Measures

Hyperuricaemia was defined as serum uric acid level ≥7·0 and ≥6·0 mg/dl in men and women, respectively. The method of detecting serum uric acid level was described elsewhere⁽²⁴⁾; serum uric acid concentration was detected on a Beckman Synchron LX20 or Beckman UniCel® DxC800 Synchron after oxidisation with the specific enzyme uricase to form allantoin and H₂O₂. The detailed measurement methods for weight and height are available in https://wwwn.cdc.gov/nchs/data/nhanes/2011-2012/manuals/Anthropometry_Procedures_Manual.pdf. The detailed measurement methods for blood pressure are available in https://wwwn.cdc.gov/Nchs/Nhanes/2011-2012/BPX_H.htm; Dietary vitamin A intake in retinol activity equivalents (mcg) was the main measure of retinol intake and was calculated through two 24-h dietary recall interviews. The first dietary recall interview was collected in-person in the mobile examination centre, and the second interview was collected by telephone 3–10 d later. Sources of dietary retinol were identified using US Department of Agriculture Food and Nutrient Database for Dietary Studies (2009–2010, 2011–2012 and 2013–2014, respectively). Intakes of animal-derived retinol (milk and milk products; meat, poultry, fish and mixtures; eggs) and plant-derived retinol (legumes, nuts and seeds; grain products; fruits; vegetables; fats, oils and salad dressings; sugars, sweets and beverages) were calculated according to the food code. As for ‘animal-derived retinol’, 1 mcg retinol activity equivalents = 1 mcg all-trans retinol from animal foods. To calculate retinol activity equivalents for ‘plant-derived retinol’, we used the equation: retinol activity equivalents for plant-derived retinol (mcg) = 1/12 beta carotene (mcg) + 1/24 other provitamin A (mcg).