Guajava - Guajava demonstrates antifungal, antibacterial, anti-inflammatory, laxative, and analgesic properties. It is used to treat herpes ulcers, ringworm, dermatitis, and urinary tract infections. Guajava is related to cassia and senna, and contains anthraquinones found in those other species as well.
Vitamins B, particularly biotin (vitamin B7) and B12 are important to keep healthy nails. If your nutrition lacks in vitamins B, nails will be dry and brittle. Vitamin C deficiency is associated to brittle nails and may cause infections of nail tissues.
Estrogen, among its other functions is responsible for the moisture retention in the body. Less estrogen causes the skin to dry. Moisture: soaking the nails in water softens the keratin. If you work requires you to have wet hands, it is probably the main cause of your brittle nails. For the same reason, biting nails causes brittle nails. The nails are touched by saliva many times during the day and remain moist most of the time.