You didn't offer details as to wear the rash is located but I'm going to guess it's either under your arms or in the creases behind your elbow or knees. If the rash is under your arms I'd be suspecious of the deoderant you're using. If that's the case I'd recommend you try using Milk of Magnesia under your arms for awhile. It's actually a natural way to use deoderant. It will not only help keep you dry but it will deoderize and dry the rash as it heals. I know that probably sounds weird but a lot of people that are developing rashes from the chemicals in deoderant have tried it and are getting good results.
If the rash is elsewhere like mentioned above or maybe on your chest you could make a homemade ointment by mixing equal parts of water, glycerine and aloe vera. Just apply is freely and often to relieve the itch.
You asked about a food remedy, I know chamomile is very good for skin rashes, try mixing;
Use 1 teaspoon of this mixture in 1 cup of water, let it steep 5 to 10 minutes and strain. Drink several cups through the day. This herbal mix relieve rashes, boils and acne.
Good luck and thank you for visiting Grandma's Wisdom. I'm always thrilled to hear from visitors from other countries.