Fats

During pregnancy, fat plays an important role not only as an important energy source but also as a contributor of the fat-soluble vitamins (A, D, E and K) and various polyunsaturated fats required for growth and development.

Linoleic acid, an omega-6 fat, and alpha-linolenic acid, an omega-3 fat, are essential fats required for growth and development. These fats cannot be synthesised by the body and therefore must be consumed in our diet from foods (Table 21.2).

Table 21.2 Dietary sources of essential and long-chain polyunsaturated fats

One of the primary functions of these fats is to produce long-chain polyunsaturated fatty acids (LCPs), which are found in particularly high concentrations in the central nervous system and retina. LCPs are also consumed in our diet, particularly in meat, dairy and fatty fish. Vegetarians who exclude all animal products from their diet must therefore ensure an adequate intake of essential fats in order to produce LCPs.

Each of the essential fats is responsible for producing one or more specific LCPs (Fig 21.1). Two of the LCPs, specifically arachidonic acid (AA) and eicosapentaenoic acid (EPA), are precursors to a group of hormone-like compounds called eicosanoids. Eicosanoids have an important role in maintaining blood pressure and haemostasis, in inflammation and in parturition. The eicosanoids produced from AA promote vasoconstriction, platelet aggregation and inflammation, whereas the eicosanoids produced from EPA promote vasodilation, inhibit platelet aggregation and reduce inflammation. During parturition, the eicosanoids produced from AA appear to enhance the breakdown of collagen fibres within the cervix, whereas the eicosanoids produced from EPA tend to inhibit this process (Allen & Harris 2001).

Figure 21.1 Conversion of the essential fats into long-chain polyunsaturated fats

(based on Allen & Harris 2001)

In Northern European countries, where the intake of fatty fish (and therefore EPA) is high, the incidence of preterm birth is low. This has sparked considerable research into the effect of EPA on parturition, specifically in women at risk of preterm labour. The results of these studies suggest that supplementation with EPA or consumption of fish 2–3 times per week may reduce the risk of preterm labour in some groups of women (Cetin & Koletzko 2008; Makrides 2008).

One concern that arises with recommendations to increase fish consumption during pregnancy is the amount of mercury present in some fish species. Larger fish tend to accumulate mercury. Food Standards Australia New Zealand (FSANZ) has specific recommendations regarding fish consumption, mercury levels and pregnancy (Table 21.3) (FSANZ 2004).

Table 21.3 Recommendations for fish intake during pregnancy

^1. Source: Food Standards Australia New Zealand 2004.

^2. Source: New Zealand Food Safety Authority 2008.

Carbohydrates

Carbohydrates are a major energy source during pregnancy and lactation. There are various classes of carbohydrates, including simple sugars and complex carbohydrates. Simple sugars are present in fruit (fructose), milk (lactose) and table sugar (sucrose). Glucose, another simple sugar, is the body’s major energy source and the most abundant carbohydrate. Glucose is also the building block for complex carbohydrates or, more specifically, the starches. Starches are a storage form of carbohydrate present in plants such as grains, legumes and root vegetables. In addition to simple and complex carbohydrates, dietary fibre may be considered a type of carbohydrate. Unlike simple sugars and starches, the human body lacks the necessary enzymes required to break down dietary fibre. Dietary fibre can be classified as either soluble or insoluble. Foods rich in soluble fibre include fruits and vegetables, oats, barley and legumes, whereas foods rich in insoluble fibre include wheat, corn, whole grains and most vegetables. The benefits of dietary fibre include increased satiety, regular bowel movements and a reduction in blood cholesterol levels.

Although carbohydrates should account for the majority of energy intake during pregnancy, the type of carbohydrate consumed is also important to prevent large fluctuations in blood glucose levels. The rate at which dietary carbohydrate, specifically glucose, is absorbed into the bloodstream can be measured, and is referred to as the glycaemic index (Table 21.4). Some carbohydrates present in food are rapidly digested in the digestive tract, thereby entering circulation quickly. This can result in a large increase in blood glucose followed by a sudden drop in blood glucose. Foods containing these types of carbohydrates are referred to as having a high glycaemic index value. In contrast, some carbohydrates are digested more slowly, allowing for a slow and steady rise in blood glucose that is not followed by a sudden drop in blood glucose levels. Foods containing these types of carbohydrates are referred to as having a low glycaemic index value. For sustained energy, foods with a low glycaemic index value are recommended.

Table 21.4 Glycaemic index (GI) values for a number of common foods

Protein

Protein, like carbohydrate and fat, is a macronutrient and therefore can provide energy. The body, however, spares protein for energy in order to utilise it for its many other roles. These include its role as a structural component of muscle, connective tissue and bone, ensuring fluid balance and blood pressure, enzymatic activity, transporting substances throughout the body, and formation of hormones and antibodies.

During pregnancy the requirement for dietary protein is slightly higher than in pre-pregnancy, to account for the development of maternal and fetal tissues. The protein requirements are based on bodyweight; for every kilogram of bodyweight a non-pregnant woman should consume between 0.8 and 1.0 g of protein, approximately 60 g per day (NHMRC 2006).

Despite limited evidence, high-protein diets have been suggested to help minimise the risk of preeclampsia. The World Health Organization does not support these diets and clearly states that in the absence of a protein deficiency, protein supplementation is unlikely to reduce a woman’s risk of developing preeclampsia (Kramer & Kakuma 2004). The current intakes of protein in both Australia and New Zealand are sufficient to meet the increased requirements during pregnancy, and therefore for most supplementation is not necessary (ABS 1995; Ministry of Health 1999).

Dietary intake of protein is important to ensure that the body has the correct mix of amino acids, the building blocks of protein. There are 20 amino acids, of which nine are considered essential. The body is unable to produce essential amino acids in adequate amounts and therefore they must be obtained from dietary sources. Animal proteins contain all nine essential amino acids, whereas plant proteins are limited in at least one. In the past, vegetarians were encouraged to combine protein sources within a meal to ensure intake of all the essential amino acids. It is now known that the body pools the essential amino acids within a 24-hour period and draws from that pool when needed. Therefore, combining proteins at each meal is not necessary, and by consuming a variety of plant sources such as whole grains, nuts, seeds, legumes and vegetables throughout the day, an adequate intake of essential amino acids should be achieved.

Iron

Iron is an important mineral present within the haem structure of haemoglobin. In any anabolic condition, such as pregnancy, the increased demands for haemoglobin result in increased demands for iron. During pregnancy, as well as in childhood, intestinal absorption of iron increases to help meet these demands. Despite enhanced absorption during pregnancy, iron deficiency and iron deficiency anaemia still occur in many pregnant women. Iron deficiency is particularly prevalent in indigenous populations—among Māori 40% (Ministry of Health 1999) and among Aboriginal Australians 55% (WHO 2007) are iron deficient—and is also a problem for immigrant populations.

Iron is present in food (Table 21.5) in two forms, haem and non-haem iron. Haem iron is the form present in meat, poultry and fish, as a component of haemoglobin. Non-haem iron is also present in meat, poultry and fish but is the only form found in plant foods, fortified foods and supplements.

Although iron is present in many foods, not all the iron is absorbed and used by the body. Haem iron is more readily absorbed in the intestine than non-haem iron. Up to one-third of haem iron is absorbed, whereas the absorption of non-haem iron is usually less than 10% (Zimmerman & Hurrell 2007). Including foods rich in vitamin C increases the absorption of non-haem iron by keeping iron in a more readily absorbable form—ferrous iron. Consumption of meat, poultry or fish together with non-haem iron also enhances iron absorption.

In addition to these enhancers or promoters of iron absorption, there are several factors that inhibit iron absorption. Tannins, present in black teas and red wine, have been reported to reduce iron absorption by up to 40%; therefore, it is recommended that these beverages be consumed two to three hours before or after a meal. Another inhibitor of iron absorption is phytates, phosphate-containing molecules present in legumes, nuts and whole grains that bind iron, rendering it unavailable for absorption. Soaking legumes, roasting nuts and fermenting whole grains decreases the phytate content of these foods, thereby decreasing their effect on iron absorption (Hallberg 2002). Finally, high doses of calcium (300 mg) or zinc (15 mg), such as the level present in supplements, consumed with iron-rich foods or iron supplements have been reported to inhibit iron absorption (Zimmerman & Hurrell 2007).

Calcium

Calcium is the most prevalent mineral in our body, with most of it contained within our skeleton. In addition to its structural role, it is involved in neural transmission, haemostasis and muscular contraction. Calcium may also play a role in regulating blood pressure. A Cochrane review of calcium supplementation and its role in preventing hypertensive disorders and problems found that in studies where women were high risk or had a low baseline calcium intake, supplementing with at least 1 g of calcium daily resulted in a significant reduction in pre-eclampsia (Hofmeyr et al 2006).

Calcium is present in many animal and plant foods (Table 21.5). As with iron, the bioavailability of calcium is much higher in animal foods such as milk, yoghurt and cheese than in plant foods. The calcium present in plant foods may also interact with various inhibitors, including phytates, high doses of minerals such as iron, and oxalates. Oxalates are compounds present in foods such as spinach, rhubarb and chocolate. Like phytates, they bind calcium, rendering it unavailable for absorption in the intestinal tract.

Table 21.5 Dietary sources of iron and calcium

Zinc

Zinc is an important component of over 80 enzymes involved in many reactions, including cell replication and growth, regulation and expression of genes, optimal immune function and hormone activity. Findings from several observational studies suggest that zinc deficiency during pregnancy increases the incidence of neural tube defects, fetal death and malformations, low birthweight, premature rupture of membranes and increased antenatal and intrapartum complications. There have been a number of randomised controlled trials of zinc supplementation during pregnancy; only half of these have reported an effect in reducing adverse birth outcomes or complications. It has been suggested that zinc supplementation is beneficial in developing countries where zinc deficiency is widespread (Cole & Lifshitz 2008).

Dietary sources of zinc include protein-rich foods such as red meats, seafood, oysters and clams as well as whole grains, legumes, nuts, seeds and wheatgerm. Fruits and vegetables are poor sources of zinc.

Iodine

Iodine is an essential component of triiodothyronine and thyroxine, more commonly known as the thyroid hormones. These hormones regulate body temperature, basal metabolism, reproduction and growth. In pregnancy, iodine deficiency can affect both mother and fetus. Maternal goitre, an enlargement of the thyroid gland, develops when intakes of iodine are habitually low. If iodine deficiency occurs during pregnancy, neonatal cretinism can occur, resulting in stunted growth, mental deficiency, deafness, motor rigidity and possibly hypothyroidism.

Iodine is present in a limited number of foods. Fish, seafood and seaweeds are the richest natural sources of iodine in Australia and New Zealand. In Tasmania and inland regions of New Zealand, the iodine content of soil is very low. Therefore, plants grown in these areas tend to be poor sources of dietary iodine compared with other regions of Australia and New Zealand. With increasing prevalence of iodine deficiency (including goitre) in Australia and New Zealand, the use of iodised salt in the manufacturing of bread has recently become mandatory, the exception being organic bread. This is due to the reduction in the use of iodised table salt within the home.

Folate

Folate is a generic term used to describe a collection of compounds, of which folic acid is the most stable and absorbable form. Folate is involved in cell replication and division, maturation of erythrocytes and amino acid metabolism. Folic acid also reduces the incidence of neonatal neural tube defects when taken prior to and following conception. Folate insufficiency has also been associated with cleft lip, cleft palate, placental abruption, low birthweight, miscarriage and Down syndrome. The recommendations for folate intake in women of childbearing age are the same for Australia and New Zealand (Table 21.6).

Table 21.6 Folate recommendations for Australian and New Zealand women

(Sources: NHMRC 2006; Ministry of Health 2006)

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