Lifestyle

5 Signs of Vitamin D Deficiency Most People Mistake for Seasonal Fatigue

By Curtis Jones · August 10, 2026

One in four American adults is low on vitamin D. Most of them think they’re just tired.

Vitamin D deficiency is one of the most common nutritional deficiencies in the world, and one of the most underdiagnosed — because every symptom it produces has a more obvious explanation. You’re tired, so you blame your schedule. Your back hurts, so you blame your mattress. Your mood drops, so you blame the season. Doctors say the real cause often goes unchecked for years.

You’re exhausted no matter how much you sleep. Vitamin D helps your cells produce energy at the mitochondrial level. When levels drop, the fatigue isn’t the kind that responds to an extra hour of sleep — it’s a persistent, low-grade exhaustion that makes ordinary tasks feel disproportionately draining. Research has shown that supplementation significantly reduces fatigue severity in people with confirmed deficiency.

Your bones or lower back ache without injury. Vitamin D is essential for calcium absorption. Without enough of it, bones weaken — a process that produces dull, persistent pain most people attribute to aging or poor posture. Lower back pain, hip pain, and generalized bone tenderness — especially when pressed — are classic signs. In severe cases, the deficiency leads to osteomalacia, a softening of the bones that increases fracture risk.

You’re getting sick more often than usual. Vitamin D plays a direct role in immune function. Multiple studies have linked deficiency to increased susceptibility to respiratory infections, colds, and flu. If you’re catching every virus that circulates through your household or workplace, your immune system may be underpowered by a nutrient gap.

Your mood has dropped — especially in winter. The connection between vitamin D and serotonin production is well documented. Low levels are associated with increased rates of depression and anxiety, particularly during months with less sunlight. Many people diagnosed with seasonal affective disorder also have vitamin D deficiency — and supplementation has been shown to improve symptoms in some cases.

Your hair is thinning or your wounds heal slowly. Hair loss and delayed wound healing are less commonly recognized symptoms, but both are linked to vitamin D’s role in cell growth and immune regulation. If cuts take longer to close than they used to, or your hair is thinning without a clear hormonal explanation, a blood test may reveal the connection.

The people most at risk are adults over 50, anyone with darker skin (which reduces vitamin D synthesis from sunlight), people who spend most of their time indoors, and residents of northern latitudes where winter sunlight is insufficient. A simple 25-hydroxyvitamin D blood test costs roughly $30 to $50 and is covered by most insurance when ordered with a clinical indication. If you’re experiencing three or more symptoms on this list, ask for the test at your next appointment — because the fix is often a $10 bottle of supplements.