Vitamin D is known to be essential to many aspects of human health — but a new study suggests that taking a certain form of it can have a negative effect.

The more potent and longer-lasting form, vitamin D3 (cholecalciferol) is produced naturally when the body is exposed to sunlight and is also found in animal products, while vitamin D2 (ergocalciferol) comes from plant or fungal sources, per the National Institutes of Health.

Researchers from the University of Surrey in the U.K. found that taking vitamin D2 can cause reduced levels of vitamin D3 in the body.

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They made this determination by analyzing 11 randomized controlled trials including 655 adults, which showed that people taking vitamin D2 supplements had less vitamin D3 than those who did not take D2, according to a university press release.

The findings of the study — conducted along with the John Innes Centre and the Quadram Institute Bioscience in Norwich, England — were published in the journal Nutrition Reviews.

“Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the U.K.,” lead researcher Emily Brown, a Ph.D. research fellow from the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, stated in the press release.

“However, we discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements. This study suggests that, subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2.”

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Previous research has suggested that the two forms of vitamin D are not interchangeable. A study published in Frontiers in Immunology, also conducted by the University of Surrey, found that vitamin D3 is more effective in boosting the immune system and protecting the body from viral and bacterial diseases, the release noted.

Some foods containing vitamin D3 include fatty fish, egg yolks, cod liver oil and fortified dairy and animal foods, health sources confirm.  (iStock)

“We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signaling system in the body – a key part of the immune system that provides a first line of defense against bacteria and viruses,” stated Professor Colin Smith, who led the above study. “Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body.”

Professor Martin Warren, chief scientific officer at the Quadram Institute, confirmed in the release that vitamin D deficiency is a “significant public health concern,” particularly during the winter months. 

“This collaborative research effort aligns well with the Quadram Institute’s mission to deliver healthier lives through food innovation to enhance the nutrient density of the food we eat,” he said. “Tackling this with the most effective form of vitamin D supplementation or fortification is of the utmost importance to the health of the nation.”

More research is needed to determine whether vitamins D2 and D3 affect the body differently — and whether that could change doctors’ medical advice on which type to take, the researchers concluded. (iStock)

More research is needed to determine whether vitamins D2 and D3 affect the body differently — and whether that could change doctors’ medical advice on which type to take, the researchers concluded.

There were several limitations to the analysis, the study findings noted.

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For example, there were only a small number of studies with many differences among them, such as their duration, the amount and timing of doses, and how results were measured. There was also the potential for biases due to incomplete reporting of details.

The results could also be affected by factors such as the amount of sunlight exposure, as well as some people taking supplements and others eating vitamin-fortified foods, the researchers noted.

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Some foods containing vitamin D3 include fatty fish, egg yolks, cod liver oil and fortified dairy and animal foods, health sources confirm. Vitamin D2 can be found in mushrooms, fortified foods and some plant/fungal-based supplements.

The study was supported by the Biotechnology and Biological Sciences Research Council (BBSRC).

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A new UC Irvine study may have finally uncovered how dietary fiber protects the body from sugar’s damage, and how protecting yourself starts in the gut.

Researchers at the UCI School of Medicine found that inulin, a naturally occurring fiber found in vegetables like onions, garlic and artichokes, can reshape the bacteria living in your gut to stop fructose before it hits your liver.

“We found that consuming a type of dietary fiber called inulin… changes the bacteria in the gut to promote the consumption of harmful dietary fructose,” said lead researcher Cholsoon Jang, PhD, of UCI’s Nutrient Metabolism & Disease Lab. 

The findings, published in Nature Metabolism, reveal a new level of protection offered by fiber, not just in digestion, but in how the body handles sugar at the molecular level.

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Jang and his team discovered that when people eat fructose (a common sugar in fruit and sweetened foods) gut bacteria in the small intestine can metabolize it before it reaches the liver. But without enough fiber, too much fructose “spills over,” overwhelming the liver and triggering fat buildup.

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Fructose is a sugar found in fruit that can ‘spill’ into the liver, leading to liver disease. (iStock)

By feeding gut bacteria with inulin, researchers found that the microbes essentially burn through fructose early, preventing that cascade of damage. 

Even more striking, once these bacteria were “primed” by inulin, they were able to reverse signs of fatty liver disease, reducing fat accumulation and boosting the liver’s natural antioxidants.

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Jang says the research proves not all calories are created equal.

He says their work gives “insight into how fiber protects our health from harmful nutrients like fructose.”

The Nature Metabolism study identified how specific gut bacteria metabolize sugar before it reaches the liver. (iStock)

The study focused on non-obese participants: people who might otherwise slip under the radar yet still face hidden risks from high-sugar diets. 

Jang noted that metabolic damage isn’t limited to those who are overweight. Even people who appear healthy can experience liver stress and insulin resistance if their gut microbes aren’t equipped to handle excess fructose.

“By identifying specific gut bacteria and metabolic pathways involved, our findings can guide personalized nutrition strategies,” Jang said.

Vegetables like garlic, artichokes and onions can be a good source of inulin, the helpful fiber studied by the research team. (iStock)

He added that future research will explore whether other common fibers beyond inulin can trigger similar protective effects.

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The implications stretch far beyond the lab. If certain fibers can train gut microbes to neutralize sugar before it harms the liver, it could open the door to new treatments for fatty liver disease, diabetes, obesity and even cancer.

For now, researchers explain that the findings indicate that fiber not only aids digestion but can be a defender of metabolic health.

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As Jang put it: “For example, by checking how well someone’s gut bacteria clears fructose before the body absorbs it, we can choose the right prebiotic or probiotic supplement for that person to improve results and reduce side effects.”