"I think the challenge for me was really seriously thinking about 'am I going to be around to enjoy the next 40, 50 years of my life with my children and grandchildren?' Obviously I'm getting close to my 50s now and that's a big question," he said.
"I've put a long term goal in place that by the time I'm 50 - which is in 2019 - I want to do an Ironman. I want to do the Coast to Coast before I'm 50."
Clarke and his wife Siala had tried other diets before and had talked about going vegan.
But it was a conversation with a colleague - another Pacific Island man who is a vegan - that really sparked his interest.
"It kind of really resonated with me. Meat is something to me that's been a big part of my diet for so long [and] what I really needed a complete change."
Siala agreed it might be the right lifestyle for their family and the couple decided to make the change today - the first day of World Vegetarian Month when animal rights charity Safe's Eat Kind challenge, which encourages people to embrace a plant-based lifestyle, begins.
When they broke the news to their kids the older three looked at them with "huge bewilderment".
"[They were] thinking 'wow' - with this look of 'are you kidding?' That's a complete 180 turn around," Clarke said.
With help from his vegan colleague and a nutritionist, Clarke and Siala came up with a meal plan this week.
The couple have also signed up to be Safe ambassadors, who speak at media and attend events on behalf of the charity and speak out about issues affecting animals.
Their adult children - 22-year-old Shekynah, Eroni Junior (EJ), 19, and Caleb, 18 - had chosen not go vegan, but the two younger boys Jireh, 8, and Zion, 6, would be eating plant-based meals with their parents.
Although cutting out meat would be a big change, Clarke was excited about the new foods he'd be trying.
"The biggest comfort I think too is that I'm not doing this alone. It's my wife and I that are doing this together and she's usually the chef of our family, so that might help things out as well," he said.
"It's going to get both of us looking at new types of menus and the way that we cook and that we prepare and other things like that. I think that's going to be the real cool thing for us - is the experimentation of foods that I've never tasted before."
While they were certain to get some raised eyebrows and questions when they told their friends and whanau about going vegan, the people they'd told so far - including some of his rugby mates - had been supportive, Clarke said.
"There's that respect. They understand and they know we all need to make changes."
What's on the menu
Breakfast: 3/4 cup of oats , 2 scoops of protein powder (Vegan), 1/2 cup of berries.
Morning snack: 250ml coconut milk, 2 scoops of protein powder & banana
Lunch: 250g of Tofu or 3 x Vegan Sausages, 200g vege salad, 200g Kumara or 1/2 cup rice.
Afternoon snack: 2 macaroons, 20 almonds, 4 rice cakes
Dinner: Same as lunch with 30g of mozzarella on tofu.
Evening snack: 3 tablespoons Grawnola, 3 tablspoons oconut yogurt, 2 scoops of protein.
