But there isn't any need to live in the gym - slabs of muscle mass can be developed with three or four intense sessions a week targeting different body areas.
"The idea is within one workout you're exhausting a particular area, rather than trying to get through the entire body in one go."
Healthy Food Guide nutritionist Claire Turnbull has worked with the North Harbour Rugby Union and said without a rigid diet gym work was wasted. "The biggest thing I found with the guys who got the best bodies were the ones who were consistent."
That meant meals high in fibre, fruit, vegetables and lean protein.
The All Blacks eat up to 2 grams of protein for every kilogram they weigh a day, which is the maximum and shouldn't be exceeded.
Both Ms Turnbull and Mr Harris said the usefulness of supplements such as protein powders was debatable, and even steroids were no "magic potions". "The higher you get up in rugby, ironically the fewer supplements people use."
Getting as in-shape as Williams would require a rigid discipline out of step with ordinary daily life, Ms Turnbull said. "It's a huge amount of work - and that's his job.
"You have to give up a lot of things to look like that."
Ms Turnbull said genetics and age meant regardless of effort or money spent such a transformation would be beyond some "skinny white men".
But Mr Harris said he had seen plenty of "average mums" and "corporate overweight guys" achieve dramatic change. Still, Williams' 110kg of bulk would look very different attached to a frame shorter than his 1.91 metres.
"Obviously you're genetically disposed to look a certain way, even if you do build muscle tissue."
