Biomarkers of Milk Fat Intake

To assess the impact of dairy food on health, measures to objectively assess intake are critical. As stated earlier in this chapter, one of the major limitations of nutrition research is the utilization of self-reported dietary intake data. An increase in awareness of healthy eating patterns as a result of advertising and nutrition education may exacerbate bias associated with self-reported dietary intake methodology (Johansson et al., 1998). This is especially the case of at-risk groups. For example, obese people are more prone to underreporting dietary intakes (Livingstone and Black, 2003). 

Fatty acid biomarkers reflecting milk intake help address issues associated with self-reported dietary intake data such as memory bias, over-and underreporting, and issues with methodological tools utilized to acquire dietary data (Trabulsi and Schoeller, 2001). 

Data on the proportions of different fatty acids in plasma lipid esters (cholesteryl esters, phospholipids, free fatty acids, or triacylglycerol), erythrocyte membranes, or adipose tissue may provide a more objective and accurate path to evaluating dietary fatty acid composition (Arab, 2003 Baylin and Campos, 2006). The fatty acid composition in blood and body tissues reflects the fatty acid composition of the diet at different time points after ingestion. Short and medium-term changes in the composition of dietary fatty acid intake are reflected in plasma lipids and erythrocyte membranes, weeks and months after intake, respectively. The incorporation of fatty acids in adipose tissue reflects long-term changes in the diet (years) (Baylin and Campos, 2006 Katan et al., 1997 Ma et al., 1995 Zock et al, 1997). 

Two saturated fatty acids, pentadecanoic acid (15 0) and heptadeca-noic acid (17 0), in adipose tissue (Baylin et al., 2002) and serum lipids (Smedman et al., 1999 Sun et al., 2007a Wolk et al., 1998) have been proposed and validated as biomarkers of dietary ruminant fat intake, that is, mainly from milk fat and to lesser extent from ruminant meat. The human body is unable to synthesize fatty acids with an uneven number of carbon atoms, whereas ruminal microbes of cows have this ability (Wu and Palmquist, 1991). To measure the content of 15 0 and/or 17 0 in plasma lipids or adipose tissue is consequently a way to estimate the milk fat intake. It is known that the proportion of 15 0 and 17 0 

Consequently, a more objective way to measure the habitual intake of milk fat would be the fatty acid composition of adipose tissue. However, this is not routinely performed in larger cohort studies, due to cost and that the procedure is invasive and less tolerated by study participants. Analysis of plasma fatty acid composition is thus a more feasible option for examination to determine dairy intake in the study population. While some groups have separated plasma into its constituent phospholipids and cholesterol esters to analyze serum 15 0 and 17 0 as markers of dairy intake (Smedman et al., 1999), Baylin et al. (2005) found that plasma that was not separated into its constituent cholesteryl ester, phospholipids, and triacylglycerols was still able to reflect habitual dairy intakes comparably to adipose tissue. Thus, whole plasma is an acceptable alternative to fractionated plasma in the absence of adipose tissue for analysis to reflect habitual dairy intakes and may be a cost effective option for consideration when conducting future intervention studies to assess the affect of dairy products on health outcomes. 

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