If your membership was started through the Diet Doctor Eat app via the App Store using a mobile device, then you must cancel the membership either through your iTunes account.
Canceling via your iTunes account
Please visit https://apple.co/2Th4vqI to see the status of, or cancel an active subscription.
Request a refund from Apple
You may ask Apple directly for a refund if the subscription is not in the free trial period: https://support.apple.com/en-us/HT204084