3.2 Nature of fat samples in studies on fat-soluble compounds

Section 3.1.5.1 of the FAO Manual on the Submission and Evaluation of Pesticide Residues Data for the Estimation of Maximum Residue Levels in Food and Feed describes the information and data required from livestock transfer (feeding) and external animal treatment studies.

In particular the Manual draws attention to additional requirements for studies on fat-soluble pesticides:

"For fat-soluble pesticides in both feeding and direct animal treatment trials, the fat samples analysed should be fully described because residue levels may vary in fat from several fat depots within the body of the same animal. The fat description should include:

· the nature of the fat (e.g., peri-renal, subcutaneous);
· location in the animal body (if more than one possibility); and
· lipid content (rendered or extracted fat may be assumed as 100% lipid).

Residue levels of fat-soluble pesticides may also depend on the condition of the animal, which should also be recorded."

The Meeting considered further the requirements for animal studies on fat-soluble compounds with a view to ensuring that MRLs are recommended to cover the types of fat which may be sampled in monitoring and compliance programmes. Ideally the types of fat taken in the initial studies should match the types taken by regulatory authorities. In low-fat animals and poultry, where it may not be possible to trim off sufficient fat for residue analysis, the analyst may have to extract fat from the sample tissues.

Reports of metabolism and transfer studies and trials involving external treatment with fat-soluble compounds on cattle, goats and pigs in JMPR Evaluations for 1993-1995 were examined for the recorded descriptions of fat. The results are summarized in Table 1. It is clear that a variety of types of fat were analysed in the studies and trials. In an individual study one or more types of fat may be taken and analysed separately or as a composite. In the past it has been assumed that the levels of residues are approximately the same in the different fat depots within an animal (except at the site of a direct treatment), but this is not necessarily the case.

Table 1. Summary of descriptions of different types of fat classified according to animal and route of intake for studies on fat-soluble pesticides recorded in JMPR Evaluations, 1993-1995.

Animal	Intake	Fa1	SC2	Abdomen3	Renal	Back	Tr4	Non-tr5	TOTAL
Cow	direct		4	8	9	6	1	1	29
Cow	oral	3	2	3		1			9
Goat	direct								0
Goat	oral	1	1	4	3				9
Pig	direct			1			1	1	3
Pig	oral	1							1
TOTAL		5	7	16	12	7	2	2	51

1 Unspecified or composite fat
² Subcutaneous fat
³ Includes omental, peritoneal and mesenteric fat
⁴ Fat from treatment site
⁵ Fat from non-treatment site

Information was also obtained on the nature of the fat samples taken in monitoring and compliance programmes. At the abattoir there is the option to sample all the different fat depots in a carcase but at later points in the distribution chain the possibilities are limited or the depot sampled may not be known. In practice, any type of fat may be sampled.

The information obtained from feeding and direct treatment studies must allow an MRL to be recommended which is suitable for the various types of fat which may be subsequently sampled by regulatory authorities.

Livestock transfer (feeding) and external animal treatment studies with fat-soluble compounds should provide information on the highest residue levels likely to occur in any fat depot when the directions for registered uses of the pesticide are followed. The highest levels would be the basis for a recommendation for an MRL. In such studies fat samples from the various fat depots need to be analysed separately.

The residue levels of fat-soluble compounds in the fat of lean animals are likely to be higher than in fat animals for the same feeding regime. Ideally in feeding studies the animals should be lean rather than fat.

The Meeting noted that the description of "fat" in some studies is not always clear. It could be taken to mean "trimmable fat" containing moisture and possibly some other tissue or it could mean the lipid portion.

The Meeting recognized that JECFA faced similar problems when evaluating the veterinary uses of fat-soluble compounds, and considered that information should be obtained on the approach used by JECFA for dealing with the issues described above.

The Meeting recommended as follows.

1. In livestock transfer (feeding) studies with fat-soluble pesticides samples of sub-cutaneous, abdominal (omental, peritoneal, mesenteric) and renal fat should be taken from an animal and analysed separately.

2. In external animal treatment studies a sample of the fat at the treatment site, in addition to the three fat types required in the feeding studies specified in recommendation 1, should be taken and analysed separately.

3. In animal studies residue levels should be expressed on the lipid content of the fat (rendered or extracted fat may be assumed to be 100% lipid). The lipid content of the fat in trimmable fat or fatty tissue should also be reported.

4. The CCPR ad hoc Working Group on Methods of Analysis and Sampling should include a more precise description of carcase fat in the tables of "Portion of Commodities to which Codex Maximum Residue Limits Apply and which is Analysed." Carcase fat could be taken to mean "trimmable fat" containing moisture and possibly some other tissue or it could mean the lipid portion.

5. In recommending an MRL the JMPR should estimate the maximum residue level which might occur in any fat depot in the animal, recognizing the possibility that a regulatory authority may take a sample of any type of fat.

6. The recommendations for changed JMPR practices would initially apply only to evaluations of data on new and periodic review compounds.

7. The content and recommendations of this section of the report should be referred to JECFA for information and comment, with the intention of harmonising requirements and procedures relating to the nature of fat samples in studies with fat-soluble compounds.