Cautious approach

I often take the cautious approach, currently doing ~5,000 IU of D with some co-nutrient like mag and zinc. Sunlight is the best choice over supplementing D. I personally don't get enough sun due to a night work schedule. There's a lot of reading that state, for example, 100 IU for person A may be equivalent of 1,000 IU for person B. The absorption factor depends on the individual, so a 5,000 IU dose may only be effective at 10%, or 500 IU. Hence, testing may be necessary to rule out deficiency.