The following literature supports the aforementioned trickle effect of obtaining vitamin D through sunlight:
1. Studies show that 5-30 minutes of sun exposure in the summer can increase skin synthesis of vitamin D3 by approximately 10,000-25,000 IU, far exceeding the dosage found in any supplement. [Source: Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011]
2. Research confirms that the vitamin D3 synthesized by the skin can be stored in the body for 2-3 months, maintaining stable serum vitamin D levels. In contrast, the half-lives of ingested vitamin D2 and D3 in the body are only about 15 days and 30 days, respectively. [Source: Jones G. Pharmacokinetics of vitamin D toxicity. Am J Clin Nutr. 2008;88:582S-6S.]
3. Studies indicate that volunteers who are exposed to light (summer solstice) and those not exposed (winter solstice), after ingesting the same dose of vitamin D2, exhibit greater and longer-lasting increases in serum 25(OH) vitamin D levels in the light-exposed group. This suggests that skin-synthesized vitamin D3 has higher bioactivity and conversion efficiency within the body. [Source: Trang HM, et al. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998;68:854-8.]
4. Findings reveal that during seasons with less sunlight in the winter, skin-synthesized vitamin D3 from sunbathing can sustainably elevate serum vitamin D levels for 2-3 weeks and reach a stable state, whereas the same oral dose of vitamin D2 only lasts for about a week. This indicates that skin-synthesized vitamin D3 has a longer half-life in the body, providing more sustained organ supply. [Source: Trang HM, et al. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998;68:854-8.]
Moderate sunlight exposure can generate and store large amounts of highly active vitamin D3 in the body, releasing it slowly to achieve stable and sustained serum levels and organ supply. This trickle effect is the optimal source of vitamin D, beneficial for its comprehensive nutritional and physiological regulatory functions in the body. Combining sunlight with food or supplements helps achieve the optimal state of vitamin D nutrition.

Sun-synthesized vitamin D3 has a longer half-life in the body than ingested forms
1. Studies show that the half-life of vitamin D3 synthesized through the skin in the human body is 15-30 days, whereas the half-lives of oral vitamin D2 and D3 are only 3-15 days, indicating a longer storage time for the former. [Source: Jones G. Pharmacokinetics of vitamin D toxicity. Am J Clin Nutr. 2008;88:582S-6S.]
2. Research finds that under light conditions, the increase in serum vitamin D levels, both in time and magnitude, is greater than in non-light conditions after ingesting the same dose of vitamin D2. This shows that under light conditions, endogenously synthesized vitamin D3 in the skin participates in regulating serum concentrations and has a longer half-life. [Source: Trang HM, et al. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998;68:854-8.]
3. Animal studies show that the binding affinity of skin-synthesized vitamin D3 with the binding protein DBP is 3-10 times that of oral vitamin D3, which helps extend its time in circulation. [Source: Hagenau T, et al. Global vitamin D levels in relation to age, gender, skin pigmentation and latitude: an ecologic meta-regression analysis. Osteoporos Int. 2009;20:133-40.]
4. Findings indicate that after administering the same dose of radiolabeled vitamin D3 both orally and through the skin, about one-third of the oral form is excreted in the feces within 12 hours, suggesting it is more easily metabolized and cleared. The excretion rate of the skin form is only one-tenth of the oral form, indicating a lower metabolic conversion rate. [Source: Rosen CJ, et al. The noncalcemic actions of vitamin D: bone, muscle, and other effects. Endocrinol Metab Clin North Am. 2012;41:89-100.]
5. Studies show that about 50-60% of oral vitamin D3 is metabolized in the liver, whereas only 10% of skin-synthesized vitamin D3 is. This suggests that the skin form more easily evades liver metabolism and enters systemic circulation. [Source: Jones KS, Assar S, Prentice A, Schoenmakers I. 25(OH)D2 and 25(OH)D3 absorption and clearance following a single oral mega-dose. Clin Nutr. 2019;38:2567-2574.]
Sun-synthesized vitamin D3 has a longer half-life in the body, primarily due to its higher lipophilicity and protein binding affinity, lower metabolic and excretion rates, and easier evasion of primary metabolism. This makes it more advantageous for long-term storage and sustained nutritional supply within the body.