Latitude, Sunlight & Health: Vitamin D, Depression, MS — Correlation vs. Causation
Epidemiologic gradients exist for several conditions by latitude, but causality is multifactorial.
Observed Associations
- Multiple Sclerosis: Strong latitudinal gradient (↑ prevalence >40° N/S). UV-B → vitamin D → immune regulation (Treg, Th17). Migration studies: risk set by age 15.
- Depression (SAD): Seasonal pattern at high latitudes; light therapy effective. Non-seasonal MDD link weaker.
- Vitamin D Deficiency: UV-B insufficient Oct–Mar >35° latitude. Dietary sources limited (fatty fish, fortified). Supplementation often needed.
Confounders
- Genetics, diet, pollution, lifestyle, healthcare access, socioeconomic status.
- Equatorial regions: High infectious burden, different genetic backgrounds.
Practical Advice
- Test 25(OH)D; supplement to 40–60 ng/mL if low.
- Morning light exposure (10k lux × 30 min) for circadian/mood.
- Don't relocate for health; optimize modifiable factors.
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