It's a complex question, and of course the answer is not simple.
However, there is wide consensus among the experts that we live in increasingly obesogenic environments. That is, environments that make it easy for us to get fat - where making a healthy choice is harder than making an unhealthy choice.
Recently I reread the report from the World Health Organisation Commission on Ending Childhood Obesity, chaired by Sir Peter Gluckman, the Prime Minister's chief science advisor.
The report details what needs to happen to address this urgent, global issue. Its recommendations are comprehensive and the language is firm.
"The greatest obstacle to effective progress on reducing childhood obesity is a lack of political commitment and a failure of governments and other actors to take ownership, leadership and necessary actions," it says.
For years health experts in New Zealand have called on the government for comprehensive action on obesity, meaning a combination of legislation, education and community-based initiatives. It's fair to say there has been a muted response.
And so it is left to non-government groups to take action. Fortunately, some good things are happening.
One of these, about to start, is the Empower programme, a partnership between two non-profit groups, Life Education Trust and Garden to Table - for which I'm proud to be an ambassador.
The programme's goal is to fight childhood obesity, through a combination of nutrition education and hands-on learning in the garden and kitchen.
This has the potential to instil lifelong knowledge, skills and habits in 32,000 kids in 170 schools around NZ.
Tellingly, one of the Who's recommendations for education is about teaching kids to cook.
Giving kids these skills - not just what's healthy but also how to make healthy happen - has the potential to create a healthier generation, maybe one with more years in its life and life in its years than ours.
Niki Bezzant is editor in chief of Healthy Food Guide.