Turns out I was one of them. Were you?
According to the National Institutes of Health Office of Dietary Supplements, vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert vitamin D to its active form, or absorption of vitamin D from the digestive tract is inadequate.
While rickets and osteomalacia are classic vitamin D deficiency diseases, in my case, it was osteopenia/osteoporosis.
Risk groups include pregnant women, breastfed infants, older adults, people with limited sun exposure, people with dark skin, people with fat malabsorption, and people who are obese or have undergone gastric bypass surgery. Then there's me -- I fell into the used to tan obsessively and now slather sunscreen obsessively, coupled with surgically induced menopause category.
My doctor recommended sticking with the sunscreen and adding 1,000 IU D3 daily. Easy enough.
If you're worried (or curious) about your vitamin D levels, ask your doctor for the simple blood test that will indicate whether or not you need to supplement your vitamin D intake.