Kalantar-Zadeh compares the lack of consensus about weight and fat to the evolution of thought about alcohol consumption.

Alcohol was thought to be detrimental to a person's health, but studies started to show that alcohol consumption in moderation could have some health benefits, he said.

"The fat-is-bad principle is a very recent approach," he said. "Body-stored fat has helped us for hundreds of thousands of years to survive hardships. That should tell us evolutionarily there was something good in that."

A higher body mass index can be protective in certain situations, he said.

"Once you are in your 70s, 80s or 90s, or if you have chronic disease like heart failure, rheumatoid arthritis, chronic lung and kidney disease, a larger body size gives you longevity," he said.

Not all fat is bad, but belly fat is more harmful than fat in the arms, legs or buttocks, he said.

Fitness expert John Siracuse said people shouldn't get caught up with numbers.

"I always got people focused on their bodies rather than on a number and make them more aware of their muscle tissue, their shape," he said. "Are they getting stronger? Faster? Can they pedal longer? If you listen to your body more, you will know the symptoms before your body starts to break down. We tend to forget about our bodies and that's when we start getting fat."

Servoss said she can see where a little bit of extra fat could be good for people facing a serious illness, but said the more weight you have, the harder it is on your joints. Excess weight also comes with numerous health challenges, including circulatory problems, high blood pressure and diabetes, she said.

"I think the numbers have their place," she said. "They do give us some reference but it does ultimately come down to how you are feeling, your exercise tolerance, are you able to do the things you love to do without any difficulty?

"That's the kind of thing I should be focusing more on, rather than that my jeans come from the back of the rack rather than the front," she said.