Nails are made up mostly of protein, I think. I think it's keratin. Same kind of protein that hair and the outside (dead) layer of skin are made of, I think. People have recommended eating gelatin to improve the strength of nails, because it is a similar protein, made from the nails and skin, etcetera, of animals. However I don't think there is any evidence that it is any better for your nails than any other protein, animal or vegetable. We break proteins into their constituent amino acids, and then reassemble them. Since the same amino acids figure in different kinds of proteins, it shouldn't make any difference what kind of protein you get, as long as you get all the dietarily essential amino acids. I don't think you need the dea amino acids in "balanced" ratios, either -- just as long as you get enough of all of them. So yes i think a protein deficiency can contribute to cracking nails. I think calcium may be involved, in small amounts, as well as other micronutrients. For some reason my finger nails are excellent, but my toenails tend to be rubbery or something. When I was younger they used to be strong like my fingernails. I think it must be due to wearing socks and shoes. My fingernails are as strong as ever.