Jennifer Aniston was once forced to eat a Big Mac.

The health-conscious star still shudders at the memory of having to eat a greasy McDonald's burger, which she claims wreaked havoc on her ''purified system''.

She told New York magazine: ''I'll never forget when [fiance] Justin [Theroux] and I were on a road trip and we were so hungry. The only thing around was McDonald's. I think I ordered a Big Mac.

''Wow, my body did not react well to that! It was like putting gasoline in a purified system.''

The former 'Friends' actress - who is expected to tie the knot with Justin this year - has followed a strict all-organic diet for years and was left queasy after her junk food slip up.

Jennifer said: ''I am always trying to eat organic and natural foods, so that just made my stomach turn and made me feel terrible. And I think what you put in your body, as well as stress, is reflected in the quality of your skin.''

The 44-year-old star shuns all processed foods in a bit to keep her shiny locks and her skin in good condition for her cosmetics deals, which include Aveeno skincare and Living Proof haircare.

Jennifer has previously said her best beauty tip is lots of rest and drinking water, explaining: ''My best tip is sleep. Honestly. Hydration drink a lot of water - and get your sleep.''