Determining the appropriate dosage and duration for vitamin D supplementation can depend on various factors, including individual needs, existing deficiencies, and underlying health conditions. Here is an explanation supported by recent researched references:

1. The first study mentioned found that taking 1,000-2,000 IUs of vitamin D 1-2 times per week for 1 month did not increase vitamin D levels at all. This suggests that this dosage and frequency may not be sufficient to correct a deficiency.
2. The second study mentioned showed that taking 1,000-2,000 IUs of vitamin D took 4 months to elevate vitamin D levels. This indicates that it may take a few months to reach optimal levels with this dosage.
3. In the third study mentioned, it took 3 months to achieve normal vitamin D levels by taking 1,600 IUs of vitamin D. This suggests that a higher dosage may be required to achieve optimal levels within a shorter time frame.

Considering these findings, it is important to note that individual responses to vitamin D supplementation can vary. The recommended daily allowance (RDA) for vitamin D is typically set at 600-800 IUs, but for individuals with deficiencies or specific health concerns, higher doses may be required.

It is worth noting that the RDA for vitamin D set by the Institute of Medicine (IOM) is generally considered to be adequate for most individuals. The RDA for vitamin D is typically set at 600-800 IUs (15-20 micrograms) per day for adults.

However, for individuals with deficiencies or specific health concerns, higher doses of vitamin D may be required to achieve optimal levels. In such cases, supplementation with higher doses, such as 1,000-2,000 IUs per day, may be recommended.

It is important to note that supplementation recommendations should be individualized based on factors such as baseline vitamin D levels, age, underlying health conditions, and sun exposure. Regular monitoring of vitamin D levels through blood tests is recommended to ensure that supplementation is adequate.

It is crucial to consult with a healthcare professional, such as a doctor or registered dietitian, who can assess your specific needs and provide personalized recommendations for vitamin D supplementation. They can take into account your individual factors and guide you on the appropriate dosage and duration for achieving and maintaining optimal vitamin D levels.