Our nails can reflect our general state of health. No one wants to have weak brittle nails. Yet having brittle nails, also known as onychorrhexis, is a very common condition affecting approximately 20 percent of the population and is more common in women.
Human fingernails and toenails are primarily composed of a protein called keratin and also contain an array of minerals. Keratin is an extremely strong protein.
Like other proteins, keratin is composed of amino acids. Keratin has large amounts of the sulfur-containing amino acid cysteine. Cysteine is required for giving nails their strength and rigidity.
The mineral composition of a human nail includes: calcium, copper, iron, magnesium, sodium, sulphur and zinc. A deficiency in any one of these minerals can affect the health and growth of our nails.
Aside from having a deficiency in the building blocks of nails, brittle nails can be caused by a variety of different things some of which include: chronic dehydration, a deficiency in omega-3 fatty acids and a biotin deficiency. 
Several chronic health conditions can also cause brittle nails, including: anorexia, bulimia, iron deficiency anemia, and underactive thyroid. Furthermore, psoriasis has been linked to brittle nails. Brittle nails can even be a side effect of certain pharmaceutical medications.
And let’s not overlook the fact that nails which are excessively exposed to water, soap or solvents can also become brittle.
If you are wondering why some of your nails grow slower than the rest, there is no need to worry. In humans, the nail of the index finger may grow faster than that of the little finger and fingernails may grow up to four times faster than toenails.
Provided you do not have a medical condition, keeping well-nourished with all the building blocks that nails need can result in strong healthy nails. A balanced diet composed primarily of a variety of unprocessed whole food is a great start.
To take your nails to the next level try supplementing your diet with a high quality protein powder, a good multi mineral complex, biotin and omega-3 fatty acids.
 Rapini, Ronald P.; Bolognia, Jean L.; Jorizzo, Joseph L. (2007). Dermatology: 2-Volume Set. St. Louis: Mosby. ISBN 1-4160-2999-0  http://www.wisegeek.org/what-is-keratin.htm  https://www.ncbi.nlm.nih.gov/pubmed/20620759  http://www.ncbi.nlm.nih.gov/pubmed/8477615 [5[ James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0  Ravosa, Matthew J.; Dagosto, Marian (2007). Primate origins: adaptations and evolution. Springer. pp. 389–90. ISBN 0-387-30335-9