Dairy food is the richest natural source of ‘harmful’ fats like long chain saturated and trans fatty acid in the diet . Changing dietary intake should significantly affect plasma levels of these fats, especially long chain fatty acids, and rTFA thought to be almost exclusively from dairy food. In this study cow’s milk was analyzed to assess fatty acids present in dairy food. Fatty acid composition in milk is mainly dependent on the feed and seasonality  and varies between countries. It was therefore essential to confirm fatty acids levels concurrent with the randomized study to allow for interpretation of study findings. Milk was considered the most useful dairy food to analyze as participants increased dairy food intake mainly by increasing intake of whole fat milk, yogurt and ice cream rather than cheese.
Intake of dairy food significantly changed for all those randomized to increase and decrease dairy food intake, with those that increased dairy food consuming on average 5.5 serves per day more than those that decreased dairy food intake. However, whilst there was a small increase in plasma levels of myristic, pentadecanoic acid and margaric acid with increased dairy consumption, plasma levels of most fatty acids present in milk did not change with diet. After multiplicity testing, only pentadecanoic acid changed significantly.
Dietary guidelines recommend consumption of low fat dairy food to reduce intake of long chain fatty acids like myristic and palmitic acid that are associated with increased cardiovascular risk , and stearic acid that is associated with reduced HDL cholesterol . However the effects of pentadecanoic acid, which increased in those asked to increase dairy food intake, are not known. Some studies have suggested that margaric acid and pentadecanoic acid are markers of dairy food intake , however this study suggests that in the real world, large changes in dairy food intake marginally changes fatty acids. Moreover, reducing dairy food intake had no effect on plasma levels of these fats suggesting that these may be present in other food sources. Participants increased uptake of food that are good sources of saturated fats such as foods with palm and coconut oils (such as table spreads, processed foods) and almond milk. Ultimately, separating the health effects of these fatty acids is difficult to justify when giving dietary advice, as they are highly correlated because they are found in the same food (e.g., beef and dairy products).
Similarly, rTFA did not change significantly between groups. Dietary advice to reduce dairy food intake in patients with cardiovascular disease is not aimed at reducing plasma rTFA levels. Consumption of rTFA is thought to be too low to have biological effects  and some studies suggest that vaccenic acid may not be harmful for human health . Previous studies have identified palmitelaidic acid as exclusively from dairy food [35, 36]; however, this study suggests that these assumptions may be misplaced.
It is possible in persons who decreased dairy food, rTFA from other sources increased. Oils containing precursors to vaccenic and palmitelaidic acid are increasingly used for processing and cooking food. Increased heat or pressure during cooking can increase rTFA levels in these oils . Vaccenic acid is found in dairy free processed foods such as takeaway chips in the United Kingdom . Dairy is also added to products like margarines, often assumed to be dairy- free. When these are heated, TFA levels can significantly be increased . A less likely possibility is that humans are able to catalyze a reaction to convert small amounts of cis to TFA. Whilst humans are able to convert vaccenic acid to conjugated linoleic acid by delta-9 desaturase in the liver [39, 40], bioconversion of cis to trans fatty acids has not been demonstrated.
Changing dairy food intake for one month could be too short to affect plasma TFA levels. However, in feeding studies changes in plasma TFA occur within 2–3 weeks [41, 42], and other dairy fatty acids like pentadecanoic and margaric acid , changed with a change in dairy intake within 3 weeks. There was a large difference in dairy food intake within and between groups. In contrast to some observational studies [35, 36], no correlation was observed between vaccenic acid and palmitelaidic acid either at baseline or 1 month (r = 0.053 and 0.089 respectively) suggesting they are not exclusively from the same source.
It is possible participants did not adhere to the randomized diet. However, food frequency questionnaires suggested good adherence and that consumption of other sources of TFA such as processed food, and goat’s milk or meat were not increased.
Food frequency questionnaires may not accurately reflect dietary intake [43, 44]. Studies comparing food frequency measures with repeated dietary recalls generally show correlations of the order of 0.4–0.7 .