為什麼曬太陽有好處?

曬太陽, 特別是冬天曬太陽,對身體有很多好處，這應該是由於我們需要吸收太陽光的能量。

大家可能覺得，植物才需要直接吸收太陽光的能量，人只要吃飯就行了。人身上大部分能量確實都源於食物，食物分解後，最終為人直接利用的能源是ATP。但是ATP有一個問題，單分子的能量較低。

人體內部有一些微量分子，缺少它們不行，這些分子統稱維生素,其中一種叫維生素D.研究表明，人體內部，維生素D需要陽光中的波長為300納米的紫外線參與才能製成。為什麼最常用的能源ATP不行呢？因為這種紫外線的單個光子能量比單個ATP能量高出十多倍，如果使用ATP，就需要很多中間過渡的化學反應，效率低多了。

那麼，維生素D和少數其它得到研究證實的產品是否是得益於陽光的僅有產品？應該不是。如果人體利用高能的紫外線製造一種產品，必定也會用來製造很多其它產品，只是研究人員還沒有發現罷了。

冬天天寒地凍，細菌大都凍死了，為什麼人們還容易得病？應該是人體的免疫系統變弱。免疫細胞都是高能量的戰士，很可能需要高能量的紫外線幫着製造。冬天曬太陽少了，導致免疫細胞供應不足。冬天去打流感疫苗防流感，效果可能不如多曬太陽，以增強自身的免疫力。

當然能量高的紫外線，對人體細胞的殺傷能力也強。夏天就不用特地出門曬太陽了。但是冬天，還是要多曬太陽。

下面是英文版，更具體一點。

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.