Wayne R. Wolf U.S. Department of Agriculture Agricultural Research Service Nutrition Institute Nutrient Composition Laboratory Beltsville, Md., 20705
T h e rapid, fairly recent develop ments of trace element research in the area of nutrition have led to a need to accurately and precisely know the content of these micronutrients in the diet. In the past several decades the analytical chemistry community has made great advances in improvements in sensitivity, selectivity, and accuracy of analytical methodology. In general, however, these advances have not been routinely applied to the determi nation of trace element content of foods. T h e problems involved in food analysis at the parts-per-million or parts-per-billion levels required for many of the newer essential trace ele ments have not been well defined. T h e present state of analytical and nutri tional knowledge varies with each of the elements. Since neither resources nor scientific knowledge indicates the feasibility of a full attack on each ele ment, priorities must be established on the basis of immediate scientific need and the possibility of success. I will limit the present discussion to those trace elements and problems of a nutritional nature and defer discus sion of those trace elements that act, as far as is presently known, only in toxic or detrimental modes and t h a t were discussed in a recent issue of this JOURNAL
(/).
For definition of the problems in volved in food analysis, it is necessary to consider the data required for foods and to then ask if these requirements can be met for each element. Analyti cal data in nutrition are used to: es tablish the essentiality of a nutrient; establish requirements; identify ade quate, subadequate, or marginal in takes in selected populations; and pre vent nonoptimal intakes (2). T h e present state of knowledge of the essentiality of trace elements in nutrition is shown in Table I. T h e
knowledge t h a t small amounts of met als are needed in the diet goes back several hundred years to the discovery of a requirement for iron and over a hundred years to the discovery of an iodine requirement. Requirements for the trace elements copper, manganese, zinc, and cobalt were discovered in the 1920's and 1930's in conjunction with t h e discovery and identification of an other class of micronutrients, the vita mins. Not until the 1950's were three more trace elements—molybdenum, selenium, and chromium—added to the list. Very recently, the six "newer trace elements", tin, vanadium, fluo rine, silicon, nickel, and arsenic, were discovered to have a nutritional re quirement. These discoveries parallel advances in analytical determination of small amounts of metals. Emission spectroscopic methods were developed
in the 1920's and 1930's, atomic ab sorption spectroscopy in the 1950's, and several very sensitive techniques, such as activation analysis and new advances in spectroscopic techniques, by the 1970's. These new, highly sensi tive analytical techniques, in conjunc tion with the development of isolator techniques for carrying out growth studies in controlled atmospheres, al lowed the discovery and identification of the "newer trace elements" (θ). T h e second need for analytical data in foods is to define requirements or amounts needed in the diet. Normally, requirements are defined by assessing balance studies on controlled popula tions and gaining some knowledge of the biological role of the trace ele ment. Total dietary intake of the trace element is monitored under experi mental conditions to determine the
Discovery of requirementa
RDA established6
Subadequate intakes Identified
Fortification recommendations
17th Century 1850 1928
Yes Yes Pending
Yes Yes ?
Yes Yes More data needed
Manganese Zinc Cobalt Molybdenum Selenium Chromium Tin Vanadium Fluorine Silicon Nickel
1931 1934 1935 1953 1957 1959 1970 1971 1972 1972 1973
No Yes No(B 12 ) Pending Pending Pending No No Pending No No
No Yes No(B 1 2 yes) No Yes Yes Experimental Experimental Experimental Experimental Experimental
No Yes No No More data needed More data needed Unknown Unknown Unknown Unknown Unknown
Arsenic
1975
No
Experimental
Unknown
Element Iron Iodine Copper
«Ret. 3. "Ref. 5.
190 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
This article not subject to U.S. Copyright Published 1978 American Chemical Society
Report
Nutrient Trace Element Composition of Foods: Analytical Needs and Problems amount required to balance the amount excreted. Alternatively, diets with different levels of trace elements are fed, and an identified biological parameter affected by the trace elem e n t is monitored to find dietary levels for optimum function. An example of such a parameter is blood hemoglobin levels which are used to monitor t h e effect of dietary iron. Recommended Dietary Allowances (RDA) have been set for iron, iodine, and zinc where knowledge is sufficient and a need for a requirement was shown for each (Table I). For copper the issuance of an RDA is pending. Selenium, chromium, and fluorine are under discussion; further study is needed before an RDA can be established. Only tentative recommendations for the "newer" trace elements in the form of a range of values can be presently proposed. To more firmly establish these recommendations, knowledge of the content of each of these trace elements in the diet is necessary. Knowledge of the trace element content of foods is also necessary to identify adequate, subadequate, and marginal intakes in selected populations or trace element deficiency-related diseases. Several large-scale surveys and epidemiological studies indicated t h a t trace element levels in water might be potentially correlated with major diseases such as cancer and heart disease (4). Further exploration of these correlations and expansion of epidemiological studies depend upon accurate, extensive knowledge of trace element distribution in the food supply. T h e final application of data on trace element content of food is to provide optimal intakes of these nutrients for the population. T h a t application could take the form of public health programs, dietary recommen-
dations such as the RDA's, dietetic services, possible fortification recommendations and actions, and educational programs and food labeling. For example, the last column in Table I shows the trace elements for which specific subadequate intakes have been identified and fortification programs recommended. T h e occurrence of iron deficiency and t h e iron fortification program of bread plus the occurrence of iodine deficiency in the United States "goiter belt" and the iodization of salt are well documented. More recently, the recognition of possible marginal zinc deficiency in the U.S. has led to some recommendations for zinc fortification by the Food and Nutrition Board Subcommittee (5). These recommendations have been implemented by most manufacturers of infant foods (6). Recent analytical methods have produced data indicating t h a t the average daily copper intake is probably 1 mg or about half the estimated h u m a n requirement (7, 8). More data generated by modern methods are needed to correctly assess the requirements and intakes of copper. For selenium, soil and plant deficiencies are known, and supplementation of animal feeds is practiced widely. Marginal h u m a n intakes have been identified in New Zealand (9). Selenium content should be determined for additional foods so t h a t the nutritional status, as regards selenium, of the U.S. population can be evaluated. Chromium deficiency occurs during protein-caloric malnutrition (10) and is suspected to occur marginally in elderly people in the U.S. T h e human requirement for chromium cannot be defined at present because of problems of analysis and incomplete knowledge of availability. Additional d a t a are needed to assess chromium status. For the "newer" trace ele-
ments, deficiencies in animals have been produced experimentally. H u m a n requirements and distribution in foods are unknown and must be determined before h u m a n nutritional status can be evaluated. Available data on food composition are presented in Agriculture Handbook No. 8, "Composition of Foods, Raw, Processed, P r e p a r e d " (11). This handbook gives some food composition data on iron. A recently updated section of Handbook 8 (12) gives some data on zinc, but the data are, in general, of a provisional nature because they are based upon a limited number of analyses on a limited number of foods. Data are not presented for the other trace elements listed in Table I. T h u s , additional food composition data are needed for all the trace elements known to have dietary requirements, particularly for those elements of greatest current interest in the nutrition community: iron, zinc, copper, selenium, and chromium. Such data can only be generated by the use of critically evaluated, well-defined analytical methodology.
Evaluation of Methodology Criteria must be developed for the evaluation of analytical methodology. Basic criteria are specificity, precision, sensitivity, accuracy, and suitability for quality control and automation. T h e methodology must be capable of a specific analysis of the nutrient of interest and be relatively free of interferences. This criterion can be attained by either separation of the nutrient from the bulk food or a specific detection system or both. T h e precision of the method must be sufficient to study and define food variations associated with processing, geographical, seasonal, and genetic factors plus oth-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 · 191 A
Table II. Recovery of ^ Samples •
•
...
••
;•
.
....
••
Λ:.-
—~~— ^w^myefc
. . · . . . . .
.COPPER X
Sample
Percentage recovered SDa
X
S© a
09-265
96.8
5.6
103.2
0.5
09-281
98.3
0.6
104.5
2.0
09-281
97.2
5.5
96.8
1.6
09-271
98.6
2.7
100.4
0.3
09-266
100.3
3.9
99.3
1.5
09-265
96.8
3.2
98.4
1.8
07-119
101.4
2.2
104.3
3.5
08-175
96.4
0.8
98.6
1.5
07-143
96.1
1.4
99.2
3.0
98.6 4.3 X = 9 8 . 0 5 ± 1.7 SD
07-141
100.6 4.6 100.5 ± 2.6 SD
* Each analysis done in triplicate.
on by Method of Standard Additions vs. Calibration Curve
08-175
1.13
1.18
+4.4
COPPER ( j i g / i m Curve % calibration Deviation 0.239 0.204 + 17.2
07-119
I3.66
0.67
+ 1.5
0.211
0.241
07-141
ι3.89
0.91
+2.2
0.158
0.189
+ 19.6
07-143
! 3.97
1.02
+5.2
0.339
0.375
+ 10.6
09-281
ι3.73
0.73
0
0.282
0.300
+6.4
09-271
1.05
1.10
+4.8
0.200
0.217
+8.5
09-266
ι3.57
0.61
+7.0
0.351
0.371
+5.7
09-265
1.43
1.32
-8.3
0.199
0.217
+9.0
09-281
0.75
0.74
-1.4
0.174
0.172
-1.2
NBS SRM 1 5 7 1
ι3.89
0.85
-4.7
0.421
0.458
+8.8
NBS S R M 1577
ι3.87
0.84
-3.6
1.182
1.269
+7.4
Wheat sample AACC-1
1.60
1.77
+ 10.6
0.304
0.350
+ 15.1
St andard atIdttlon
Sampli ι digest
ZINC (Mg/mL Curve calibration Deviation
Standard addition
Day-to-Day Reproducibility of Analysis of
+14.2
^Iffi
Date
COPPER
