Background

Long-chain polyunsaturated fatty acids (LC-PUFAs) are fatty acids with a backbone of greater than 20 carbons, and are of either of the omega-3 (e.g., docosahexaenoic acid or DHA, 22:6n-3 and eicosapentaenoic acid or EPA, 20:5n-3) or omega-6 series (e.g., arachadonic acid or ARA, 20:4n-6). Humans are able to synthesis these LC-PUFAs from fatty acid precursors via a series of elongation and desaturation steps at all stages of the life cycle. DHA and EPA, for example, are synthesized from the shorter, less unsatu-rated omega-3 fatty acid, alpha-linolenic acid (ALA, 18:3n-3), and ARA is synthesized from linoleic acid. LC-PUCFAs are essential for the development and maturation of the fetal and neonatal brain as well as eicosanoid metabolism, fluidity in membranes, and gene expression. Whether pregnant and lactating women and infants can convert enough ALA to DHA and EPA to meet physiological requirements is uncertain and future research in this area is urgently required. Further, the 18-carbon fatty acids, lino-leic acid (omega-6 series), and ALA (omega-3 series) compete for the same enzymatic machinery to synthesize ARA and DHA. The trend toward higher dietary intakes of the 18-carbon omega-6 versus the omega-3 series of fatty acids may likewise contribute to inappropriately low levels of LC-PUFAS of the omega-3 series. As has been shown in studies using stable isotopes, even infants have the enzymatic machinery to convert ALA acid to DHA and linoleic acid to ARA [93-97]. These studies alone, however, provide insufficient data to assess whether sufficient quantities of DHA and ARA are synthesized to meet the infant's requirements. Infants fed formulas without DHA and ARA, but containing adequate levels of alpha-linolenic and linoleic acid, have lower levels of DHA and ARA in their blood compared with either breastfed infants or infants fed formulas supplemented with these fatty acids [96]. As with DHA and ARA in breast milk, the profile and concentration of these fatty acids in the blood will reflect dietary intake and do not provide sufficient data to assess whether endogenous biosynthesis of

DHA and ARA is adequate to meet requirements. Results from clinical trials with term-born infants designed to evaluate whether preformed DHA and ARA need to be added to infant formula in addition to the precursor fatty acids (ALA and linoleic acid) are mixed with some showing at least a short-term benefit [98-105] on either visual or cognitive development and others showing no benefit at all [106-111].

The US Institute of Medicine assumes that the fatty acid composition of breast milk meets the requirements of most infants. However, the concentration of DHA in breast milk globally ranges widely from 0.1 to 1.4% of total fatty acids due to the fat composition of the mother's diet [112, 113]. Furthermore, Innis et al. [114] have reported a 50% decline in human milk concentrations of DHA since the late 1980s in Canada and Australia. As anticipated, maternal supplementation with DHA appears to increase breast milk DHA content in a dose-dependent manner [115, 116]. While maternal supplementation with ALA tends to increase ALA content of human milk, it appears to have little effect on milk DHA concentrations [117]. At present, there is insufficient evidence to determine whether the variation in DHA content of human milk has clinical implications for the breast-fed infant including visual function or neurodevelopment [118]; however, there are a number of interesting studies to suggest there may be, and hence, further research in this area is important [93-95, 119-122]. As is the case with other nutrients, it will be difficult to untangle the possible relative impact of DHA consumption during pregnancy versus lactation on infant development. New evidence does suggest that supplementation of women prenatally with DHA may affect maturation of the visual system of infants and their ability to problem solve [123, 124]. There is some evidence to suggest a potential role for omega-3 fatty acids in the prevention of depression during the postpartum period, but again more research needs to take place to confirm this relationship [118] and see Chap. 19, "Postpartum Depression and the Role of Nutritional Factors".

Gaining Weight 101

Gaining Weight 101

Find out why long exhausting workouts may do more harm than good. Most of the body-building workout and diet regimens out there are designed for the guys that gain muscle and fat easily. They focus on eating less and working out more in order to cut the excess fat from their bodies while adding needed muscle tone.

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