Scientists discovered that exposing dairy cows to artificial ultraviolet (UV) light for just 30-60 minutes per day can significantly increase the amount of vitamin D in their milk. In two separate experiments, cows exposed to UV-B light produced milk with 44-112% more vitamin D3 compared to cows without UV exposure. This finding is important because vitamin D deficiency is becoming more common in Europe, and milk is a major source of this essential nutrient for many people. The study suggests that using UV light in dairy barns could be a simple, natural way to help people get more vitamin D without changing what cows eat or how much milk they produce.

The Quick Take

  • What they studied: Whether shining artificial UV-B light on dairy cows living indoors could increase the vitamin D content in their milk
  • Who participated: Two separate groups of 32 Holstein dairy cows each (a common dairy breed) that were in the later stages of their milk-producing cycle. The cows were housed indoors and received the UV-B light treatment for 12 weeks
  • Key finding: Cows exposed to UV-B light for 30-60 minutes daily produced milk with significantly more vitamin D—between 44% to more than double the vitamin D content compared to cows without UV exposure. The effect was consistent across two different experiments
  • What it means for you: If dairy farms adopt this practice, milk could become a richer source of vitamin D, potentially helping people meet their daily vitamin D needs more easily. However, this is still an experimental approach, and farms would need to invest in UV-B lighting systems. More research is needed to confirm long-term safety and effectiveness in real-world farm settings

The Research Details

Researchers conducted two separate but similar experiments with dairy cows. In each experiment, they divided 32 cows into different groups based on two factors: whether they received UV-B light or not, and whether they received a dietary additive (L-cysteine) or not. This created four different treatment combinations in each experiment. The first experiment exposed cows to UV-B light for 30 minutes per day, while the second experiment used 60 minutes per day. Both experiments lasted 12 weeks, and researchers regularly tested the milk and blood of the cows to measure vitamin D levels.

The researchers carefully measured different forms of vitamin D in the milk and in the cows’ blood. They tracked how these levels changed over time and compared results between the UV-B treated cows and the control cows that received no UV-B light. They also monitored whether the UV-B light affected other aspects of milk quality, such as how much milk was produced or the levels of fat, protein, and lactose (natural milk sugar).

This type of study design—using two separate experiments with the same basic approach—is valuable because it allows researchers to see if their findings are consistent and reliable across different conditions.

This research approach is important because it tests a practical, non-invasive way to naturally increase vitamin D in food. Rather than adding vitamin D supplements to milk (which is already done in many countries), this method uses the cows’ own biology to produce more vitamin D. By using two separate experiments, the researchers could verify that their results weren’t just a one-time occurrence but a consistent effect. This makes the findings more trustworthy and suggests the approach could work in real dairy farms

Strengths of this study include the use of two separate experiments that produced similar results, which increases confidence in the findings. The researchers carefully controlled variables and measured multiple forms of vitamin D. The study was long enough (12 weeks) to observe meaningful changes. However, the study was conducted in controlled laboratory conditions with a relatively small number of cows, so results may differ in real farm settings with different cow breeds, climates, or management practices. The study didn’t examine potential long-term effects or whether consumers would accept milk produced this way

What the Results Show

The main finding was clear and consistent: UV-B light exposure significantly increased vitamin D in milk. In the first experiment with 30 minutes of daily UV-B exposure, milk vitamin D3 levels increased by 44.2%. In the second experiment with 60 minutes of daily exposure, the increase was even larger at 112%—meaning the milk had more than double the vitamin D. The researchers also measured a related form of vitamin D called 25-(OH)-D3, which increased by about 20-24% with UV-B treatment.

Interestingly, the effect wasn’t limited to just one form of vitamin D. In the second experiment, vitamin D2 (a different type of vitamin D) also increased by 33% with UV-B treatment. These increases were substantial enough to potentially make a real difference in people’s vitamin D intake from drinking milk.

The vitamin D levels in milk changed naturally over the 12-week study period, suggesting that seasonal or time-related factors influence vitamin D production. However, the UV-B treatment consistently boosted vitamin D levels throughout the study period, showing that the effect was reliable and not just a temporary response.

Cows exposed to UV-B light also showed increased vitamin D in their blood (plasma 25-(OH)-D3), at least in the first experiment. This indicates that the UV-B light was actually being absorbed by the cows’ skin and triggering vitamin D production in their bodies, which then appeared in their milk

An important secondary finding was that the UV-B light treatment did not negatively affect milk production or milk quality. The amount of milk produced remained the same, and the levels of fat, protein, and lactose (the natural sugars in milk) were not changed by the UV-B exposure. This is crucial because it means farms could implement this strategy without sacrificing milk production or quality. Additionally, the dietary additive (L-cysteine) that was tested had no effect on any of the outcomes, suggesting that changing cow diet alone wouldn’t be an effective way to boost milk vitamin D. The UV-B light itself was the key factor driving the vitamin D increase

This research builds on growing interest in naturally enhancing nutrients in food through animal management rather than chemical fortification. Previous research has shown that vitamin D deficiency is increasingly common in Europe, particularly in populations with limited sun exposure. This study provides evidence for a practical solution that works with cows’ natural biology. The approach of using UV-B light to stimulate vitamin D production is based on well-established science—the same process that creates vitamin D in human skin when exposed to sunlight. This study is among the first to demonstrate that this principle can be applied to dairy cows in indoor farming settings to enhance milk nutrition

Several important limitations should be considered. First, the study was conducted in controlled laboratory conditions with a small number of cows (64 total across both experiments), so results may differ in real commercial dairy farms with different environments, cow breeds, or management practices. Second, the study only lasted 12 weeks, so we don’t know if the effect would continue over longer periods or if cows might adapt to the UV-B light over time. Third, the study didn’t examine whether the increased vitamin D in milk would actually improve vitamin D status in people who drink it, or whether consumers would accept milk produced this way. Fourth, the study didn’t assess potential costs of installing and maintaining UV-B lighting systems in dairy barns or any potential effects on cow welfare or behavior. Finally, the study didn’t test whether the UV-B light might have any unintended effects on other nutrients or milk components that weren’t measured

The Bottom Line

Based on this research, UV-B light exposure appears to be a promising strategy for naturally increasing vitamin D in milk from indoor dairy cows (moderate confidence level). The approach is simple, doesn’t require changing cow feed, and doesn’t reduce milk production. However, before farms widely adopt this practice, additional research is needed to: (1) confirm results in real commercial farm settings, (2) determine the most cost-effective UV-B exposure duration and intensity, (3) assess long-term effects on cows and milk quality, and (4) evaluate whether the increased vitamin D actually improves vitamin D status in milk consumers. For now, this research suggests a promising direction for future development rather than a ready-to-implement solution

This research is most relevant to: dairy farmers and agricultural companies looking for ways to enhance milk nutrition, public health officials concerned about vitamin D deficiency in their populations, and consumers interested in getting more vitamin D from natural food sources. People with vitamin D deficiency or those at risk (including those with limited sun exposure, darker skin tones in northern climates, or certain medical conditions) should be particularly interested. However, individual consumers cannot directly apply these findings—they would need to wait for dairy farms to adopt the practice. People who are already meeting their vitamin D needs through sun exposure, supplements, or fortified foods may not need this enhancement

If dairy farms were to adopt UV-B lighting today, consumers could potentially see increased vitamin D in milk within weeks. However, the realistic timeline for widespread adoption is likely several years, as farms would need to invest in equipment, conduct their own trials, and adjust their operations. Even then, the actual impact on individual vitamin D levels would depend on how much milk people drink and their overall vitamin D intake from other sources. People should not expect immediate changes but could look for this as a long-term option for enhancing their milk’s nutritional value

Want to Apply This Research?

  • Track daily milk consumption in ounces or milliliters, and note whether the milk is from UV-enhanced sources (once available). This allows users to monitor their vitamin D intake from milk and correlate it with their overall vitamin D status if they’re getting blood tests
  • Users can set a goal to consume a specific amount of milk daily (such as 2-3 cups) and track it in the app. Once UV-enhanced milk becomes available in stores, users can switch to that option and track the change. This creates a simple, measurable behavior change that could improve vitamin D intake without requiring supplements
  • Implement a monthly tracking system where users log their milk consumption and any vitamin D-related symptoms (fatigue, muscle weakness, bone pain). Users could also note when they switch to UV-enhanced milk and monitor whether they notice any changes in energy levels or overall wellness over 2-3 months. This long-term approach helps users understand the real-world impact of dietary changes on their vitamin D status

This research demonstrates a promising laboratory technique for increasing vitamin D in milk, but it is not yet widely available in commercial dairy products. Individuals concerned about vitamin D deficiency should consult with their healthcare provider about appropriate testing, dietary sources, and supplementation strategies. This study was conducted in controlled conditions with a small number of cows and may not reflect real-world farm results. Do not rely solely on this research for medical decisions regarding vitamin D intake or supplementation. Always speak with a doctor or registered dietitian before making significant changes to your diet or starting new supplements, especially if you have existing health conditions or take medications