Fatty acid composition of adipose tissue and blood in humans and its use as a biomarker of dietary intake

Abstract

Accurate assessment of fat intake is essential to examine the relationships between diet and disease risk but the process of estimating individual intakes of fat quality by dietary assessment is difficult. Tissue and blood fatty acids, because they are mainly derived from the diet, have been used as biomarkers of dietary intake for a number of years. We review evidence from a wide variety of cross-sectional and intervention studies and summarise typical values for fatty acid composition in adipose tissue and blood lipids and changes that can be expected in response to varying dietary intake. Studies in which dietary intake was strictly controlled confirm that fatty acid biomarkers can complement dietary assessment methodologies and have the potential to be used more quantitatively. Factors affecting adipose tissue and blood lipid composition are discussed, such as the physical properties of triacylglycerol, total dietary fat intake and endogenous fatty acid synthesis. The relationship between plasma lipoprotein concentrations and total plasma fatty acid composition, and the use of fatty acid ratios as indices of enzyme activity are also addressed.

Introduction

The extent to which epidemiological research methods can be exploited to elucidate and substantiate the relation between dietary fat and the risk of chronic diseases such as heart disease and cancer is dependent heavily on the accurate and precise measurement of usual intake of dietary fats. Methods of dietary assessment that require participants to record or recall their food and beverage consumption over a fixed period of time have been and remain the most common measure of exposure to dietary fat in epidemiological studies. However, these techniques have a number of limitations that affect both the accuracy and precision of the measurement. Respondents often under-report consumption, a practice more common amongst those who are overweight than lean [1], moreover, respondents are known to alter their usual diet, whether consciously or sub-consciously, during the recording period [2]. Interviewer bias, incorrect portion size estimations, coding and computational errors, and errors associated with the use of food composition databases are also known problems. Often, an exact match of food is not available and very few food composition databases include a complete set of up to date values for individual fatty acids [3], [4], [5]. Relative intakes of individual fatty acids in the diet are therefore extremely difficult to estimate from reported dietary intakes. Last but not least, the accurate reporting of food intake imposes a large respondent burden so that a subject needs to be well motivated.

Given these limitations, there has been considerable interest in using tissue and blood fatty acid composition as biological markers of fat intake to improve on dietary assessment and there have been a number of good reviews on this subject [6], [7], [8]. However, there are no comprehensive summaries of the fatty acid composition of the different lipid fractions typically analysed or on the way that fatty acid composition can be interpreted. Therefore, it now seems timely to review the use of fatty acids as biomarkers of dietary intake in order to further study relationships between diet, metabolism and disease. We have systematically reviewed the scientific literature to identify and summarise population-based studies that show the fatty acid composition of adipose tissue, plasma, erythrocyte, and platelet lipids - most commonly used as biomarkers.

There are a number of immediate and pertinent issues that deserve consideration and which we review in this paper. Firstly, we address the strength of association as well as quantitative relation between dietary fat intake and fatty acid biomarkers – in other words how good are biomarkers at predicting dietary fat intake. We also identify biomarkers of short and long-term dietary fat intake and examine the use of fatty acid product to precursor ratios as indices of enzyme activity. Finally, confounding effects of non-dietary determinants of tissue and blood fatty acids such as the selectivity of the various lipid-metabolising enzymes, transport systems to tissues and the flux into and out of various pools in the body are discussed.