i am on fasting today vs i am on fast today

Some examples from the web:

• Do you think Bethenny Frankel's fasting today?
• One of the hunger strikers, after 44 days of fasting, has today been transferred to the Hôtel de Dieu Hospital in Paris in extremely poor health after losing 24 kilos.
• See how long they can go on fasting.
• The thoughtless child that has decided on fasting has started a juvenile war?
• She's fasting for my long life today
• We are gathered here today to celebrate, by the ancient rite of hand fasting, the union of Arthur Pendragon and Princess Elena of Gawant.

Some examples from the web:

• i am on fast today
• Crush injury with liver hematoma on FAST scan.
• Several algoritms with papers on Fast kernel density estimation, Improved Fast Gauss Transformation, and Fast ranking.
• It's on fast-forward, Sweets.
• This guy binged on fast food before he ordered more.
• (sarcastically): You two catch on fast.
• Helicopters coming in, gunships, ninjas on fast ropes.
• Yes, you catch on fast.
• MenuetOS development has focused on fast, simple, efficient implementation.
• Lives on fast food row on third.
• And her power comes on fast...
• The screen will be updated often, this is only useful on fast terminals.
• Type 17 point cut is optional for customer with requirement on fast material engagement.
• So much on fast. Know what you find.
• And then couldn't wait to put your suit on fast enough.
• In an interview with Box Office, Lin revealed that he had, after discussions with Diesel, storyboarded, previsualized and edited a twelve-minute finale for Fast Six before filming was completed on Fast Five.
• My husband grew up on fast food He'll eat anything
• Since it came on fast, you have the acute version.
• When I wake up, I watch the footage on fast forward.
• And the plague is coming on fast Ready to kill-
• They're going to lose a lot of money on fast women and slow nags.