down the proteins further. The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin and elastase, which break complex proteins into smaller individual amino acids. The amino acids are then transported across the intestinal mucosa to different tissues of the body where they are either used in replacing damaged tissues or in the synthesis of proteins. Excess amino acids may be converted by liver enzymes into keto acids, which are used as sources of energy via the citric acid cycle, or converted into glucose or fat for storage, and urea which is excreted in urine and sweat.
All cells in the body contain proteins; certain hormones such as insulin and glucagon, as well as antibodies and almost all enzymes, are proteins. Proteins transport nutrients and oxygen in the blood and also help maintain the acid-base balance of blood and tissue fluids.
Proteins are essential for growth and repair and maintenance of health. However, a number of health concerns are associated with protein originating primarily from animal sources including: cardiovascular disease, due to the high saturated fat and cholesterol associated with animal proteins, and bone health, from bone resorption due to sulphur-containing amino acids associated with animal protein [11]. Generally, the quality of proteins in the diet depends on their constituent amino acids. Compared with plant proteins, the nutritional value of animal proteins is higher because the distribution of amino acids in animal cells is similar to that in human cells [11]. Low intake of animal-sourced proteins during late pregnancy is believed to be associated with low birth weight [12]. Meat-based diets have also been shown to cause a significantly greater net protein synthesis and greater gains in lean body mass compared to vegetarian diets, which could be a function of reduced breakdown of protein with the former [13].
Fats and Lipids
In biology, lipids have been loosely defined as a group of organic compounds that are insoluble in water but soluble in non-polar solvents. Contrasting with carbohydrates, lipids are not polymers but smaller molecules extracted from the tissues of plants and animals [8]. Dietary fat includes all the lipids in plant and animal tissues that are eaten as food. Meats and dairy foods are the most obvious sources of fat, but most foods contain some fat. Vegetable sources rich in dietary fat are nuts and seeds, olives, peanuts and avocados.
Although lipids cover an extremely heterogeneous collection of molecules from a structural and functional perspective, all lipids have a polar, hydrophilic “head” and a non-polar, hydrophobic, hydrocarbon “tail.” According to their insolubility in water, lipids are commonly categorised into 3 major groups of simple, compound and miscellaneous lipids in some nutrition text books [8] (Fig. 3). However, in the LipidBank database [14], lipids are classified based on their response to hydrolysis as: “simple lipids”, those yielding at most 2 distinct types of compound upon hydrolysis (e.g., acylglycerols: fatty acids and glycerol), “complex lipids” yielding 3 or more products upon hydrolysis (e.g., glycerophospholipids: fatty acids, glycerol and headgroup), and “derived lipids”, alcohols and fatty acids resulting from hydrolysis of simple lipids. However, the LIPID MAPS classification system has been created based on the concept of 2 fundamental “building blocks”: ketoacyl groups and isoprene groups [15]. Consequently, lipids are defined as small hydrophobic or amphipathic (or amphiphilic) molecules that may originate entirely or in part through condensations of thioesters and/or isoprene units [15]. Based on this classification system, lipids have been divided into 8 categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids and polyketides (derived from the condensation of ketoacyl subunits); and sterol- and prenol-lipids (derived from the condensation of isoprene) [15].
Fatty acids are the key constituents of lipids in food and the body and are categorised into 3 types: saturated, monounsaturated and polyunsaturated, according to the number of carbons, the number of double bonds and the position of double bonds in the molecular chain. Based on the nutritional need, fatty acids are also categorised as essential and non-essential. The essential fatty acids are a-linolenic (a type of omega-3) and linoleic (a type of omega-6), which cannot be synthesised in the body and, therefore, must be obtained through the diet. The most prevalent form of dietary fat are the triglycerides, which are composed of 3 fatty acids and a glycerol molecule.
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
