While there are several potential causes—such as age, diet, or certain medical conditions—the main culprit during winter is the reduced exposure to sunlight.

Nutritional therapist Eve Kalinik explained that one of the first signs of deficiency is a sudden “susceptibility to infections and viruses,” since vitamin D plays a key role in immune defense. She also noted that symptoms can include “bone pain or muscle weakness (as vitamin D helps regulate calcium levels in the body), as well as low mood and fatigue, which are often linked to Seasonal Affective Disorder (SAD).”

Overlapping Symptoms and Hidden Signs

Fatigue is often the earliest symptom, but it can easily be overlooked during the chaotic winter months. Factors like disrupted sleep from the time change, stress, or hormonal shifts can mask vitamin D deficiency. This tiredness can then spiral into other problems, such as frequent colds, poor mood, and weight gain.

If left unchecked, low vitamin D levels can contribute to bone loss, leading to conditions like osteoporosis or osteomalacia—both of which increase the risk of fractures later in life. In severe cases, deficiency may even cause seizures.

Testing for Vitamin D Levels

If you suspect you might be deficient, an at-home blood test can help. These kits are widely available and provide a clear picture of your vitamin D status. The results can also show just how low your levels are, allowing you to adjust your supplements or diet accordingly.

As winter sets in and sunlight grows scarce, keeping an eye on your vitamin D levels can make a major difference to your health, mood, and energy throughout the colder months.