Milk and dairy products are major sources of fat in the human diet, but there are few detailed reports on
the fatty acid composition of retail milk, trans fatty acids in particular, and how these change throughout
the year. Semi-skimmed milk was collected monthly for one year from five supermarkets and analysed
for fatty acid composition. Relative to winter, milk sold in the summer contained lower total saturated
fatty acid (SFA; 67 vs 72 g/100 g fatty acids) and higher cis-monounsaturated fatty acid (MUFA; 23 vs
21 g/100 g fatty acids) and total trans fatty acid (6.5 vs 4.5 g/100 g fatty acids) concentrations. Concentrations
of most trans-18:1 and -18:2 isomers also exhibited seasonal variation. Results were applied to
national dietary intakes, and indicated that monthly variation in the fatty acid composition of milk available
at retail has limited influence on total dietary fatty acid consumption by UK adults.