Aim The maternal diet during pregnancy and lactation may affect the long‐term health of the offspring. Our aim was to study how a fish or meat diet perinatal and after weaning affects body composition, insulin sensitivity and the profile of n‐3 and n‐6 polyunsaturated fatty acids (PUFAs) in breast milk, fat depots, skeletal muscle and liver in male adult mice offspring. Methods During gestation and lactation, C57BL/6 dams were fed a herring‐ or beef‐based diet. Half of the pups in each group changed diets after weaning. In offspring, body composition measured by DEXA, plasma lipid profile and insulin sensitivity measured by euglycemic clamp or QUICKI were monitored to adulthood. Analysis of total FAs by GC‐MS were performed in the diet, breast milk and in different tissues. Results At 9 week of age, offspring of herring‐fed dams had less body fat than offspring of beef‐fed dams. Mice fed herring after weaning had increased insulin sensitivity at 15 week of age, reduced total plasma cholesterol and triglyceride levels, and compared with beef‐fed mice, larger interscapular brown adipose tissue depots. The FA composition of the maternal diet was mirrored in breast milk, and the herring diet significantly affected the FA profile of different tissues, leading to an increased content of n‐3 PUFAs. Conclusion A herring‐based maternal diet reduces body fat in the offspring, but the insulin sensitivity, plasma lipids and amount of brown adipose tissue are affected by the offspring's own diet; the herring diet is more beneficial than the beef diet.