The battle against the bulge should start while you're still a toddler, according to an obesity study in Auckland which has spanned nearly two decades.
Researchers have found if obesity can't be curbed before a child turns 4, attempts to shed the fat could prove relatively ineffective later in life.
The University of Auckland study, funded by Cure Kids, has tracked the health of 871 Pakeha from birth to their teens, measuring the impact on obesity of everything from TV, diet, stress and exercise to genetic factors.
With renewed funding from charity Cure Kids, researchers will now revisit each of their subjects - now aged 16 - and collect fresh data with an emphasis on diet and physical activity.
The study has so far found:
Rates of overweight and obesity have continued to increase as the children have become older, with nearly a quarter of the sample either overweight or obese at 11 years.
Higher body-fat percentages in children at 7 and 11 years are associated with their mother's weight and age at time of birth.
Less sleep and the number of hours watching TV affect body weight.
More than half of the children do not eat the recommended daily servings of fruit and vegetables.
Half of the children had been chosen for being small for their gestational age, which has been long thought a key risk factor for obesity and metabolic diseases later in life.
Results from the study so far has challenged this assumption, with many of these children remaining shorter and lighter through life.
Following a pattern of steady weight gain, nearly one quarter of the group had grown overweight or obese by the age of 11, while more than half of the children were not eating the recommended intakes of fruit and vegetables earlier in childhood.
"We are looking for interactions between particular genes and dietary patterns and physical activity," said study leader Dr John Thompson.
"For example, if you have a particular gene and a bad diet, does that make things worse?"
Another crucial question was at what age was obesity most easily curbed. The study had identified 3 years as a deadline.
"There is evidence that exercise and dietary intervention programmes once children and adults have become obese are relatively ineffective," Dr Thompson said.
"Because we've collected information on physical activity and diets over time, what we start to do now is look and see whether it's diet and physical activity earlier in life that is affecting obesity now, rather than just the current diet and physical activity.
"You've got this big cycle where, once you get obese, you stop doing much exercise - and it's a matter of which comes first."
The next round of results would be analysed next year, but Dr Thompson said he was keen to follow the participants through into adulthood, when potential cardiovascular problems could be monitored.
Cure Kids chief executive Vicki Lee said the charity was committed to finding solutions to obesity, which had risen to epidemic proportions globally and was now the fifth leading risk factor for mortality.
The charity had backed the project for more than a decade and she was keen to continue support.
"It's harder and harder to get research funding for these long-term studies, which will actually provide greater outcomes and evidence."