According to recent evidence, diets high in fish oil may lower an individual’s body fat levels by increasing the rate of fat oxidization in the body.
One study, published in the International Journal of Obesity in 1997, looked at the effect of high doses of fish oil on total weight, body fat, and the rate of fat oxidization. The research involved six volunteers who were fed a control diet over a three week period followed by a high fish oil diet where 6 g/day of fat was replaced by 6 g/day of fish oil.
The researchers found that body fat decreased by an average of 0.88 kg during the fish oil diet compared to 0.30 kg during the control diet. The resting metabolic rate of the participants increased by 3.8% on average during the fish oil diet and the rate of fat oxidization increased by more than 20%.
Fat oxidization is the process by which fat in the body is converted into usable energy. When fat oxidization rates are low, some of the dietary fat consumed cannot be converted into usable energy so the individual must consume more food to cover the energy deficit. The unoxidized fat is then stored in the body in the form of adipose tissue.
The researchers calculated that the increase in fat oxidization due to the fish oil supplementation would lead to the loss of almost half a kilogram (1.1 pounds) of adipose tissue over a three week period.
The results are backed up by similar studies in mice which have found that mice fed diets high in fish oil tend to have lower overall body fat levels than mice fed conventional diets.
The 6g/day of fish oil used in this study equates to around 100-150g of oily fish per day. Oily varieties of fish include salmon, sardines, trout, mackerel, herring, and fresh tuna. Canned tuna is not classified as oily fish because the canning process removes much of the oil.