为什么晒太阳有好处?

晒太阳, 特别是冬天晒太阳,对身体有很多好处，这应该是由于我们需要吸收太阳光的能量。

大家可能觉得，植物才需要直接吸收太阳光的能量，人只要吃饭就行了。人身上大部分能量确实都源于食物，食物分解后，最终为人直接利用的能源是ATP。但是ATP有一个问题，单分子的能量较低。

人体内部有一些微量分子，缺少它们不行，这些分子统称维生素,其中一种叫维生素D.研究表明，人体内部，维生素D需要阳光中的波长为300纳米的紫外线参与才能制成。为什么最常用的能源ATP不行呢？因为这种紫外线的单个光子能量比单个ATP能量高出十多倍，如果使用ATP，就需要很多中间过渡的化学反应，效率低多了。

那么，维生素D和少数其它得到研究证实的产品是否是得益于阳光的仅有产品？应该不是。如果人体利用高能的紫外线制造一种产品，必定也会用来制造很多其它产品，只是研究人员还没有发现罢了。

冬天天寒地冻，细菌大都冻死了，为什么人们还容易得病？应该是人体的免疫系统变弱。免疫细胞都是高能量的战士，很可能需要高能量的紫外线帮着制造。冬天晒太阳少了，导致免疫细胞供应不足。冬天去打流感疫苗防流感，效果可能不如多晒太阳，以增强自身的免疫力。

当然能量高的紫外线，对人体细胞的杀伤能力也强。夏天就不用特地出门晒太阳了。但是冬天，还是要多晒太阳。

下面是英文版，更具体一点。

How sunlight affects our health?

Exposure to sunlight, especially in winter time, is beneficial to health. But the extent and mechanisms are not entirely clear.

In the case of vitamin D, we have more knowledge. Sunlight with wavelength 300nm, which is in the ultraviolet zone, participates in the chemical reactions that produce vitamin D. The most universal energy source inside human body is ATP. Why our body doesn't rely on ATP to produce vitamin D? Each photon with wavelength 300nm has more than ten times of energy than an ATP molecule can release. Suppose human body utilizes ATP to build vitamin D. It could take many steps, if it can construct these intermediate chemical reactions. So utilizing UV light is a great way to tap into solar energy when our bodies need high energy input in chemical reactions.

Are vitamin D and few other substances confirmed from the research the only products that require high energy input in their production in our body? It is highly unlikely. Probably many chemical processes utilize high energy UV light as well. If this is the case, there can be some significant lifestyle and health care implications.

In cold winter weather, it is difficult for bacteria to survive. Yet people get sick easily in winter time. This indicates our immune system becomes weak during winter. Since immune cells are energetic warriors against invaders, it is possible that some immune cells are produced with the help of high energy UV light. Hence the reduced exposure to sunlight may reduce the production of immune cells and other cells. This suggests several research questions.

During winter time, we are often advised by the health authority to have flu vaccine. Would promotion of regular sunlight exposure a more effective means to reducethe probability of flu and other illness?

Health authority often recommends the intake of vitamin D. Would a recommendation of regular sunlight exposure provides a more systematic way to obtain vitamin D and other necessary materials? Sunlight exposure probably provides more health benefits than ingesting individual supplements.

For people who are sick, should regular sunlight exposure become part of the treatment as a means of boosting immune system? This may be especially helpful in winter time in northern areas. It is also possible to make the treatment more targeted. The production of vitamin D requires UV light with wavelength of 300 nm. The production of other needed molecules may require light of specific wavelengths as well. One way to identify the required wavelengths is to record the wavelengths of the emitted light during the decomposition of these molecules. These wavelengths may provide clues for determiningthe required wavelengths in producing the needed molecules.

When we stay in sunshine in winter, our mood improves. This psychological response suggests sunshine provides great health benefits. We shall do more research to further explore the mechanisms of these benefits and utilize such benefits.

Appendix:calculation of the energy levels of ATP and uv light of 300 mm,which produces vitamin D

ATP -> ADP + 30kJ/mol

For each ATP molecule, it is 30kJ/(6*10^23) = 5*10^(-20)J

The energy of a photon is e= hv, where h is the Planck constant, roughly 6*10^(-34), v is the frequency of the light. The UV light that reacts with cholesterol is of wave length about 300nm, which is 300*10^(-9)m = 3*10^(-7)m. The speed of light is c = 3*10^8m/s.The frequency v =c/wave length = 3*10^8/(3*10^(-7))= 10^15.

The energy of a single UV photon with wave length of 300nm is e = hv =6*10^(-34)*10^15 = 6*10^(-19)J, which is more than 10 times higher than the energy provided by an ATP molecule.