what do I have?

What do you think I have? I've noticed over the years that often when I exercise or run that after about 10 minutes I get very itchy, usually on my thighs where the shorts end, or my arms. When I itch it it burns.  I once asked an aerobics instructor about it and she said something about a possible niacin deficiency? I was wondering if that sounds like heat or sweat rash? Also, My face has always been red and blotchy, I was wondering if that's a sign of rosacea. One more thing since I'm asking about skin conditions, sometimes my neck itches. When I barely touch it, it turns bright red.  

[Dr Santos]

Hi,

Miliaria or sweat rash is a skin disease marked by small and itchy rashes. This is common ailment in hot and humid conditions or activities that may induce sweating. It occurs when the sweat gland ducts get plugged due to dead skin cells or bacteria and leads to  irritation, itching and to a rash of very small blisters most specially to the face, neck, under the breasts or  skin folds, that may rub against clothing, such as the back, chest, and stomach.  This can be  prevented by avoiding activities that induce sweating, using air conditioning to cool the environment,wearing light clothing and in general, avoiding hot and humid weather. On the other hand, Rosacea,  is  a  chronic, inflammatory skin disorder  which presents as  facial redness across the nose, cheeks, chin and forehead.  Potential triggers  include external  factors such as the sun, wind, food or beverages, hot environments, cold environments, vasodilator medications, physical irritation, and even cosmetics or skin care products. Stress, anxiety, strong emotions, and hormone-related conditions  may also lead to this condition. If it persists, have this evaluated by your doctor for proper management. Direct examination is important to determine the diagnosis. Treatment will also depend on the underlying cause.

Take care and keep us posted.