Carbohydrates

Simple carbohydrates, also known as sugars, are termed ‘monosaccharides’ because they are the single sugar units that form the basis of all sugar structures. Glucose is a monosaccharide and the primary source of energy for most human cells. Most glucose is not ingested through the diet, but rather synthesised from sucrose (common sugar) in the liver (Wardlaw 1997). More complex forms of carbohydrates, such as starches and fibres, are composed of many units of smaller carbohydrates found in grains, vegetables and fruit and are termed ‘polysaccharides’. Overall, carbohydrates are important as a fuel. They are considered protein-sparing, as they prevent the breakdown of proteins in the muscles, heart and liver into amino acids and ultimately glucose to produce fuel.

Lipids

The term ‘lipid’ is a generic one used to describe a number of chemicals that share two main characteristics: they are insoluble in water and contain fatty acids. When they are solid at room temperature, they are called fats, whereas those that are liquids are called oils. Fatty acids are the simplest form of lipids, and more than 40 different types occur in nature (Wahlqvist et al 1997). Of all the various classes of lipids, only precursors of the omega-3 and omega-6 fatty acids are considered essential, meaning they are necessary in the diet to maintain health. The amount of dietary fat ingested and the ratios between saturated, monounsaturated and polyunsaturated fatty acids are important influences on overall health and disease.

Protein

Protein is formed by linking individual amino acids together, with the order of the amino acid sequence determining the protein’s ultimate form and function. Proteins are crucial to the regulation and maintenance of many vital body processes, such as blood clotting and fluid balance, cell repair, and hormone and enzyme production.

Amino acids can be divided into two broad groups: essential or non-essential. There are nine essential amino acids that must be ingested through the diet to maintain health. Protein foods that contain all nine essential amino acids are also referred to as complete protein sources (e.g. animal protein). By contrast, almost all plant foods are incomplete protein sources, with the possible exception of spirulina and soy. During times of growth, tissue repair or pronounced catabolism, such as after surgery, the body requires additional protein for efficient new tissue growth to occur. There is also emerging evidence that supplementation with individual amino acids produces a variety of pharmacological effects that can be manipulated to provide therapeutic benefits. Several of these amino acids are reviewed in the monograph section.

Minerals

Some minerals fall into the category of macronutrients because they are required in gram amounts every day, although they do not directly produce energy. These include sodium, potassium, calcium, phosphorus and magnesium.

Vitamins

Vitamins are organic substances (i.e. they contain a carbon atom) essential for normal growth and functioning of the body. They have been divided into two broad groups: vitamin C and the eight members of the B complex (B₁, B₂, B₃, B₅, B₆, B₁₂, biotin and folic acid) are defined as water-soluble vitamins, whereas vitamins A, D, E and K are fat-soluble. Levels in the body need to be replenished on a regular basis, as only vitamins A, E and B₁₂ are stored to any significant extent. Just as a vitamin deficiency causes adverse outcomes, high doses of these supplements can also do so if used inappropriately.

Essential trace minerals

These are minerals required by the body, usually in milligram or microgram amounts. They include iron, iodine, fluoride, zinc, chromium, selenium, manganese, molybdenum and copper. All trace minerals have the potential to be toxic when consumed in high doses.

Phytochemicals

Phytochemicals are components in plant foods that appear to provide significant health benefits, yet are not essential. Considerable research

TABLE 3.1 The Chemical Complexity of Food

has been undertaken in the past few decades to understand the role they play in maintaining health and preventing certain diseases, such as cancer, but there is still much to learn. Some of the better known phytochemicals include isoflavones (e.g. genistein), bioflavonoids (e.g. procyanidins), carotenoids (e.g. lycopene), glutathione and alpha-lipoic acid. Many of the phytochemicals found in food are also found in medicinal herbs and may partly explain their therapeutic qualities.

Inadequate dietary intake

The Australian National Nutrition Survey (NNS) conducted in 1995 found that approximately 30% of 2–7-year-old children ate no fruit or vegetables. Among 8–11-year-olds, 44% of boys and 38% of girls ate no fruit on the previous day and approximately one in four older children consumed no vegetables. Additionally, neither age group consumed the minimum number of serves of dairy products (Cashel 2000). Later results from the Australian Institute of Health and Welfare in 1998 reported that approximately one in two Australian children under 12 were not eating fruit or fruit products, and more than one in five consumed no vegetables in a typical day.

The dietary intakes of adults are also far from ideal. The 1995 NNS also found that approximately 44% of adult males and 34% of females did not consume fruit in the 24 hours preceding the survey, and 20% of males and 17% of females did not consume vegetables (Giskes et al 2002).

Interestingly, dietary intakes were influenced by income level, with lower-income adults consuming a smaller variety of fruits and vegetables than their higher-income counterparts. Additional research has consistently identified other groups at risk of poor dietary habits, such as the elderly, institutionalised individuals and the indigenous population.

Surveys of individual vitamin and mineral intakes also identify a number of at-risk subpopulations. Calcium intakes for 1045 randomly selected Australian women (20–92 years) as estimated by questionnaire showed that 76% of women aged 20–54 years, 87% of older women and 82% of lactating women had intakes below the recommended dietary intake (Pasco et al 2000). Of these, 14% had less than the minimum requirement of 300 mg/day and would, therefore, be in negative calcium balance and at risk of bone loss. The 1997 New Zealand National Nutrition Survey found that 20% of all New Zealanders and one in four women had calcium intakes below the UK estimated average requirements, and 15–20% of women aged 15–18 years were considered to have frank deficiency (Horwath et al 2001).

Surveys in Australia and New Zealand report that many adolescent girls have insufficient dietary intakes of iron and zinc to meet their high physiological requirements for growing body tissues, expanding red cell mass, and onset of menarche (Gibson et al 2002).

Evidence of deficiency in Australia and New Zealand

It is a common perception that although dietary intakes are not ideal, vitamin and mineral deficiencies are not a major health concern; however, research has shown that several subpopulations, such as children, pregnant and lactating women, older adults, institutionalised individuals, indigenous peoples and vegetarians, are at real risk.

For example, several local surveys have found that mild to moderate iodine deficiency is more prevalent than once thought (Li et al 2001). This is of great concern because mild iodine deficiency during childhood and pregnancy has the potential to impair neurological development. One survey of Sydney schoolchildren, healthy adult volunteers, pregnant women and patients with diabetes found that all four groups had urinary iodine excretion values below those set by the World Health Organization for iodine repletion (Li et al 2001). Another survey of 225 children in Tasmania identified evidence of mild iodine deficiency in 25% of boys and 21% of girls (Guttikonda et al 2002). A research group at Monash Medical Centre in Melbourne screened 802 pregnant women and found that 48.4% of Caucasian women had urinary iodine concentrations below 50 micrograms/L compared to 38.4% of Vietnamese women and 40.8% of Indian/Sri Lankan women (Hamrosi et al 2005). These figures are disturbing when one considers that normal levels are over 100 micrograms/L, mild deficiency is diagnosed at 51–100 micrograms/L and moderate to severe deficiency at < 50 micrograms/L (Gunton et al 1999). A study conducted at a Sydney hospital involving 81 women attending a ‘high risk’ clinic found moderate to severe iodine deficiency in 19.8% of volunteers and mild iodine deficiency in another 29.6% (Gunton et al 1999).

In Australia and New Zealand, the prevalence of vitamin D deficiency varies, but it is now acknowledged to be much higher than previously thought (Nowson & Margerison 2002). The groups at greatest risk are dark-skinned and veiled women (particularly in pregnancy), their infants, and older persons living in residential care. The study by Nowson andMargerison found marginal deficiency in 23% of women, and another frank deficiency in 80% of dark-skinned women in Australia. A study conducted in a large aged-care facility in Auckland identified frank vitamin D deficiency in 49% of elderly participants in midwinter and in 33% in midsummer (Ley et al 1999). Even in studies of ‘healthy’ adults, vitamin D insufficiency has been found to affect more than 40% of residents in Queensland (Kimlin et al 2007, van der Mei et al 2007) and over 65% of Tasmanians (van der Mei et al 2007).