If you consumed the foods and drinks that celebrities plug — such as Pepsi (Britney Spears) and Coke (Justin Timberlake) — you'd be headed for some serious health problems.

One investigation by researchers at NYU Langone Medical Center found that 71% of the beverages they advertise have sugar added, and more than 80% of the food products they recommend are nutrient-poor.

It can be hard to tell when you're nutrient-poor. But there are some telltale signs that you need to see your doctor for a blood test to evaluate your nutritional well-being. These include:

• Brittle hair and nails. That may indicate an essential fatty acid deficiency. Aim for more omega-3s from salmon, sardines, walnuts, and flaxseeds; omega-6 from walnuts, cashews, almonds, and soybean oil; and omega-9 from vegetable and seed oils, seeds, and nuts.

• Brain drain. That can be caused by a lack of omega-3s, which are essential for cognitive function.

• Digestive problems. These may come from a lack of fiber, which is found in whole grains and fruits and vegetables.

• Aging skin. Those extra wrinkles may come from a lack of vitamins A, C, D and E, according to a review of data published in the journal Dermatology-Endocrinology. 

Supplements can be lifesavers — literally. But your first line of defense against poor nutrition should always be to get your vitamins and minerals from food.

Let your doctor guide you about needed supplements after you get that blood test.