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Articles

Fortification Contributed Greatly to Vitamin and Mineral Intakes in the United States, 1989–1991¹

Louise A. Berner^*2, Fergus M. Clydesdale and Judith S. Douglass^**

^* Food Science and Nutrition Department, California Polytechnic State University, San Luis Obispo, CA 93407; Department of Food Science and Nutrition, University of Massachusetts, Amherst 01003; and ^** ENVIRON International Corporation, Arlington, VA 22203

²To whom correspondence should be addressed. E-mail: lberner{at}calpoly.edu

	ABSTRACT

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The objective of this work was to quantify the contribution of fortification (defined here as adding nutrients beyond traditional enrichment standards) to dietary nutrient intakes in the United States. A list of fortified foods was developed that was relevant at the time of the analyses, and prefortification (naturally occurring) nutrients in the fortified foods were determined from industry-supplied data. Using dietary data from the 1989–1991 Continuing Survey of Food Intakes by Individuals (CSFII), intakes of nine nutrients were determined both as reported in the CSFII (i.e., postfortification) and also by using prefortification nutrient levels for the identified fortified foods. We report data for the total population age 1 y based on respondents (n = 11,710) with 3 d of dietary data, as well as select age/gender subgroups. All data were weighted. Fortification substantially increased the intakes of all nutrients examined except calcium, in all age/gender groups but especially in children. In numerous cases, fortification was responsible for boosting median or 25th percentile intakes from below to above the RDA. The breakfast cereal category was responsible for nearly all the intake of nutrients from fortified foods, except vitamin C for which juice-type beverages made as great or a greater contribution. These data from 1989 to 1991 serve as a useful baseline with which to compare contributions of fortification as the practice expands. The large contribution of fortification even in 1989–1991 suggests that continued monitoring of fortification practices, using methods such as those presented here, is important.

KEY WORDS: • fortification • dietary nutrient intakes • CSFII 1989–1991

	INTRODUCTION

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Nutrients have been added to foods in the United States since iodine was added to salt in Michigan in 1922 (1) . In the past, scientists and policymakers have used the terms "enrichment" to refer to the addition of nutrients lost during processing (e.g., adding B vitamins and iron to refined flour) and "fortification" to refer to addition of nutrients at levels beyond those naturally present in the foodstuff (e.g., adding vitamin C to breakfast cereals). Currently, the terms enrichment and fortification are often used interchangeably to indicate simply the addition of nutrients to foods. In this article, however, we generally use the terms enrichment to refer to nutrient additions based on Food and Drug Administration standards of identity and fortification to refer to other voluntary additions of nutrients to foods.

National policy for enrichment of flour was established > 50 y ago, and today there are standards of identity for a variety of foods with added nutrients (such as flour, pasta, rice, milk and salt). For most fortified foods, however, manufacturers are guided only by general principles set forth for the addition of nutrients to food. The current Fortification Policy of the Food and Drug Administration, detailed in 21CFR104 Subpart B, can be summarized in part as follows: 1) there should be a documented need for adding the nutrient; 2) the food to be fortified should be a suitable vehicle to help correct a dietary insufficiency; 3) the nutrient should not be present at excessive levels; 4) the nutrient should be stable under customary conditions of storage, distribution and use; and 5) the nutrient should be bioavailable. Although the original intent of adding nutrients to foods was to address widespread nutrient deficiencies, the picture is different now because many of the formerly epidemic nutrient deficiencies, such as iodine and niacin, are no longer common in the United States (at least in part because of the success of enrichment/fortification programs). At the same time, suboptimal intakes of other nutrients, such as calcium and folic acid, seem to be widespread (2 3 4) . In addition, our focus has shifted from prevention of classic nutrient deficiency diseases to optimization of nutrient intakes for prevention of chronic disease and for overall health and well being (5) , although the appropriate nutrient intakes to achieve these outcomes are often unknown.

Nevertheless, enrichment and fortification have been and will continue to be important means to improve the nutrient intakes of a wide cross-section of the population (1 ,6) . To make good judgments on the benefits of current fortification practices and the opportunities for fortification in the future, it is important to assess how fortified foods contribute to nutrient intakes across the population. In the past, researchers have reported on the impact of adding nutrients to grain products. For example, Cook and Welsh (7) reported that based on the 1977–1978 Nationwide Food Consumption Survey of the U. S. Department of Agriculture (USDA),³ cereal grains enrichment and fortification accounted for 18–32% of the intakes of thiamin, riboflavin, niacin and iron. Enriched flours accounted for approximately two-thirds of the added nutrients, while breakfast cereals contributed approximately one-fifth of the added nutrients at that time. Popkin et al. (6) used the same data and found that enrichment and fortification of cereal foods impacted nutrient intakes across sociodemographic categories. Subar et al. (8) reported that ready-to-eat cereals were among the top 10 food sources for 18 of 27 nutrients that they studied, presumably due to fortification, in the diets of U. S. adults from 1989 to 1991. Years earlier, Block et al. (9) reported that cold cereals were among the top 10 contributors to vitamin A, thiamin, riboflavin, niacin and iron—again, presumably due to fortification. However, these research groups did not report on the specific contributions of fortification versus naturally occurring nutrients from foods. Moreover, we could find no published research on the specific contributions of fortification to a wide range of nutrients and from a variety of food categories. Therefore, the purpose of the study reported here was to quantify the contribution of fortification per se (i.e., the addition of nutrients beyond the traditional enrichment nutrient levels that are part of the standards of identity for numerous foods) to dietary nutrient intakes in the U. S. population.

	METHODS

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Under contract with the International Life Sciences Institute-North America (ILSI-NA), TAS-ENVIRON (now ENVIRON International Corporation, Arlington, VA) carried out the data analyses on which this article is based. Data from the 1989–1991 Continuing Survey of Food Intakes by Individuals (CSFII) were used for this study because they were the most recently available data from a national survey at the time that this research was initiated. ENVIRON worked with ILSI-NA to identify a list of fortified foods. Then, levels of nutrients in diets both pre- and postfortification were calculated based on USDA and industry data, and the contribution of fortified foods to intake of numerous vitamins and minerals was determined. Methods are described in more detail below.

Food consumption data source.

For this project, food consumption data from the 1989–1991 CSFII were used, as provided on CD by the National Technical Information Service (Springfield, VA) (10) . The CSFII results included 3 d of dietary intake data for 11,710 noninstutitionalized people ages 1 y living in the United States, based on one 24-h recall conducted by a trained interviewer and two subsequent days of intake from respondent dietary records. Only respondents with three full days of intake were used for this study. Combined 3-y data were used, and USDA sampling weights were applied so that the data can be considered representative of the U. S. population at the time of the survey. The CSFII 1989–1991 survey has been described in detail elsewhere (10) .

Categories of fortified foods examined.

The first step in these analyses was to identify fortified foods to be examined for their contributions to nutrient intakes. Here, the definition of fortified means addition of nutrients to foods beyond levels specified in standards of identity for enriched foods. In other words, for this study, products such as enriched flour, bread, rice, pasta and milk were not considered fortified because the nutrients added to them are part of enrichment or other regulations. Food codes used during the 1989–1991 CSFII were reviewed to see what fortified foods were included. Table 1 summarizes the four main categories of fortified foods identified for this study. There were 183 different USDA food codes in the breakfast cereal category; 38 different codes for vitamin- and mineral-fortified drinks; 21 codes for meal replacements and supplements; and 4 codes for calcium-fortified beverages. In total, 246 different USDA food codes were included as fortified foods for the purpose of this study.

Total nutrient intakes of nine different nutrients (vitamin A, vitamin C, thiamin, riboflavin, niacin, folate, calcium, iron and zinc) for 16 population subgroups (children: 1–3, 4–6 and 7–10; men: 11–14, 15–18, 19–24, 25–50, 51–69 and 70 y; women in the same age groups; and the population total) were determined. Nutrient data were from the USDA Nutrient Data Base for Nationwide Surveys, Release 7. Some nutrients (such as vitamin E) were not included simply because they were not typically added to foods at the time of the survey.

We have called these nutrient intakes the postfortification nutrient levels, and they represent the estimated intakes based on the three full days of intake data for each respondent to the 1989–1991 CSFII. In other words, postfortification nutrient levels include the naturally occurring nutrients in foods and any nutrients added during processing. Data are presented both as the absolute amount of the nutrient in the diet (e.g., µg retinol equivalents for vitamin A or mg for vitamin C) and as the percentage of the RDA for the given population subgroup. For this project, the 1989 RDA values (11) were used because these were in effect at the time that the survey and analyses were carried out.

Postfortification nutrient intakes were also determined for the subsets of individuals who consumed one or more of the fortified foods from the categories in Table 1 over the 3-d survey. These people will be referred to as users of fortified foods.

Determination of theoretical prefortification nutrient intakes from the diet.

To know how much fortification per se contributed to nutrient intakes, it was necessary to calculate prefortification nutrient levels in the foods identified in Table 1 . ILSI-NA, working with key individuals in the food industry, provided ENVIRON with industry data on prefortification nutrient levels in most of the relevant foods reported as having been consumed by CSFII respondents. For example, ENVIRON provided a breakfast cereal manufacturer with USDA Release 7 data for all nutrients in a defined amount of cereal X. The manufacturer then provided ENVIRON with the prefortification level of any relevant nutrients in the specific cereal X. Differences between the USDA Release 7 data and the prefortification nutrient levels were assumed to be due to fortification.

In a few cases in which no manufacturer data were available, the naturally occurring (i.e., prefortification) nutrient levels were estimated by using a similar product for which industry data were available. For example, USDA has a food code for generic corn flakes; if the code for generic corn flakes was recorded for a CSFII respondent, the prefortification nutrient concentrations in brand name corn flakes were entered instead (with a coded notation that the nutrient levels for the generic cereal had been imputed based on a similar product), because manufacturer data on the prefortification nutrient levels were available only for the brand name cereal.

As for the postfortification nutrient data, the prefortification data are presented both as the absolute amount of the nutrient (e.g., µg retinol equivalents for vitamin A or mg for vitamin C) and as the percentage of the RDA for the given population subgroup. Again, the 1989 RDA values were used (11) .

Contribution of fortified foods to nutrient intakes.

Nutrient contributions specifically from each of the four categories of fortified foods described earlier (and summarized in Table 1 ) were determined for the nine different nutrients and 16 population subgroups. This analysis provides information on the main dietary sources of fortification nutrients at the time of the survey.

Statistical analyses.

As mentioned, USDA sampling weights for the full 3-y CSFII survey were used. For each respondent, data represent the average of three full days of intake. For the total population and the age and gender subgroups, the mean nutrient intakes were determined, as well as the 25th, 50th, 75th, 90th and 95th percentiles of intake from the total diet postfortification and from the total diet prefortification. Here, we present only the 25th, 50th and 95th percentiles of nutrient intakes because of space limitations and because the data we include show both typical and extremes of intake during the 3-d survey. In addition, we present the contribution of fortified foods per se to intakes of nutrients among users of fortified foods. Finally, mean nutrient intakes from each of the four categories of fortified foods were calculated.

	RESULTS

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Nutrient intakes from all foods for the total population 1 y of age are shown in Table 2 . The postfortification intake levels represent the estimated average daily intakes by the population and include both naturally occurring and added nutrients. The prefortification intake levels represent the theoretical intakes if nutrients had not been added to fortified foods. By comparing prefortification and postfortification data, it is clear that fortification substantially increased the intakes of all nutrients except calcium. Few foods were fortified with calcium during the study; in fact, only four USDA food codes were available for calcium-fortified beverages at the time, and breakfast cereals were not routinely fortified with calcium.

View this table: [in this window] [in a new window]   Table 2. Average daily nutrient intakes pre- and postfortification by total population 1 y of age, based on CSFII 1989–1991123

View this table: [in this window] [in a new window]   Table 3. Average daily nutrient intakes pre- and post fortification by total population 1 y of age, based on CSFII 1989–1991, as a percent of 1989 RDA123

	DISCUSSION

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The data reported here are consistent with earlier published reports of the major impact of fortification and/or enrichment programs on nutrient intakes, based on data from the 1977–1978 Nationwide Food Consumption Survey (6 ,7) . Fortified breakfast cereals, in particular, have been shown to contribute significantly to intakes of a variety of nutrients (8 ,12 ,13) . In a comparison of data from 1987 and 1992 National Health Interview Surveys, Norris et al. (13) found that fortification of breakfast cereals was entirely responsible for preventing a decline in intakes of vitamin C, folate and vitamin E from 1987 to 1992. McNulty et al. (12) studied Irish schoolchildren and found that subjects who did not consume fortified breakfast cereals had inadequate intakes of several B vitamins and iron, but cereal consumers had intakes in line with recommended daily intakes.

We should note that most analyses, including ours, only consider nutrient levels in foods and cannot address issues like bioavailability of naturally occurring versus fortification nutrients. In addition, when these analyses were performed, the dietary folate equivalent concept had not yet been articulated, and nutrient databases, such as the USDA Nutrient Database, had not provided dietary folate equivalent numbers. Suitor and Bailey (14) recently addressed this issue and cautioned that folate values for fortified foods are underestimated using current databases. Thus, data such as ours actually underestimate the contribution of fortification to food folate values expressed as dietary folate equivalents. Recently, Malinow et al. (15) tested whether breakfast cereal fortified with folic acid increased plasma folic acid and decreased homocysteine levels in adults with coronary artery disease and found a modest effect of folic acid at enrichment standard levels and a major impact when folic acid was provided at 100–150% of the RDA of 400 µg. This published report goes a step beyond the consistent findings that cereal fortification is effective in increasing nutrient intakes for consumers.

Although it has been reported that fortified foods are consumed across sociodemographic barriers (6) , it is still somewhat disconcerting that our data show that approximately one-fourth of children and more than one-half of adults did not consume a fortified food during the 3-d CSFII 1989–1991 survey. It is possible that analysis of more recent data would reveal different consumption of fortified foods than we report here.

To be safe and effective, fortification should reach populations in need and, at the same time, not contribute to excessive intakes. We assume that nutrient intakes from fortified foods presented in this article are underestimates of actual nutrient intakes from fortified foods over the 3-d survey, because most food composition data for fortified foods are based on label claims rather than on laboratory analyses. In other words, because regulations require that class I (added) nutrients be present at least at 100% of label claim, manufacturers need to add enough of a nutrient to ensure label compliance at the time of purchase. Food composition data only reflect the label claim amount, so the actual nutrient intakes are likely higher than what food composition data predict.

On the other hand, the 95th-percentile intakes reported here from a 3-d survey are likely higher than the true long-term daily intakes at the 95th percentile (5) . Thus, there are two possible limitations—each with opposite effect—to using these data to estimate extremes of exposures, and we cannot be absolutely certain the extent to which we are underestimating nutrient exposure (because USDA food composition data for fortified foods are largely based on label claim and not analytical data) or overestimating usual 95th-percentile exposures (because of the nature of variability in nutrient intakes). Based on data presented in our article, dietary intakes for the nutrients studied here seemed within safe limits at the time of the survey.

The analyses presented here do not include intake of nutrients from supplements, but USDA reports from the 1994–1996 CSFII indicate that 47% of the U. S. population are supplement users. Forty-seven percent of children age 5 y and under, 42% of men 20 y and over and 56% of women 20 y and over use vitamin/mineral supplements of some kind (16) . Subar and Bowering (17) reported that supplement use was unrelated to the use of highly fortified foods, but the possibility of excessive intakes among some subgroups of the population has not been explored fully. Several studies over the past decade have shown that supplement users are more likely to be women; to be older; and to have higher incomes, more education and more nutrient-rich diets than nonusers (18 19 20 21 22) . It will be important to assess the intake of fortified foods in these people and to characterize the vitamin/mineral supplementation practices of heavy consumers of fortified products.

The data presented here confirm that fortification made a major contribution to intakes of numerous vitamins and minerals for all age and sex groups, but especially for children. In addition, the data can serve as a useful baseline with which to compare contributions of fortification as the practice expands. Several lines of research would be fruitful to add to knowledge about fortified foods. First, because the number and types of fortified foods have skyrocketed in recent years, an analysis similar to the one reported here would be useful with current data (and with new food categories, such as sport and energy bars). We know that calcium-fortified foods are much more widely available now than when these analyses were performed, but how such foods contribute to total calcium intakes is not clear. Also of interest would be an analysis of the demographics of fortified product users. Popkin et al. (6) indicated that use of enriched and fortified products crossed socioeconomic lines back in the 1970s, but there are no current analyses on a large cross-section of the U. S. population. At the same time, several research groups have shown that vitamin and mineral supplement use is more prevalent among people with higher education levels, higher incomes and even better diets, suggesting that supplement use may not be as effective a strategy as fortification can be for reaching people in need. Finally, the effects of establishing standards for the rational fortification or enrichment of select commonly consumed foods should be examined for nutrients, such as calcium and vitamin E, which are currently consumed at low levels by many subgroups of the U. S. population but have not been influenced to a great extent by fortification in the past.

	FOOTNOTES

 
¹ Supported by the Project Committee on Fortification, ILSI North America.

³ Abbreviations used: CSFII, Continuing Survey of Food Intakes by Individuals; ILSI-NA, International Life Sciences Institute-North America; USDA, U. S. Department of Agriculture.

Manuscript received February 8, 2001. Initial review completed March 29, 2001. Revision accepted May 15, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

 

1. Mertz W. Food fortification in the United States. Nutr. Rev. 1997;55:44-49[Medline]

2. National Institutes of Health Optimal Calcium Intake: National Institutes of Health Consensus Statement 1994:1-31 National Institutes of Health Bethesda, MD.

3. Anonymous Folic acid fortification. Nutr. Rev. 1996;54:94-95[Medline]

4. Rimm E. B., Willett W. C., Hu F. B., Sampson L., Colditz G. A., Manson J. E., Hennekens C., Stampfer M. J. Folate and vitamin B6 from diet and supplements in relation to risk of coronary heart disease among women. J. Am. Med. Assoc. 1998;279:359-364[Abstract/Free Full Text]

5. Food and Nutrition Board Dietary Reference Intakes: Applications in Dietary Assessment, A Report of the Subcommittees on Interpretation and Uses of Dietary Reference Intakes and Upper Reference Levels of Nutrients, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes 2000 National Academy Press Washington, DC.

6. Popkin B. M., Siega-Riz A. M., Haines P. S. The nutritional impact of food fortification in the United States during the 1970s. Fam. Econ. Nutr. Rev. 1996;9:20-30

7. Cook D. A., Welsh S. O. The effect of enriched and fortified grain products on nutrient intake. Cereal Foods World 1987;32:191-196

8. Subar A. F., Krebs-Smith S. M., Cook A., Kahle L. L. Dietary sources of nutrients among US adults, 1989–1991. J. Am. Diet. Assoc. 1998;98:537-547[Medline]

9. Block G., Dresser C. M., Hartman A. M., Carroll M. D. Nutrient sources in the American diet: quantitative data from the NHANES II survey: vitamins and minerals. Am. J. Epidemiol. 1985;122:13-26[Abstract/Free Full Text]

10. National Technical Information Service Continuing Survey of Food Intakes by Individuals and 1989–1991 Diet and Health Knowledge Survey on CD-ROM 1989–1991 U. S. Department of Commerce, National Technical Information Service Springfield, VA. Accession Number PB96-501747.

11. Food and Nutrition Board Recommended Dietary Allowances 10th ed. 1989 National Academy Press Washington, DC.

12. McNulty H., Eaton-Evans J., Cran G., Woulahan G., Boreham C., Savage J. M., Fletcher R., Strain J. J. Nutrient intakes and impact of fortified breakfast cereals in schoolchildren. Arch. Dis. Child. 1996;75:474-481[Abstract]

13. Norris J., Harnack L., Carmichael S., Pouane T., Wakimoto P., Block G. US trends in nutrient intake: the 1987 and 1992 national health interview surveys. Am. J. Public Health 1997;87:740-746[Abstract/Free Full Text]

14. Suitor C. W., Bailey L. B. Dietary folate equivalents: interpretation and application. J. Am. Diet. Assoc. 2000;100:88-94[Medline]

15. Malinow M. R., Duell P. B., Hess D. L., Anderson P. H., Kruger W. D., Phillipson B. E., Gluckman R. A., Block P. C., Upson B. M. Reduction of plasma homocyst(e)ine levels by breakfast cereal fortified with folic acid in patients with coronary heart disease. N. Engl. J. Med. 1998;338:1009-1015[Abstract/Free Full Text]

16. U. S. Department of Agriculture, Agricultural Research Service Food and Nutrient Intakes by Individuals in the US, by Sex and Age, 1994–1996, Nationwide Food Surveys Report 96-2 1998:197 U. S. Department of Agriculture, Agricultural Research Service Washington, DC.

17. Subar A. F., Bowering J. The contribution of enrichment and fortification to nutrient intake of women. J. Am. Diet. Assoc. 1988;88:1237-1245[Medline]

18. Subar A. F., Block G. Use of vitamin and mineral supplements: demographics and amounts of nutrients consumed. Am. J. Epidemiol. 1990;132:1091-1101[Abstract/Free Full Text]

19. Kim I., Williamson D. F., Byers T., Koplan J. P. Vitamin and mineral supplement use and mortality in a US cohort. Am. J. Public Health 1993;83:546-550[Abstract/Free Full Text]

20. Slesinski M. J., Subar A. F., Kahle L. L. Dietary intake of fat, fiber and other nutrients is related to the use of vitamin and mineral supplements in the United States: the 1992 National Health Interview Survey. J. Nutr. 1996;128:3001-3008

21. Houston D. K., Johnson M. A., Daniel T. D., Poon L. W. Health and dietary characteristics of supplement users in an elderly population. Intl. J. Vitam. Nutr. Res. 1997;67:183-191

22. Lyle B. J., Mares-Perlman J. A., Klein B. E., Klein R., Greger J. L. Supplement users differ from nonusers in demographic, lifestyle, dietary and health characteristics. J. Nutr. 1998;128:2355-2362[Abstract/Free Full Text]

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