Fig. 3. Classification of lipids [8].

The digestion of dietary fat starts in the stomach as its churning action helps to form an emulsion. After entering the intestine, the partially emulsified fat is mixed with bile and is further emulsified. The emulsion is hydrolysed by lipases secreted by the pancreas, converting triglyceride to monoglycerides and free fatty acids which are then absorbed by the enterocytes of the intestinal wall. Fatty acids with a chain length of <14 carbons enter directly into the portal vein system and are transported to the liver; whereas fatty acids with 14 or more carbons are re-esterified within the enterocyte and enter the circulation via the lymphatic route as chylomicrons. Fat-soluble vitamins (vitamins A, D, E and K) and cholesterol are delivered directly to the liver as part of the chylomicron remnants [16]. Fatty acids are transported in the blood as complexes with albumin or as esterified lipids in lipoproteins.

      From the endogenous fat, liver produces very-low-density lipoproteins (VLDL) which are the main carriers of triglycerides, supplying fatty acids to adipose and muscle tissues. The end-products of VLDL metabolism are low-density lipoproteins, which carry approximately 60–80% of cholesterol in plasma. High-density lipoproteins remove fat molecules (phospholipids, cholesterol, triglycerides, etc.) from the cells and tissues and transport them back to the liver [16].

Fats are an important component of diet, being the most energy-dense macronutrient. They are an essential component of cell membranes and internal fatty tissues that protect the vital organs from trauma and temperature change by providing padding and insulation. In recent years, lipid nutrition research has been focused on the role of specific fatty acids in the metabolism of cholesterol, lipoprotein and glucose. The type and amount of fatty acids in the diet have been shown to affect the plasma concentrations of low-density lipoprotein, VLDL, high-density lipoprotein, cholesterol and triglyceride [17], as well as insulin sensitivity and glucose metabolism [18]. In respect of human health, essential fatty acids are precursors to the formation of prostanoids, thromboxanes, leukotrienes and neuroprotectins, which in turn regulate key physiologic functions such as blood pressure, vessel stiffness/relaxation, thrombocyte aggregation, fibrinolytic activity, inflammatory responses and leukocyte migration [19].

Although early studies suggested that dietary saturated fats increase the risk of coronary artery disease, several recent analyses have shown that saturated fatty acids, particularly in dairy products, can improve health [20]; whereas the evidence of omega-6 polyunsaturated fatty acids (PUFAs) promoting inflammation and some diseases is growing. Oxidation of PUFAs, as well as sugars, produces various aldehydes which are known to initiate or intensify several diseases, such as cancer, asthma, type 2 diabetes, atherosclerosis and endothelial dysfunction [20].

      Micronutrients

      Vitamins and minerals are nutrients that are found in small amounts in most foods and are essential, in minute amounts, for normal metabolic function.

      Vitamins are a group of organic compounds that cannot be synthesised by humans and should be provided by the diet; otherwise their deficiency could cause adverse health conditions. While vitamins have few chemical similarities, their metabolic functions have been defined in 1 of 4 general groups: (i) membrane stabilisers, (ii) hydrogen and electron donors and acceptors, (iii) hormones, and (iv) coenzymes [8]. They also have important roles in gene expression in humans. Vitamins are classified into 2 groups based on their solubility: fat-soluble and water-soluble. The main functions and dietary sources of vitamins are presented in Table 2; with more details provided in Chapter 6. The fat-soluble vitamins are absorbed passively by intestinal mucosae in the presence of fat. This group of vitamins is mainly found in the lipid portions of the cells such as membranes and lipid droplets. Fat-soluble vitamins are excreted with faeces via the enterohepatic circulation [8]. The water-soluble vitamins are also mainly absorbed by passive diffusion, except for vitamin B₁ (thiamine) which is absorbed by a sodium-dependent transport mechanism and vitamin C which is absorbed by both passive diffusion and sodium-dependent active transport. Vitamin B₁₂, a collective term for a group of cobalt-containing compounds known as corrinoids, is absorbed by active transport in the terminal ileum after binding to salivary haptocorrin and “intrinsic factor” (a protein cofactor secreted by the parietal cells of the stomach). The water-soluble vitamins are not stored in significant amounts in the body and are excreted in the urine [8].

      In nutrition, minerals are a group of inorganic elements that cannot be made by the body and are necessary for a variety of functions, including: the formation of bones and teeth, as essential constituents of body fluids and tissues, as components of enzyme systems and for normal nerve function. Based on the quantity needed by the body, minerals have been categorised into 2 major groups: macrominerals with a requirement of =100 mg/day and microminerals (trace elements) with a requirement of <15 mg/day, which include ultra-trace elements that are necessary at a level of µg/day, although specific requirements have not been established for some of them (Table 3) [8]. Minerals take up 4–5% of the body weight of an average adult. Although macrominerals occur mainly in the ionic state in the food and the body, some minerals also exist as components of organic compounds such as phospholipids and haemoglobin. They are usually absorbed in the ionic state, with heme iron being exceptional, by an active transport mechanism. The unabsorbed minerals are excreted in the faeces [8]. For more detail, the reader is referred to Chapters 3, 4 and 5.

Table 2. Vitamins – key information