We previously reported changes in gene expression in mammary tissue from non-inflamed mammary glands adjacent to an inflamed gland challenged with lipopolysaccharide (LPS). We determined if changes in the expression of selected genes in non-inflamed glands would be replicated in RNA isolated from milk fat. Cows were milked twice daily prior to experiment. Per cow, one mammary gland (QLPS) was randomly assigned to receive an intramammary infusion of 50 µg LPS immediately after morning milking on d-0. The ipsilateral (QI) and contralateral (QC) mammary glands adjacent to QLPS remained untreated. Quarter foremilk samples from all mammary glands were collected on d-1 and d-0 for milk composition and isolation of RNA for quantification of selected genes via quantitative polymerase chain reaction. Symptoms of clinical mastitis developed only in QLPS and were apparent within 3 h post-challenge. In QI and QC, lactose percentages were lower at 12 h post-challenge compared to d-1, but milk fat and protein contents were not different. For gene expression, 7 of 13 selected genes were differentially regulated in non-inflamed glands. In QI but not QC, LALBA expression was lower at 12 h post-challenge than on d-1. One gene of interest, LPIN1, was significantly upregulated in QI and QC but downregulated in QLPS at 12 h post-challenge. Five additional immune or stress-related genes were significantly upregulated in QLPS and, to a lesser but significant degree, in QI and QC compared to d-1. Notably, expression of two immune genes (NFKBIA, PTX3) was significantly greater in QI than QC despite QI having a numerically lower somatic cell count. Minor changes in the composition of milk secreted by non-inflamed mammary glands were linked to several immune and stress responses in those glands. Further, individual non-inflamed mammary glands responded differently depending on their position relative to the mastitic gland.

Milk fat is a high-value component of the U.S. dairy market. It is the major energy component of milk and is responsible for many organoleptic and technological characteristics of milk and dairy products. In addition, milk fat is unquestionably distinctive among all dietary fats that humans consume, as it is not only comprised of several hundred different fatty acids (FAs) but also contains a wide and unique array of bioactive lipids. Milk fat is dispersed in milk primarily in the form of fat globules. These cytoplasmic lipid droplets originate from mammary epithelial cells (MECs) and are secreted into the alveolar lumen surrounded by a membrane. Many advances in our knowledge of specific enzymes involved in milk lipid synthesis, the selectivity of the triacylglyceride (TAG) synthesis enzymes for specific FAs, the molecular mechanisms behind the uptake of long-chain FAs into the cells and the milk lipid secretion process have led to an improved understanding of the biology of milk fat synthesis. However, research to provide deeper insights into the mechanism of lipid synthesis in MECs is warranted and might lead to novel strategies to alter milk fat content and quality to benefit the dairy industry and meet dietary recommendations and consumer demands for foods that positively impact health. In this review, we aimed to provide a general overview of our current knowledge of the molecular aspects of milk lipid synthesis in MECs, from the uptake of blood-derived precursors to the intracellular formation of TAG-rich fat droplets secreted into milk as milk fat globules. We also highlight some current gaps in the knowledge that warrant further exploration. Given the importance of dairy food in the human diet, a better understanding of these processes could help develop novel strategies to alter milk fat composition in ways that benefit both human health and dairy producers.

Rumen microbial biohydrogenation (RBH) is the major factor responsible for the bovine milk rich in saturated fatty acids (FAs). Here, we evaluated the effects of nutritional manipulation of ruminal propionogenesis and methanogenesis, two primary hydrogen sinks, on the RBH and milk FA profiles in vivo and in vitro using three propionogenesis enhancers (fumarate [FUM], biotin and monensin) and one methanogenesis inhibitor (N-[2-(nitrooxy)ethyl]-3-pyridinecarboxamide [NPD]). The in vivo results showed that inclusion of FUM in lactating dairy goat diet could protect dietary unsaturated FAs against RBH with increased proportions of C18:2n − 6 (by 33.5%), C18:3n − 3 (by 38.1%) and RBH intermediates (e.g. trans-10 C18:1 and trans-11 C18:1) in rumen contents. Additionally, FUM supplementation increased the milk Δ9 desaturase index (by 15.5%) with higher cis-9 monounsaturated FAs in the milk. As a result, FUM increased the proportions of polyunsaturated and monounsaturated FAs in the milk with lower atherogenicity index (by −15.3%) and thrombogenicity index (by −19.5%). Conversely, supplementing NPD increased RBH completeness (by 7.4%) with higher milk atherogenicity index (by 10.5%) and thrombogenicity index (by 8.7%). The adverse effects of NPD on the milk FA profiles can be eliminated when supplemented in combination with FUM. The metagenomic analyses showed that neither FUM nor NPD affect the rumen microbial α- or β-diversity at the strain or gene level. The in vitro study showed that the conversion rate of FUM to propionate was increased from 54.7% to 80.6% when FUM supplemented in combination with biotin and monensin, resulting a higher anti-RBH potential. Accordingly, manipulation of ruminal methanogenesis and propionogenesis can redirect hydrogen toward or away from RBH and thereby influence the milk FA profiles. FUM is a promising feed additive in ruminant not only to reduce the methane emissions as previously proved but also to improve the nutritional desirability of the milk FA profiles for human health.