Before heading to your next manicure, take a moment to inspect your nails. Are they dull or do they have horizontal ridges? Either may be a sign of a protein deficiency or malnutrition, says Kate Kanner, a registered dietitian with Fairfield Medical Center in Lancaster, Ohio. Horizontal grooves may also develop as a result of an acute illness, such as pneumonia, she notes. By comparison, nails with vertical ridges are fairly normal, especially as we age, says Kanner.