Mahaffey et al. 1975 — FDA Total Diet Study: Heavy Metal Exposure from Foods
This 1975 landmark paper presents the first multi-year, nationally representative US dietary monitoring data for six metals (Pb, Cd, Hg, Zn, As as As2O3, Se) using the FDA’s Total Diet Study (Market Basket Survey), which composited 117 foods into 12 food classes and prepared them as normally consumed. The authors report detection frequencies and mean concentration values by food class for 1968–1974, compare estimated daily intakes against WHO/FAO provisional tolerable intakes, and note that while average intakes for Pb, Cd, and Hg fell below adult tolerable levels, Cd was “close enough to the tolerable intake so that further increases in the cadmium content of foods should be avoided.” The paper is the defining methodological and baseline reference for the US market-basket heavy metals surveillance approach.
Key numbers
Lead (Pb): Pb was found in a large number of food composites; fruits and vegetables were the dominant dietary source, accounting for 78% of total Pb intake despite comprising only 23% of the diet by weight. Estimated daily intake for 1973 was approximately 60 µg/day (Method: zeros treated as zeros), ranging up to 233 µg/day under alternative censoring assumptions. This underestimation problem was specific to Pb because many composites were near or below the method detection limit; authors note the 1965–1970 average was 63 µg/day.
Cadmium (Cd): Cadmium had the most widespread distribution of the six metals — detected at high frequency across dairy, grains, potatoes, leafy vegetables, root vegetables, garden fruits, and fats. Highest detection rates were legume vegetables (100%, mean 0.26 ppm), potatoes (100%, 0.046 ppm), and leafy vegetables (100%, 0.051 ppm). Cereals and grains provided the greatest percentage of total Cd intake by food class. Estimated daily intake 1973: 51 µg/day; 1974 estimate: 34 µg/day.
Mercury (Hg): Mercury had the most limited distribution — virtually 100% of dietary Hg was attributable to meat, fish, and poultry, predominantly fish. All other food classes showed no detectable Hg except trace levels in a few composites. Estimated daily intake: 2.89 µg/day (1973); neutron activation analysis required for non-fish matrices due to levels near detection limits.
Arsenic (As, reported as As2O3): Distribution narrower than Cd; dairy, meat-fish-poultry, and grain-cereal products accounted for 92% of total As intake. Estimated daily intake 1973: 10 µg/day (as As2O3, not speciated as iAs vs. organic).
Zinc (Zn): Present in all food classes; dairy, meat-fish-poultry, and grains/cereals provided 77% of Zn intake. Mean daily intake: 18 mg/day.
Selenium (Se): Essentially all from meat-fish-poultry and grain-cereal groups (>99%). Estimated daily intake: 150 µg/day.
Tabulation from Table 5 (detection frequency %, mean ppm) for selected food classes (FY 1973):
| Food class | Pb % (ppm) | Cd % (ppm) | Hg % (ppm) | As % (ppm) |
|---|---|---|---|---|
| Dairy products | 33 (0) | 17 (0.005) | 0 (0) | 3 (0.003) |
| Meats, fish, poultry | 77 (0.013) | 40 (0.009) | 97 (0.011) | 27 (0.02) |
| Grain and cereal | 73 (0.10) | 97 (0.028) | 3 (0) | 7 (0.003) |
| Potatoes | 57 (0.003) | 100 (0.046) | 0 (0) | 3 (0.003) |
| Leafy vegetables | 83 (0.050) | 100 (0.051) | 0 (0) | 7 (0) |
| Legume vegetables | 100 (0.26) | 33 (0.006) | 0 (0) | 7 (0) |
| Root vegetables | 83 (0.11) | 80 (0.021) | 3 (0) | 7 (0) |
| Garden fruits | 87 (0.12) | 80 (0.019) | 0 (0) | 0 (0) |
| Fruits | 73 (0.043) | 17 (0.042) | 0 (0) | 3 (0) |
Units: ppm = mg/kg wet weight in prepared/as-consumed composites; n values not provided per food class; data are composite averages across 30 market baskets from 4 US geographic regions.
Methods (brief)
FDA Total Diet Study protocol: 117 foods representing the diet of 15–20-year-old US males (highest calorie consumers) purchased from retail stores in Southeast, Northeast, Central, and Western US regions, 30 times per year. Foods composited into 12 food classes after preparation as normally consumed. Analytical methods not fully specified in this paper but reference made to prior Kolbye et al. (1974) publication. Pb near detection limit in many composites; trace handling assumptions substantially affect intake estimates. Cd, As, Se: minimal detection-limit problems. Hg: reliable by flameless AAS; non-fish matrices required neutron activation analysis to detect any Hg. Speciation not performed; As reported as As2O3 total; Hg as total mercury (not methylmercury).
Implications
Certification: This is the historical baseline for US dietary heavy metal exposure prior to the regulatory era. Useful for understanding the pre-CTZ landscape and for framing how far monitoring has come. Cadmium’s ubiquity in plant-based food classes established in 1973–1974 data is the same pattern documented by modern surveys. The paper’s explicit warning that Cd was approaching tolerable limits in the 1970s provides historical context for why Cd in plant foods became a regulatory priority.
Courses: The 1975 Total Diet Study methodology is the origin of the market-basket approach still used by FDA today. Explaining food safety monitoring infrastructure to course participants starts here.
App: Data are too old and use composite-class resolution to inform per-ingredient contamination profiles directly, but the finding that grains/cereals dominate Cd intake and fruits/vegetables dominate Pb intake aligns with modern ingredient-level findings.