The aim of this analysis was to examine the selection, prevalence, and relative nutritional contribution of dairy and calcium-fortified foods in the diets of a cohort of adolescent girls. Although this was a randomized controlled intervention trial with one group (HC) receiving regular and explicit information to increase dietary calcium, the food sources of calcium were nevertheless independently selected by each subject based on personal preference. Therefore this analysis is useful in quantifying the impact of calcium-fortified foods in self-selected diets within the context of regular dietary counseling to maintain a high calcium intake, and monetary reimbursement to cover the cost of the high calcium foods. In contrast, the lesser impact of calcium-fortified foods as selected by the UC group can be considered to be more representative of the general population of adolescent girls consuming their usual diets.
The mean calcium intake of the UC group was approximately 75 mg/d lower than the 2005–2006 NHANES national average of for this age group (12
). However, NHANES data include calcium consumed as dietary supplements (23
), not included in the present analysis because none of our subjects reported consumption of supplemental calcium. Irrespective of the absolute calcium content of the HC and UC self-selected diets, both groups were consistent with data reported by others showing 65–70% of total dietary calcium supplied by dairy foods (24
). 17% to 25% of total dietary calcium was contributed by cheese in the HC and UC groups, a finding that is also consistent with national survey data for adolescent girls (25
). Thus, calcium-fortified foods did not appear to displace dairy foods in the diets of our study population.
The top contributor of calcium from a calcium-fortified food source for the total cohort was calcium-fortified RTE breakfast cereal. RTE breakfast cereal has been a vehicle for nutrient enrichment and fortification for decades (15
). Calcium-fortified RTE breakfast cereal contributed approximately 2–3% of the mean daily calcium in the UC group, consistent with NHANES 1999–2002 national survey data (27
At equivalent 300 mg calcium loads, the calcium-fortified nutrient profile differed from the nutrient profile of the dairy foods. A greater contribution of energy, carbohydrate, added sugar, and sodium and a lesser contribution of protein, phosphorus, potassium and vitamin A from calcium-fortified composite was found. We also found that the calcium-fortified composite reflected a higher contribution of magnesium and vitamin D, and a lesser contribution of total fat, relative to an equivalent 300 mg calcium load from the dairy composite. There are two potential explanations for this occurrence. Both magnesium and vitamin D were supplied in the calcium-fortified composite by RTE breakfast cereals and fruit juices, many of which were fortified with vitamin D, naturally occurring sources of magnesium, and low in fat. Secondly, the dairy food composite reflected a proportional contribution from cheese representing 25–35% of the total dairy calcium. Relative to fluid milk, cheese has a lower magnesium content and a higher fat content and, unlike milk, is not fortified with vitamin D. Consequently, the dairy composite contributed lower values for magnesium and vitamin D and higher values for fat than did the calcium-fortified composite at an equivalent calcium load. A higher fat value, however, did not result in a higher energy value. An increased value for carbohydrate and added sugar in the calcium-fortified composite contributed to an energy value of 211 calories; 36% higher than the 155 calorie energy value of the dairy food composite.
Data from NHANES 2005–2006 indicate that calcium, phosphorus, magnesium, potassium, and vitamin D remain nutrients of concern for teenage girls (12
). The Dietary Guidelines for Americans (DGA) recommendation of three cups of milk and milk products/d was designed to provide 69% of the calcium, 56% of the phosphorus, 26% of the potassium, and 23% of the magnesium recommended for this age group, in addition to 54% of the protein and 61% of the vitamin A (6
). The HC group achieved each of these target nutrient adequacy goals exclusively from dairy foods. Thus, while it is useful to compare nutrient profiles of the calcium-fortified and dairy composites at equivalent calcium levels, such a comparison does not reflect total nutrient intakes or overall dietary adequacy. Due to a preference for dairy foods as a dietary calcium source, the HC group met or exceeded the nutrient adequacy values intended to be achieved by the recommended 3 cups of milk and milk products/d for calcium and each of the nutrients measured.
Implications for Nutrition Education
Calcium-fortified foods have been identified as an alternative calcium source for individuals who cannot or choose not to consume milk and milk products (13
), or as a means of augmenting low calcium intakes (16
). However, studies have shown that individuals who obtain their calcium from non-dairy sources fail to meet the nutrient profile of those who consume dairy foods (6
). This study confirms those findings.
It should be emphasized that calcium-fortified foods do not constitute a sub-group within the milk and milk products food group, and it is therefore inappropriate to position calcium-fortified foods as a nutritionally equivalent alternative to milk and milk products in public health policy or in nutrition education settings. At the present time there is not enough evidence available on the population-wide impact of calcium-fortified foods on overall nutrient adequacy, diet quality, or health outcomes to support recommending these foods as dairy alternatives. Furthermore, foods supplying calcium in amounts equivalent to dairy foods cannot be assumed to deliver equivalent calcium bioavailability. A comprehensive analysis of studies measuring the bioavailability of calcium carbonate as a fortificant reveals a span of mean fractional calcium absorption values ranging from 0.21 to 0.42 (28
). Such disparity compounds the challenge faced by nutrition professionals of accurately assessing the impact of calcium-fortified foods on calcium nutrition.
Nutritionists and health educators should be aware of the total nutrient contributions of calcium-fortified foods relative to dairy foods and tailor nutrition messages to consumers accordingly, addressing total energy intakes and nutrient adequacies beyond calcium. Our data confirm reports from others concluding that incorporating milk and milk products into the diets of adolescent girls is the best way to achieve adequate intakes of calcium and other bone-related nutrients (8
). For those who avoid milk and milk products, dietary guidance must consider strategies for replacing not only calcium but other significant nutrients supplied by milk as well, within appropriate energy levels.
Implications for Nutrition Surveillance
In nutrition surveillance data collected 1989–1991 only 2.5% of the population of teenage girls reported using a calcium-fortified food at least once (15
). Our data, gathered 10–15 years later, identified 96% of the total cohort as having selected a calcium-fortified food at least once. Thus, over the span of one decade, the penetration of calcium-fortified foods in the consumer market has become nearly ubiquitous.
The increased availability of calcium-fortified foods underscores the importance of accurately assessing calcium intakes and the impact of calcium-fortified foods on nutrient intakes and diet quality in the population. A comprehensive national database accurately reflecting available calcium-fortified foods must be continually updated, consistent with changes in the food supply (29
). Dwyer et al cite a critical need for enhanced capability to identify calcium-fortified foods in national surveys and to assess nutrient intakes from fortified food sources (23
). Thus, nationwide calcium intake data may currently underestimate intakes due to erratic market trends in calcium fortification of foods and delays in updating databases (30
Several limitations must be acknowledged. The sample size of our study was small and limited to a regional population of primarily Caucasian adolescent girls. Larger studies are needed to elucidate the impact of calcium-fortified foods on total calcium intakes, diet quality, and nutrient adequacies across the full range of ages and ethnicities, in men and women. Secondly, it was beyond the scope of this analysis to examine the contribution of dairy and calcium-fortified foods on total diet quality and nutrient intake from all dietary sources. Therefore, while our data suggest that calcium-fortified foods do not constitute a nutritionally equivalent substitute for milk and milk products, nutrient shortfalls cannot be predicted unless total dietary intake is taken into account. Nevertheless, the information gleaned from following this group of urban females gives us a better understanding of the food sources supplying calcium in the diets of contemporary adolescent girls and underscores the need to monitor the impact of calcium-fortified foods on total calcium intakes of the U.S. population.