Hi, Welcome to HCM.
Pigmentation can occur in any part of the body. Elbows, ankles, inner thighs are the most common places. The causes can vary from the skin sensitivity, external climatic condition, sweaty inner areas, hair growth and acne
prone area etc. There is absolutely no connection between the diet and the pigmentation. Check on the exercise routine if it makes your thighs sweat or makes it rubs together which can be a reason. Also there can be other physiological reasons why the colour is darkened. Continue your diet and drink lots of water throughout the day. Try not keeping your thighs too close and leave it open whenever possible to allow for aeration. Also, please consult a dermatologist
if the situation persists as there are treatments to remove pigmentation as well.