Maximum Residue Limits (MRLs) for veterinary drugs are set by comparing deterministic point estimates of consumer dietary exposure — the Estimated Daily Intake for chronic exposure and the Estimated Short-Term Intake for acute exposure, now replaced at JECFA by the Global Estimate of Chronic Dietary Exposure (GECDE) and Global Estimate of Acute Dietary Exposure (GEADE) — with health-based guidance values such as the Acceptable Daily Intake and Acute Reference Dose. MacLachlan and Mueller (2012) showed that the traditional model diet overstated mean food consumption and understated large single-meal portions, proposed survey-based revised consumption figures and a separate short-term intake calculation, and asked for probabilistic studies combining distributions of actual residue levels from monitoring data with distributions of actual food consumption to quantify the conservatism (safety margins) built into these point estimates and to determine whether that conservatism should be harmonized across drugs and commodities. More than a decade later, probabilistic exposure modeling for veterinary drug residues exists only in national surveillance risk assessments benchmarked against health-based guidance values, while the regulatory MRL-setting methodology codified in the FAO JECFA Toolbox (revised July 2026) remains fully deterministic. The open problem is to construct and apply probabilistic exposure models that map the deterministic point estimates used in MRL derivation to specific percentiles of the chronic and acute exposure distributions, quantifying their conservatism and assessing whether the level of conservatism is, or should be made, consistent across veterinary drugs and food commodities.

Background

Veterinary drugs used in food-producing animals can leave residues in edible tissues, milk, eggs, and honey. To protect consumers, regulators establish Maximum Residue Limits (MRLs) — legal maximum concentrations of a marker residue (the residue chosen for monitoring) in each food commodity. An MRL is acceptable only if the resulting human dietary exposure passes a risk assessment against health-based guidance values: the Acceptable Daily Intake (ADI), the amount that can be ingested daily over a lifetime without appreciable risk, and, for drugs that can harm after a single dose, the Acute Reference Dose (ARfD). Exposure assessment for MRL setting has historically been deterministic: fixed residue concentrations are multiplied by fixed food-consumption figures to give point estimates. The 'model diet' used by major regulators assumed a 60 kg person daily consumes 300 g muscle, 100 g liver, 50 g kidney, 50 g fat, 1.5 L milk, and 100 g eggs. MacLachlan and Mueller (2012) demonstrated that this hypothetical diet overstates mean consumption (inflating chronic exposure estimates such as the Estimated Daily Intake, EDI) while understating the large portions eaten on a single eating occasion (understating acute exposure, which matters for drugs with an ARfD). They proposed revised chronic consumption figures derived as the maximum across the 13 cluster diets of the WHO GEMS/Food programme (the Global Environment Monitoring System food-contamination database; for a 60 kg person: 130 g muscle, 32 g fat, 7.2 g liver, 7.2 g kidney, 303 g milk, 72 g eggs), and a separate Estimated Short-Term Intake (ESTI) using large-portion sizes, defined as the 97.5th percentile of consumption by consumers of that specific commodity on a single day, combined with high residue levels. Following this line of refinement, JECFA (the Joint FAO/WHO Expert Committee on Food Additives, which evaluates veterinary drug residues for the Codex Alimentarius Commission) replaced the model-diet EDI/ESTI with the Global Estimate of Chronic Dietary Exposure (GECDE), based on median total-residue concentrations multiplied by mean consumption with one commodity at high-percentile consumption, and the Global Estimate of Acute Dietary Exposure (GEADE), based on the 95/95 upper tolerance limit of residues (the concentration exceeded in no more than 5% of samples with 95% confidence) multiplied by the 97.5th-percentile single-day consumption. All of these remain point estimates that deliberately embed conservatism: actual residue monitoring data show that median and average exposures are far lower than the point estimates suggest. In parallel, probabilistic exposure assessment — Monte Carlo simulation that propagates the full distributions of residue concentrations and food consumption through the exposure calculation to produce a distribution of possible intakes — has matured for other food chemicals: EFSA issued guidance on probabilistic modelling of dietary exposure to pesticide residues in 2012, and deterministic-versus-probabilistic comparisons exist for pesticides and contaminants. MacLachlan and Mueller themselves derived one deterministic conservatism bound (an ESTI combining large portions with MRL-level residues corresponds to at least the 99.875th percentile of consumer exposure), but no such quantification exists for the point-estimate framework as a whole, for veterinary drugs specifically, or for the consistency of the embedded safety margins across different drugs and commodities.

Problem Statement

For veterinary drug residues in food, construct probabilistic dietary exposure models that combine the distribution of actual residue levels observed in residue monitoring data with the distribution of actual food consumption (for both long-term daily intake and single-day large-portion intake), and use them to quantify the conservatism of the deterministic point estimates used in deriving Maximum Residue Limits — the model-diet and revised-consumption EDI, the ESTI, and their current JECFA successors GECDE and GEADE. Specifically: (1) determine, for both chronic and acute exposure and across at least two veterinary drugs and at least two food commodities, what percentile of the corresponding simulated exposure distribution each deterministic point estimate represents, i.e., the quantitative safety margin the point estimate embeds relative to actual exposures; and (2) assess whether the level of conservatism so quantified is consistent across drugs and commodities, and whether the point-estimate framework therefore requires further refinement so that the conservatism is more uniform. The analysis must be anchored to the actual regulatory point-estimate formulas (including the GECDE and GEADE constructions currently used by JECFA) rather than to a generic exposure benchmark such as a health-based guidance value alone.

References

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