The 27-year-old was dropped by Ireland earlier this year and found his Munster place under threat from the talents of Robin Copeland, CJ Stander and captain Peter O'Mahony.
Yet the flanker redoubled his efforts over the summer and dropped four kilograms while retaining muscle mass, working alongside the club's new full-time nutritionist Catherine Norton.
And O'Donnell admits his greater mobility around the field has helped him play a starring role in five GUINNESS PRO12 appearances this campaign, leading to a return to the international arena.
"I just went back, took the summer break and used that as a new starting point. I worked on a lot of the basics, upped my work rate and the intensity that I take to the game," O'Donnell told the Irish Independent.
"You just have go away and work hard, get better, come back in and put your hand up again. That's what I concentrated on doing.
"I definitely feel my fitness has gotten better. I went away, dropped a few kilos, got my body fat down and just worked hard.
"Having Catherine there is very good because she's constantly monitoring what you're eating.
"She'll be there looking over your shoulder to see if it's quality food - was it something you bought from the local Maxol station or was it something that you actually put some time into the night before?"
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