Nutrition For Energy Part 9: Vitamin D, The “Vitamin” That Acts More Like a Hormone

Vitamin D is usually discussed in the context of bones. That is understandable, but it misses why low levels can affect how you actually feel.

Vitamin D helps regulate processes tied to energy production, muscle function, immune balance, and mood. It is also unusual among vitamins because your body can make it from sunlight, then convert it into an active form that functions more like a hormone, influencing gene expression in many tissues throughout the body. That helps explain why low vitamin D can show up as persistent fatigue, lower mood, muscle weakness, slower recovery, and a body that feels less steady than it should.[1–3]

More Like a Hormone Than a Typical Vitamin

One of the most interesting things about vitamin D is that it does not behave like a simple nutrient you eat and use directly. Vitamin D made from sunlight, food, or supplements is biologically inactive at first. The body converts it in the liver to 25-hydroxyvitamin D, then primarily in the kidney to 1,25-dihydroxyvitamin D, the active form known as calcitriol. That active form binds to the vitamin D receptor, which is present in many tissues, helping regulate a wide range of cellular processes.[1,2]

This is part of why vitamin D can feel bigger than a lab number. It sounds basic, but it influences systems women notice in daily life: energy, muscle function, immune steadiness, and mood.[1,2]

Why Vitamin D is Important for Our Energy

When women think about fatigue, they often think first about sleep, iron, stress, or thyroid function. All of those matter. But energy also depends on whether cells, especially muscle cells, can produce and use energy efficiently.

Vitamin D appears to play a role here. Reviews on vitamin D and skeletal muscle describe vitamin D signaling as relevant to mitochondrial oxidative function, ATP production, and the way muscle tissue handles oxidative stress. Reduced vitamin D receptor signaling has been linked with poorer mitochondrial function in skeletal muscle, which helps explain why low vitamin D can feel like reduced stamina, lower physical capacity, or fatigue that lingers in the background.[4,5]

This does not mean vitamin D works like caffeine. It does not create artificial energy. Its role is more foundational. It helps support some of the systems that make steady energy possible in the first place.[4,5]

Vitamin D and Muscle Weakness

Low vitamin D is also well known for its connection to muscle weakness, especially proximal muscle weakness, the kind that makes stairs, squatting, getting up from the floor, or simply carrying yourself through the day feel harder than they should.

A long-cited clinical review describes common manifestations of vitamin D deficiency as proximal muscle weakness, muscle aches, and bone pain. More recent literature continues to link vitamin D with normal muscle function and muscle mitochondrial health.[4,6,7]

This is one reason low vitamin D can be easy to miss. What looks like low motivation, poor fitness, or “just getting older” can sometimes include lower physiological capacity.

Vitamin D and Immune Regulation

Vitamin D also helps regulate the immune system. The NIH notes that vitamin D affects immune function and inflammation, and immune reviews describe vitamin D as helping coordinate both innate and adaptive immune responses.[1,8]

That matters for energy because an overtaxed or dysregulated immune system is expensive. When the body is constantly allocating resources toward inflammation or immune activation, people often feel it as fatigue, lower resilience, or a slower recovery curve. Vitamin D is not the whole story here, but it is part of the regulatory environment.[1,8]

Vitamin D and Mood

Vitamin D is also relevant to mood. That relationship is not simple, and it should not be reduced to “low vitamin D causes depression.” Mood is shaped by many things: sleep, inflammation, hormones, stress load, trauma history, social connection, and overall health.

Still, there is a meaningful signal here. The NIH fact sheet summarizes observational evidence linking lower vitamin D levels with depression, and a 2023 umbrella meta-analysis reported that lower serum vitamin D levels were associated with higher odds of depression while supplementation showed potential benefit in reducing depressive symptoms.[1,9]

That does not make vitamin D a standalone mood treatment. It does make it one more reason not to ignore when someone feels persistently flat, more emotionally fragile, or noticeably worse during seasons with less effective sun exposure.

How Much Vitamin D Can We Get From the Sun?

Sunlight really does matter for vitamin D. When UVB light reaches uncovered skin, the body can begin making vitamin D on its own. In that sense, vitamin D is one of the few nutrients we can produce rather than only consume.[1,2]

A commonly cited estimate, referenced by the NIH, is that approximately 5 to 30 minutes of sun exposure to the face, arms, hands, and legs, especially between 10 a.m. and 4 p.m., either daily or at least twice a week, can often support sufficient vitamin D synthesis in many people.[1]

So yes, 15 to 20 minutes can absolutely be meaningful. But it is not a universal dose. The amount your body makes depends on the season, time of day, cloud cover, skin tone, age, latitude, how much skin is exposed, and whether clothing or sunscreen is blocking UVB. Older adults and people with darker skin generally make less vitamin D from the same sun exposure, and UVB does not pass through glass, so bright sunlight through a window does not help much here.[1]

What Time of Day and Season Work Best?

This is the part many people do not realize: not all sun exposure is equally useful for vitamin D.

Endotext notes that peak vitamin D3 production occurs around noon, and that a larger portion of the day is capable of producing vitamin D in the summer than in winter. That means early morning or late afternoon light may be excellent for circadian rhythm, mood, and getting outside, but it is often not the most effective light for vitamin D production.[2]

Season and latitude matter a great deal as well. Endotext notes that in Edmonton there is very little vitamin D3 production in exposed skin from mid-October to mid-April, Boston has a somewhat longer but still limited effective season, while Los Angeles can produce vitamin D3 all year long.[2]

That helps explain why the same “just get 15 minutes of sun” advice works differently depending on where someone lives and what time of year it is.

Why Many Women Still Need Supplements

In theory, sunlight can cover a meaningful portion of vitamin D needs. In practice, many women do not get enough effective UVB exposure consistently enough to rely on sunlight alone.

Modern life is part of the reason. We work indoors. We spend more time in cars and buildings. We often go out early or late rather than near midday. We may cover more skin, use sunscreen, live farther north, or have darker skin that requires more exposure to generate the same amount. And vitamin D is naturally present in relatively few foods; in U.S. diets, fortified foods provide much of it.[1,2]

So while sunlight is valuable, supplements are often the more reliable strategy for maintaining vitamin D status year-round, especially for women with indoor schedules, limited midday exposure, darker skin, older age, or wintertime living.[1]

If fatigue, muscle weakness, lower mood, or low sun exposure are part of the picture, checking 25-hydroxyvitamin D is usually more useful than guessing. It is the main marker used to assess vitamin D status.[1]

The Deeper Takeaway

Vitamin D is one of those nutrients that sounds simple until you realize how many systems quietly depend on it.

It influences mitochondrial and muscle function, helps regulate immune activity, and appears to matter for mood as well. It also behaves more like a hormone than most people realize, which helps explain why low levels can show up in such broad ways.[1–5,8,9]

When energy is persistently low, strength is slipping, recovery feels poor, or mood feels less stable, it is worth looking more carefully at the foundations.

Vitamin D is one of them.

References

[1] NIH Office of Dietary Supplements. Vitamin D – Health Professional Fact Sheet.
[2] Bikle DD. Vitamin D: Production, Metabolism, and Mechanism of Action. Endotext.
[3] Pike JW, Christakos S. Biology and mechanisms of action of the vitamin D hormone.
[4] Latham CM et al. Vitamin D Promotes Skeletal Muscle Regeneration and Mitochondrial Health. 2021.
[5] Salles J et al. Vitamin D status modulates mitochondrial oxidative capacities in skeletal muscle. 2022.
[6] Bordelon P et al. Recognition and Management of Vitamin D Deficiency. 2009.
[7] Gunton JE et al. Vitamin D and muscle. 2018.
[8] Bikle DD. Vitamin D Regulation of Immune Function. 2022.
[9] Musazadeh V et al. Vitamin D protects against depression: Evidence from an umbrella meta-analysis. 2023.

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