Magnesium Intake and Its Main Food Sources in 15-18-Year-Old New Zealand Schoolgirls

Abstract

Background: Magnesium is an essential mineral. Low intakes have been widely found among dietary surveys in many industrialised countries. Magnesium intakes were not reported in the most recent New Zealand Adult Nutrition Survey (ANS 2008/09), and in the 1997 National Nutrition Survey (NNS) low intakes were consistent with that of other countries. Thus, magnesium intakes and food sources in New Zealand are not well characterized. Objective: The overall aim is to investigate dietary magnesium intake and its main food sources within the female adolescent (15-18-year-old) population of New Zealand (NZ). Methods: The Survey of Nutrition Dietary Assessment and Lifestyle (SuNDiAL) project is a nation-wide study investigating the intakes of vegetarian compared to non-vegetarian girls aged 15-18 years. The data presented in this thesis are from the preliminary round of collection. A convenience sample of 15-18-year- old girls was recruited from 8 different high schools within NZ. A team of 17 Master of Dietetics (MDiet) candidates recruited participants from each school with 2-3 MDiet candidates within each location. Anthropometric data (height, weight and ulna length) as well as two non-consecutive 24-hour recalls using a multiple pass system were collected, with BMI Z scores then calculated. Participants were required to complete a series of online questionnaires to collect demographic, health, dietary intake and supplement use information. 24- hour recall data were entered into the nutrient analysis programme FoodWorks 9 and the Multiple Score Method was used to generate estimated usual intakes. Food group analysis was used to describe the main food sources of magnesium. Results: There were 145 participants who signed up from the 8 schools with an average age of 16.7 years. Of these 145 participants, 130 had both weight and height taken with 34.6% of these participants categorised as being overweight or obese. Dietary intake data were available for 132 participants with a mean energy intake of 8066kJ/day (95% CI 7754, 8379kJ/day). Magnesium intake data were slightly positively skewed and thus median intake of 280mg/day (10th percentile of 175mg/day and 90th percentile of 428mg/day) was used for this analysis. An inadequate intake level (percent below EAR) of 58.3% of participants was found. The main food sources of magnesium were bread, grains and pasta, vegetables and fruit with a mean percent contribution of 9.5, 9.4, 7.5 and 7.5% respectively. Of the girls who completed the supplement questionnaire, 4% reported taking a magnesium containing supplement daily, with an average dose of 177mg/day. Conclusion: Data from this thesis appear to be congruent with other national surveys and convenience samples from around the world showing a mean dietary intake of magnesium below the EAR of 300mg/day, with most countries between 200-300mg/day. Most of the dietary magnesium in these other countries also came from plant-based products with some from milk, poultry and other animal-based products. Possible options to rectify a suboptimal intake include promotion of: magnesium containing supplements; magnesium rich foods; and fibre rich foods to indirectly promote magnesium intake. This research indicates further investigation is warranted to assess the magnesium intakes of this population due to these preliminary data showing a low intake of magnesium in 15-18-year-old girls in NZ.