Fingernails can curl from a B12 or iron deficiency.
More Info: One of the medical terms for curled fingernail is "koilonychia", more commonly known as "spoon nails." The main cause of this is iron deficiency anemia, or a lack of enough B12. This vitamin is found generally in meat products, not fruits and vegetables. So some vegetarian and vegan eaters, if they are not adjusting their food intake with proper supplementation, may be more likely to experience "spoon nails" than meat eaters.
There are a number of other, common sense reasons why a person's fingernails might be curled. These include: the use of nails to pick, pry and poke things, especially by men when working around the house; allowing nails to grow beyond a reasonable length and failing to cut them; wearing shoes that are too tight; not keeping fingernails or toenails properly moisturized.
The extension of fingernails with artificial (acrylic) extensions is not something that tends to curl the base portion for each finger. Rather, the main threat in that case is infection. Infections typically take root if the gap between the regular portion of the nail and the artificial section is improperly bridged and sealed. This typically happens when the work is done in unlicensed or low-level nail salons.
Longest Fingernails in the World
There is no better example of curled nails that the ten finger tip keratin coverings sported by American Lisa Redmond. Because she has not cut her fingernails since 1979, Redmond is now the Guinness world record holder for longest fingernails.
It has actually taken careful care and manicuring for Redmond to achieve her world-record feat, which includes a right-thumb fingernail length of two feet, 11 inches. The photos depicting Redmond's nails are astonishing, a whirl of curls that look more like she is performing with a set of fluttering ribbons. When Redmond's nails were measured at a special event in Madrid in 2008, the total length of her ten fingernails exceeded 28 feet.