Energy balance in pregnancy and lactation

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Energy metabolism and energy requirements during pregnancy

An individual’s requirement for essential nutrients corresponds to the amount of food they consume in relation to his/her energy needs.1 When a woman enters pregnancy she should have a consistent long-term good body size and composition and gain weight at a rate consistent with good health for herself and her child. The recommendations for energy intake for women vary depending on their background (population-specific) as they differ in body size and lifestyles. For example the energy requirements are different for well nourished women from developed societies compared to shorter women from developing societies.2

Energy Metabolism

During pregnancy, women gain weight which comprises of the products of conception (foetus, placenta, amniotic fluid), the increases of various maternal tissues (uterus, breasts, blood, extracellular extravascular fluid), and the increases in maternal fat stores. Therefore the energy cost of maintenance (also known as the basal metabolic rate, BMR), as well as physical activity, increases as a result of the increased tissue mass.2 This anabolic situation in pregnancy leads to a positive energy balance although some pregnant woman may also have a negative energy balance.1 This is due to the numerous metabolic adjustments that occur during pregnancy and lactation to support both foetal growth and milk synthesis without disrupting maternal homeostasis which requires retention of fat and protein in the mother and foetus. These adjustments ensure that a constant supply of glucose and amino acids reach the foetus. Adjustments also occur for lactation ensuring the mammary gland is the main area of nutrient utilization.3

INSERT FIGURE 1 FROM ENERGY METABOLISM DURING HUMAN PREGNANCY.

Energy is needed to synthesise the correct amount of fat and protein in new tissue and this consists of two components: 1. The energy in fat and protein retained in the body 2. The energy needed to synthesise these components. As it has been shown in figure 1 the total energy expenditure in pregnancy consists of four components including the energy costs for synthesizing the fat and protein retained.1

Recent information on BMR has found that the average increase during the first, second and third trimesters was 4%, 10% and 24% respectively although different women vary considerably with. Women from developing countries showed a much smaller increase in BMR than those from developed countries, furthermore women with high prepregnant BMI values showed larger increases in BMR which indicates a possible increase in metabolic activity of adipose tissue in pregnancy (LINK TO JO’s SECTION). Data therefore shows that a change in BMR during pregnancy is largely a function of maternal nutritional status.

Availability of substrates to the foetus

In order to sustain the foetus’ exponential growth the mother must supply it with continuous nutrients. Glucose followed by amino acids is the most important substrates crossing the placenta. Despite the placenta being almost impermeable to lipids, other than free fatty acids and ketone bodies, lipid metabolism is highly affected during pregnancy. There are two key stages during gestation; the first corresponds to the first 2/3 of the pregnancy when the foetal growth is minimal and the mother stores a great proportion of the nutrients consumed, which along with her increased food intake causes fat store accumulation. The last 1/3 of gestation is when the foetus grows very rapidly which is sustained through nutrient transfer through the placenta which means the mother switches to a catabolic condition. Lipid stores, in particular, are broken down and glucose is the most abundant nutrient that crosses the placenta at this point.4

INSERT FIGURE 2 FROM METABOLIC ADAPTATIONS IN PREGNANCY

Energy requirements

The definition of energy requirements during pregnancy can be paraphrased as “The energy requirement of a pregnant woman is the level of energy intake from food that will balance her energy expenditure when the woman has a body size and composition and level of physical activity consistent with good health, and that will allow for the maintenance of economically necessary and socially desirable physical activity. In pregnant women the energy requirement includes the energy needs associated with the deposition of tissue consistent with optimal pregnancy outcome.”2

References

1. Forsum E & Lof M. Energy metabolism during human pregnancy. Annual review of Nutrition. 2007; 27: 277-92 2. Butte NF & King JC. Energy requirements during pregnancy and lactation. Public Health Nutrition. 2005; 8: 1010-27 3. Butte NF, Hopkinson JM, Mehta N, Moon JK, Smith EO. Adjustments in energy expenditure and substrate utilization during late pregnancy and lactation. Am J Clin Nutr. 1999; 69: 299-307 4. Herrera E. Metabolic adaptations in pregnancy and their implications for the availability of substrates to the fetus. European Journal of Clinical Nutrition. 2000; 54: S47-S51.

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