The mouse study showed that those fed trans-fatty acids (TFA) with omega-6 rich maize oil had no increase of fatty acids (triacylglyceride [TAG]) in the liver.
Yet introducing trans-fats alongside olive and rapeseed oils (omega-3 and omega-9 respectively) led to increase of liver TAG, noted the researchers, led by Dr Juliana Saín of the Universidad Nacional del Litoral in Argentina.
The suggestion is that the effects of TFA on diets containing different edible oils is dependent on the fat source, the team said.
“The effects of a diet containing low levels of TFA, with a high proportion of vaccenic acid (a naturally occurring TFA) on liver and serum TAG regulation, depend on the dietary proportions of omega-3, omega-6 and omega-9 UFA,” they wrote in the British Journal of Nutrition.
The benefits of increasing omega-3 long-chain polyunsaturated fat consumption and decreasing intake of saturated fats are well documented. TFA has been linked with increasing blood fats, endothelial dysfunction, risk of cardiovascular disease and cancer.
However, this study is the first to investigate the potential effect of dietary TFA on liver and serum TAG regulation with different proportions of unsaturated fats in the same diet, Saín and co said.
“These results provide insights into some controversial findings associated with the intake of TFA and its effects on human metabolic alterations,” the team added.
The team fed two groups of 36 mice a diet containing different proportions of omega-3, omega-6 and omega-9 unsaturated fatty acids from olive, maize or rapeseed. In half of the mice, partially hydrogenated vegetable oil (high in industrially-produced trans fats) was added into the oils.
All mice were fed the test diet for 30 days before the researchers measured the effects of the trans- fatty acid substitutions on the weight and metabolic functions of liver, muscle and fat tissue.
They reported that for both olive oil and rapeseed oil, the addition of trans fats resulted in altered liver functions – including an increase in TAG levels and an imbalance between lipogenesis and oxidation. When trans fats were added to the maize oil no alterations of liver functions were seen, they confirmed.
However, the increased TAG levels with olive and rapeseed oil groups were not observed in serum. The researchers note this is likely due to increased epididymal white adipose tissue (EWAT) and lipoprotein lipase activity (LPL) in the groups.
“It is widely known that, in fasting, serum TAG levels are related to the balance between the hepatic secretion and the peripheral tissue clearance through LPL activity. Therefore, the potential discrepancy observed between the high liver TAG secretion and the normal serum TAG levels could be explained by a differential removal of TAG by LPL enzyme in extrahepatic tissues,” the team explained.
The study follows another recent publication which suggested even low consumption levels of TFA can see fatty acids incorporated into tissues, though retention is related to the amount of unsaturated fats eaten in the diet.
The trans-fat issue
Metabolic issues are not the only ones linked with trans fats. A 2015 analysis published in the British Medical Journal, is just one of a long line of studies suggesting that industrially produced trans fats are associated with heart disease and heart attack risk.
According to the European Society of Cardiology, the detrimental effect that trans fats have on heart health and mortality is "beyond dispute."
As such, many food producers have already made switching out trans fats a priority, and the presence of TFA in foods has fallen dramatically in recent years.
Last year, four major international food companies – Mars, Kellogg’s, Mondelēz and Nestlé – and several health NGOs including the European Public Health Alliance (EPHA), the European Heart Network (EHN) and the European Consumer Rights Organisation (BEUC) sent an open letter to the European Commission asking it to legislate and limit trans fats originating from partially hydrogenated oils to 2 g per 100 g of fat.
A European Commission report recently agreed that a legal limit on permitted levels of industrial trans fats in food would be the most effective, though the report did not state what that limit should be.
Source: British Journal of Nutrition,
"The effects of trans-fatty acids on TAG regulation in mice depend on dietary unsaturated fatty acids"
First published online 20 May 2016, doi:10.1017/S0007114516002415
Authors: Juliana Saín, Marcela Aída González, Jimena Verónica Lavandera et al.