What you put inside your body reflects on the outside. Your skin, your hair and nails can tell how well or poorly you're feeding your body. Too little or too much of a nutrient can affect your health. An imbalanced diet can cause your nails to become brittle and dry, and hair to break or fall outBoth hair and nails are made from the same protein, called keratin. Several specific nutrients including protein, biotin, and vitamin A are essential for keratin synthesis. A diet filled with foods rich in these nutrients can help promote keratin production in your body and keep your hair and nails healthy and strong. An imbalanced diet can affect them both. Besides diet, other unhealthy lifestyle choices can also have an impact on hair and nail health. For example, smoking can reduce circulation and cut down on the amount of biotin in the blood. Here are some foods/diet mistakes that may be responsible for your brittle nails or thinning hair.