Folk medicinal sciences such as Ayurveda differ there. According to Ayurveda, a man has particular amount of the semen and it decreases as the man grows old. It is therefore, an old man may not produce the semen. An amount of semen is sufficient to pass adolescent, young and middle age. But, there are many reasons that can decrease the quantity of the semen and then once the semen quantity has started decreasing, a man feels weak and there could be many sexual problems including erectile dysfunction, premature ejaculation and in severe cases, the sterility.
In Ayurveda, it is mentioned that one has to protect the semen and should eat some foods and herbs that can increase the semen. Herbs such as Indian gooseberry (Aamla), Asparagus (Shatavari), Guduchi, Kuchala (Mucuna), Gokshura (Tribulus) etc have been practiced to increase the libido and so the semen in the men. They can be taken as single herb therapy or you can go for some formula that contains the blend of those herbs. Many herbal pharmaceutical companies manufacture such herbal pills that can be taken as directed by physician.
Ayurveda suggest going for those all edible items that are somewhat sticky (such as okra, eggplant etc.) since they increase the semen quantity and quality. Some of the fruits like mango and banana also increase the semen according to Ayurveda. Ayurveda favors taking Shatavari (an Ayurvedic herb, available in granules nowadays) along with milk and very little sugar. Taking this daily at night will increase the sexual power and also the semen quantity.
Dark green leafy vegetables, their salads, fresh fruit juices etc are also known for their efficacy increasing the vitality and so the virility in a man. Some modern Ayurvedic scholars favor taking eggs and fish oils as two of the most effective foods for increasing the semen in a man. Latest researches and clinical trials reveal that some of the holy exercises such as Yoga and Pranayama are very beneficial treating various sexual disorders including low libido. Meditation is another holy exercise that involves the mental stability and hence can help restoring the hormonal levels and enzymes in the body.
Along with such diet and herbal supplements, one should take enough rest and should not wear any tight inner wear, especially tight underwear. Tight clothes restrict the blood circulation in reproductive organs and hence may decrease the libido and so the semen. One should also give up over masturbation that decreases the semen quantity in significant way as said in Ayurveda.