Fat

Fetal fat content as a fraction of fetal weight varies several fold among species (Figure 5). The fat content of newborns at term of almost all land mammals is 1-3% and is considerably less than that of the human (15-20%). Differences in body fat content among species are due primarily to the capacity of the placenta to transfer fat to the fetus and to the capacity of the fetus to synthesize triglycerides and

Human Guinea Rabbit Sheep Calf Cat Monkey Pig Rat pig

Figure 5 Fetal fat content at term as a per cent of fetal body weight among species. (Reproduced with permission from Hay WW Jr (1996) Nutrition and development of the fetus: carbohydrate and lipid metabolism. In: Walker WA and Watkins JB (eds.) Nutrition in Pediatrics, 2nd edn, p. 376. Hamilton: B. C. Decker.)

Human Guinea Rabbit Sheep Calf Cat Monkey Pig Rat pig

Figure 5 Fetal fat content at term as a per cent of fetal body weight among species. (Reproduced with permission from Hay WW Jr (1996) Nutrition and development of the fetus: carbohydrate and lipid metabolism. In: Walker WA and Watkins JB (eds.) Nutrition in Pediatrics, 2nd edn, p. 376. Hamilton: B. C. Decker.)

fat. Even in those species that take up fat from the placenta and deposit fat in fetal tissues, the rate of fetal fatty acid oxidation is presumed low, because plasma concentrations of fatty acids (and keto acid products such as fl-hydroxybutyrate and acetoace-tate) are low, and because the carnitine palmitoyl transferase enzyme system is not sufficiently developed to deliver long-chain fatty acids to the respiration pathway inside the mitochondria.

In the human fetus, calories produced by the complete oxidation of glucose and lactate can fully meet energy required for maintenance metabolism and for conversion of glucose and lactate to fatty acids. The portion of glucose converted into fat has been estimated to be 23 kcalkg-1 day-1. This would permit accumulation of 2.4 gkg-1 day-1 of fat. In the human fetus between 26 and 30 weeks' gestation, nonfat and fat components contribute equally to the carbon content of the fetal body. After that period, fat accumulation considerably exceeds that of the nonfat components. At 36 weeks' gestation, 1.9g of fat accumulates for each gram of nonfat daily weight gain, and by term, the deposition of fat accounts for over 90% of the carbon accumulated by the fetus. The rate of fat accretion is approximately linear between 36 and 40 weeks' gestation, and by the end of gestation, fat accretion ranges from 1.6 to 3.4 gkg-1 day-1. At 28weeks' gestation, it is slightly less and ranges between 1.0 and 1.8gkg-1 day-1. By term, fat content of the human fetus is 15-20% of body weight, ranging from less than 10% in IUGR fetuses to 25% or more in macrosomic infants of diabetic mothers.

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