1 The relationship between diet and health is now a major factor in the development of production and marketing strategies for the British meat industry, following Government recommendations that people should be encouraged to eat less fat. It has emphasised the need for accurate information on the body composition of national livestock populations and the fat content of the meat and meat products derived from them. This paper collates the information available for cattle, sheep and pigs, and provides base-line (1984) estimates of national lean and fat production. Changes that have taken place over the past ten years are also examined.

2 The basic framework for making estimates was the distribution of carcasses between fatness ranges in the national carcass classification schemes operated by the Meat and Livestock Commission (MLC). The distribution for cattle was obtained from a random 1 in 3 sample of beef carcasses classified (0.25 of all clean beef carcasses were classified).

3 A computer spread-sheet was constructed relating the midpoint means of the fatness ranges to carcass tissue proportions and chemical composition. Regressions for predicting carcass lean and fat content were calculated from accumulated data from surveys of commercial carcasses and breed evaluations for cattle (Kempster, 1986), for sheep (Kempster, Jones and Wolf, 1986a) and for pigs (Diestre and Kempster, 1985). Key regressions are given in Table 1. Carcass lean and fat are defined as in the standard MLC tissue separation procedure. Regressions for estimating carcass lipid content: were obtained using data from several studies. The principal study involved carcasses from breed comparison trials (MAFF/MLC, 1982). Details of the other data sets are given by Kempster, Cook and Grantley-Smith (1986b). Key relationships are given in Table 2.

4 Estimates of the composition of carcasses in different classification fatness ranges are given in Table 3. These are for 'clean' cattle, sheep and pigs, but estimates were also made for cull cows and cull ewes.

5 National estimates of carcass composition and the weights of lean and fat produced in 1975/77 and 1984 are given in Table 4. The carcass composition of beef was the same in 1974/76 and 1984 but the average carcass weight has increased by 20kg. The implication of this is that changes in breed and production system have created the potential for leaner carcasses but that the beef industry has preferred to exploit this potential by increasing carcass weights. An increase of 20kg is equivalent to about 15gAg separable fat in carcass on the basis of typical regressions within breed and system.

The carcasses of clean sheep in 1984 were estimated to be slightly lighter and leaner than those in 1977. Information on marketing patterns suggests that there has been little change in production methods (as far as they affect composition) and that lambs are now being slaughtered early, possibly stimulated by the pattern of Guide Prices in the EEC Sheep Meat Regime.

In marked contrast to cattle and sheep, the separable fat content of the average pig carcass has fallen from 274g/kg in 1975 to 228g/kg in 1985, with a small increase in carcass weight. It is estimated that the increase in the use of entire males has contributed 5g/kg to the overall change.

6 The estimates in the paper are considered to be the best available with current information but because of the limitations of the data may be subject to error. Possible sources of error are discussed by Kempster et al (1986b). The authors would be pleased to know of data relating physical and chemical composition that could be used to refine the estimates.