Energy Accretion in the Fetus

Fat has a high energy content (9.5kcalg-1) and a very high carbon content (approximately 78%). Thus, differences in fetal fat concentration among species lead to large differences in calculated energy accretion rates and carbon requirements of the fetal tissues for growth. The energy concentration of nonfat dry weight is fairly consistent across species and also within species at different developmental stages, indicating that the ratio of protein to nonprotein substrates in the tissues is relatively constant. Thus, energy accretion rate of any fetus can be estimated from the growth curve of the fetus in question and the changing fat and water concentrations.

Data for energy accretion and distribution in the human fetus are shown in Table 3. Because growth of fat and nonfat (protein plus other) tissues are metabolically linked through energy supply that is used for protein synthesis and the production of

Table 3 Calculation of the energy distribution in the term human infant

3 Ü

Fat

Nonfat wet weight

Nonfat dry weight

Weight (g)

3450

386

3064

511

Total calories (kcal)

5950

3650

2300

2300

Energy concentration

1.73

9.45

0.75

4.5

(kcalg-1)

From Ziegler EE et al. (1979) Body composition of the reference fetus. Growth 40: 329-341.

From Ziegler EE et al. (1979) Body composition of the reference fetus. Growth 40: 329-341.

anabolic hormones that promote positive protein, fat, and carbohydrate growth, restriction of nutrient supply is likely to produce growth deficits of all tissues, not just fat (i.e., growth retardation involves limitation of muscle growth as well as fat and glycogen). Indeed, chronic experimental selective energy (glucose) restriction in the fetal sheep leads to increased protein breakdown as well as to lower rates of fetal growth and lipid content. In contrast, as shown by the growth curves in Figure 6 from human infants born prematurely at different times

2000

1500

1000

1200/0

1500

1000

1200/0

Gestational age (days)

Figure 6 Dry weight (A) and fat content (B) plotted against gestational age in the same newborn human infants shown in

Figure 3 for LGA (■. . •••), AGA (O, —), and SGA (♦,---)

infants. (Reproduced with permission from Sparks JW (1992) Intrauterine growth and nutrition. In: Polin RA and Fox WW (eds.) Fetal and Neonatal Physiology, p. 184. Philadelphia: W. B. Saunders.)

Gestational age (days)

Figure 6 Dry weight (A) and fat content (B) plotted against gestational age in the same newborn human infants shown in

Figure 3 for LGA (■. . •••), AGA (O, —), and SGA (♦,---)

infants. (Reproduced with permission from Sparks JW (1992) Intrauterine growth and nutrition. In: Polin RA and Fox WW (eds.) Fetal and Neonatal Physiology, p. 184. Philadelphia: W. B. Saunders.)

over the last third of gestation, there is a bias towards thinner, SGA infants with less fat relative to nonfat weight and nitrogen content, raising the possibility that in a species that does lay down considerable fetal fat during late gestation, differences in intrauterine growth rate may reflect fat deposition more than the growth of nonfat, protein-containing tissues.

Mineral Accretion in the Fetus

Fetal calcium content is best correlated with fetal body length; this is true for both AGA and SGA infants. Using this index, fetal calcium content increases exponentially with a linear increase in length. Using this estimate, the human fetal rate of calcium accretion is about 85 mg kg-1 day-1. Accretion of other minerals varies more directly with body weight, and according to the distribution of the minerals into extracellular (e.g., sodium) or intracellular (e.g., potassium) spaces.

Regulation of Fetal Growth

Fetal growth is the result of interaction among maternal, placental, and fetal factors, representing a mix of genetic mechanisms and environmental influences through which the genetic factors are expressed and modulated. The single most important environmental influence that affects fetal growth is the nutrition of the fetus. Nutrient supply to the fetus and the resulting increases in fetal tissue and plasma concentrations of anabolic hormones and growth factors are regulated by maternal health, maternal nutrition, uterine growth (including uterine blood flow and endometrial surface area), and placental growth and function.

Genetic Factors

Many genes contribute to fetal growth and birth weight of the normal term fetus. Maternal genotype is more important than fetal genotype in the overall regulation of fetal growth. Table 4 presents estimates of the quantitative contribution of fetal and parental factors to fetal growth and birth weight at term. The more modest regulation by the paternal genotype, acting through the fetal genotype, is essential for trophoblast development. In fact, overexpression of the paternal genotype can produce trophoblast tumors. More specific gene targeting studies have shown the importance of genomic imprinting on fetal growth. For example, in mice normal fetal and placental growth require that the IGF2 gene be paternal and the IGF2 receptor gene be maternal, and paternal disomy producing IGF2 gene overexpression results in fetal overgrowth while

Table 4 Factors determining variance in birth weight

Per cent of total variance

Fetal

Genotype 16

Sex 2

Maternal

Genotype 20

Maternal environment 24

Maternal age 1

Parity 7

Unknown 30

From Penrose LS (1954) Proceedings of the 9th International Congress on Genetics, Part 1; and Milner RDG and Gluckman PD (1996) Regulation of intrauterine growth. In: Gluckman PD and Heymann MA (eds.) Pediatrics and Perinatology, 2nd edn, p. 285. London: Arnold.

maternal disomy producing IGF2 underexpression results in fetal dwarfism. In humans, isopaternal inheritance of IGF2 alleles is associated with the Beckwith-Wiedemann syndrome, which includes hyperinsulinism and fetal macrosomia.

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