Vitamin D

Low vitamin D can lead to weakened bones (osteopenia or osteoporosis), muscle weakness, and increased risk of falls. It may also affect immune function and mood. Maintaining adequate vitamin D levels through sunlight, diet, and supplementation when needed supports bone strength, muscle function, and overall health.

Most adults benefit from a yearly vitamin D check, ideally in late winter when stores are lowest. After starting supplementation, retest at 3 months. Once at target, annual checks are usually enough to maintain.

At baseline / for screening: Once every 12 months, ideally in late winter when stores are lowest. More frequently if you have malabsorption, take medications that affect vitamin D metabolism, or have a high BMI (body fat sequesters vitamin D).

When monitoring an intervention or change: Retest 3 months after starting or adjusting vitamin D supplementation, since that's how long blood levels take to stabilize at a new steady state. Compare values from the same time of year for a meaningful trend, since season and sun exposure shift levels substantially.

Note: Vitamin D status shifts with season, sun exposure, and BMI. For a meaningful trend, compare values from the same time of year (late winter to late winter); mixing seasons obscures the real picture. After starting supplementation, the 3-month retest establishes your individual dose-response.