[1] Victoria Univ Wellington, Sch Biol Sci, POB 600, Wellington 6140, New Zealand
[2] Massey Univ, Coll Hlth, Palmerston North, New Zealand
[3] Univ Otago Wellington, Dept Med, Wellington, New Zealand
[4] Univ Southampton, MRC, Lifecourse Epidemiol Unit, Southampton, Hants, England
[5] Victoria Univ Wellington, Grad Sch Nursing Midwifery & Hlth, Wellington, New Zealand