Your entire body - including your nails - benefit from a balanced diet with adequate protein. But preliminary evidence suggests that the B-vitamin, biotin, can help weak and brittle nails.  A study showed that supplemental biotin at 2.5 mg/day for six months strengthened nails significantly.  No side effects have been reported for biotin in amounts up to 10 mg/day because, like other water soluble B-vitamins, excess amounts pass in the urine.  Biotin is in egg yolks, wheat bran, yeast, and liver, as well as synthesized by bacteria in the intestines. And do beware of environmental causes of brittle nails: excess water exposure and harsh beauty products.  Moisturize your hands and cuticles several times a day.