I've tested thousands of people over 20 years in functional medicine.
The deficiency I find most often, across every age, every background, every fitness level?
Vitamin D.
Not slightly low. Critically low. Below 30 ng/mL when the optimal range is 60–80.
Low Vitamin D is linked to immune dysfunction, depression, insulin resistance, and accelerated aging.
Have you had yours tested recently? What was your number?