Vitamin D is the supplement many people reach for in winter. It's associated with stronger bones, better immunity and improved mood. But what is less commonly known is that vitamin D doesn't work alone. Its effectiveness relies on magnesium, a mineral many of us are deficient in.
Without sufficient magnesium, vitamin D cannot be properly activated in the body. That means even people who supplement regularly, or who show adequate vitamin D levels on blood tests, may not be receiving its full benefits.
How vitamin D works
Often referred to as the “sunshine vitamin,” vitamin D is more accurately described as a pro-hormone.
It is produced in the skin through sunlight exposure or taken in through food and supplements. Once in the body, it undergoes two conversion steps. The liver first turns it into 25-hydroxyvitamin D, and the kidneys then convert it into its active form, 1,25-dihydroxyvitamin D.
These conversion steps rely on enzymes that require magnesium. Without it, vitamin D remains largely inactive. Research published in The Journal of the American Osteopathic Association has shown that magnesium deficiency can impair this process. As a result, vitamin D may be present in the bloodstream but unable to regulate calcium, support bone strength or contribute effectively to immune function.
Why magnesium matters
Magnesium also helps regulate calcium in the body. Vitamin D increases calcium absorption, but without enough magnesium, that calcium can end up in soft tissues rather than bones, which may raise the risk of kidney stones or vascular issues. Research shows that improving magnesium intake can support vitamin D activity and bone health without needing very high doses of vitamin D. In other words, the two nutrients work together, and supplementing one without the other may limit its effectiveness.
Choosing the right magnesium
Not all magnesium supplements are worth taking. Some, like magnesium oxide or citrate, are poorly absorbed. They may claim a higher elemental level but if your body can't absorb it effectively, it's mostly pointless. Chelated forms, such as TRAACS magnesium bisglycinate, are generally better absorbed because the mineral is bound to (not just mixed) amino acids. This helps it reach the cells where it's needed and is usually gentler on the stomach. The added benefit of the glycine is a calming effect, supporting sleep and stress management.
Making it work for you
certainly hear of this and you may have experienced it yourself: taking vitamin D all winter without noticing any improvement in energy or immunity. Adding magnesium, around 240mg a day from supplements or magnesium-rich foods such as leafy greens, nuts and seeds, can make a significant difference. Over a few weeks, it can help activate vitamin D while also supporting muscle function, energy metabolism and the nervous system.
Maximising your vitamin D
As with any vitamin, mineral or indeed part of the human body, Vitamin D does not work in isolation. Magnesium is an essential cofactor, and without it, the benefits of supplementation may be limited. Most adults need 300-400mg of magnesium a day from a combination of diet and supplements. So if you're taking Vitamin D this winter, remember to keep an eye on your magnesium levels too.
