time we get home from work, the sun has set and it's dark. Who wants to go for a walk or play outside with the kids when it is nighttime earlier?”   

Of course, he overlooked the encroaching bad weather that will also make being outside a misery in certain parts of the country, and the fact that those whose work keeps them inside well beyond even daylight saving time sunsets don’t get to “play outside,” except perhaps on weekends. But he had a point. Exposure to afternoon sunshine during the school and work week in the northern tier of states will have to wait until the end of March, when the combination of longer days and a return to daylight saving time will delay the hour of sunsets again. 

The reduction in outdoor activities that may, in part, be linked to standard time has been associated with low vitamin D levels. Vitamin D, aka the sunshine vitamin, is made naturally when ultraviolet-B ("UVB") light strikes the skin. The compound made is not the final vitamin D product; it undergoes chemical reactions in the body to create the biologically active form.

In one sense, vitamin D is a nutrient that everyone should be able to acquire. All it takes is exposure to sunlight. But as a review by Nair and Maseeh points out, the amount of vitamin D that is obtained from the sun may be limited by lifestyle, season of the year, use of sunscreen, and the pigmentation of the skin. Almost half the world’s population suffers from insufficient vitamin D due to an absence of adequate skin exposure to sun along with air pollution, the use of sunscreen which significantly reduces vitamin D synthesis, and darker skin. The latter, while protective against ultraviolet radiation, necessitates a longer exposure to the sun to make vitamin D. Obesity (a BMI of 30 or above) is also associated with lower than normal levels of vitamin D.

Studies have found significant decreases in vitamin D produced by the sun during the months of limited sunlight. No one has linked this to the switch to standard time, however. The winter climate seems reason enough to explain the lack of sun exposure.

Vitamin D is found naturally in egg yolks; fatty fishes such as tuna, mackerel, and salmon; cheeses; and foods that are fortified with vitamin D, including dairy products, orange juice, soy milk, and cereals. Cod liver oil is also an excellent (although horribly smelling) source. An older relative told me that when she was five, she had to drink orange juice laced with cod liver oil every morning. She thought about running away from home.

Most physicians recommend obtaining vitamin D through supplements. A typical multivitamin contains about 400 IU (international units) of the vitamin, but it is possible to obtain much higher doses by taking the vitamin separately.

The best-known and perhaps best-studied function of vitamin D is its role in maintaining bone density. Vitamin D supports and enhances the absorption of calcium from the intestinal tract, along with phosphate. When vitamin D levels are inadequate, calcium absorption is impaired and affects the mineralization (or formation) of new bone tissue. Osteopenia and osteoporosis in adults and rickets in children can result.

The weakened bones can, in turn, fail to support muscles adequately, thereby leading to an increased possibility of falls and broken bones. The sometimes treacherous conditions of winter make people more vulnerable to falling; lower vitamin D levels and bone fragility may add to the problem.

How much supplemental vitamin D should be taken to compensate for the inadequate (though naturally acquired) stores from the sun is the subject of active investigation. According to the Institute of Medicine Dietary Reference for vitamin D and the American Geriatrics Society and the National Osteoporosis Foundation, between 800 and 1,000 IU should be taken daily. These organizations suggest that a blood test measuring vitamin D levels be done to determine whether the vitamin level is inadequate.

How much vitamin D should be taken as a supplement if the level is found to be too low to support the maintenance of bone density and strength? This may depend on whether the results of recently completed research are confirmed. A three-year study was carried out at the McCraig Institute for Bone and Joint Health at the University of Calgary to test different doses of vitamin D supplementation on bone density and bone strength. Three hundred volunteers between the ages of 55 and 70 received different doses of vitamin D per day: 400 IU, 4,000 IU, and 10,000 IU. The authors reported that there was no increased benefit for bone density from the larger doses, and perhaps even a reduction, compared to the lowest dose of vitamin D. The 400 IU dose was sufficient.

Because the now shorter hours of daylight may be initiating or even exacerbating the type of depression known as Seasonal Affective Disorder (SAD), one seemingly obvious cause is inadequate vitamin D. If so, taking vitamin D supplements would be an easy treatment component for this emotionally debilitating disorder. At present, however, there is no definitive answer. Some suffering from SAD and other types of depression have been shown to have a low level of vitamin D, and supplementation increased their level to normal. But there was no obvious cause and effect. As Zad Chow writes on the website Examine.com, depression itself may cause vitamin D level to drop if the person lacks exposure to sunlight or makes poor food choices. Someone who is depressed may not go outside to walk or engage in other outdoor activities, he suggests, limiting exposure to sunlight. Many studies are ongoing to see whether supplementation with high doses of vitamin D might help depression; the answers are not yet definitive.

Would keeping daylight saving time in effect year round fix the problem of low vitamin D? Probably not. But an awareness of a possible deficiency in vitamin D status with the coming of winter should have everyone asking: “Do I need a supplement?”


“Vitamin D: the ‘Sunshine’ Vitamin,” Nair R and Maseeh A, J Pharmacol Pharmacother; 2012; 3:118–126.

“Vitamin D Insufficiency in North America,” Hanley D and Davison K, The Journal of Nutrition; 135: 332-337.

“Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength,” Burt L, Billington E, Rose M et al, JAMA 2019; 322: 736-745.

“Vitamin D and Depression: A Systematic Review and Meta-Analysis Comparing Studies with and without Biological Flaws,” Spedding S, Nutrients 2014; 6:1501-1518.