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Starch Malabsorption in Infants

Shulman, Robert J.∗,†,‡

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Journal of Pediatric Gastroenterology and Nutrition: June 2018 - Volume 66 - Issue - p S65-S67
doi: 10.1097/MPG.0000000000001856
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Abstract

What Is Known

  • Infants commonly are fed starches in the first few months of life.
  • Pancreatic amylase activity increases very slowly over the first year of life.

What Is New

  • Starch digestion in young infants depends primarily on the activity of maltase-glucoamylase.
  • Starch digestive capacity also is a critical factor in deciding when to begin cereal feeding in infants.

The introduction of complementary (solid) feeding, such as cereal, to infants is not recommended until 6 months of age (1,2). Despite these recommendations, 30% to 40% of infants in the United States are fed cereal before this time (3,4). Some reasons for early introduction of cereal by parents or caretakers relate to the perception that the infant is not satisfied with formula and/or breast milk or that early introduction of cereal will reduce fussiness or improve sleep (5,6).

Considerations for when cereal should be introduced into the diet include the physical ability of the infant to swallow solids, the allergic potential of the components in the cereal, and the risk for the development of later allergies (7–9). An often-overlooked issue is the ability of the young infant to digest and absorb cereal.

The primary component of dietary cereal provided to infants is starch, with protein a much smaller component. Although amylase is present in saliva, in infants it is present at very low levels and is inactivated in an acidic milieu (10,11). Pancreatic amylase is the primary enzyme responsible for starch digestion in the lumen of the intestine (12). Its presence and activity are at very low, and in some cases, undetectable levels in duodenal fluid until 4 to 6 months of age, and even at 1 year of age remain well below that in children (11,13–18). Consequently, one would anticipate that the ability of young infants to digest, and ultimately, to absorb dietary cereal would be impaired whenever compared with that in older infants and children. The colonic microbiota are capable of salvaging some amount of malabsorbed dietary carbohydrate (including starch) by converting it to short-chain fatty acids, which then are absorbed and can be utilized as energy by the infant (11–16,19). Starch digestibility also is dependent on the type of starch and the degree to which it is cooked (20).

Breast milk contains amylase activity, which is greatest in colostrum and declines thereafter, but does not decrease significantly after 6 months of lactation (21–23). There is small intraindividual variation in breast milk amylase activity but great interindividual variation (21,23). Evidence suggests that depending on the amount of amylase activity present, some activity is capable of surviving passage through the stomach and remaining active in the duodenum (22,24). That said, whether breast-fed infants are able to digest starches to a greater degree than formula-fed infants, to our knowledge, remains to be determined.

The use of stable isotopes has enabled starch (cereal) digestibility studies to be carried out in young infants. Feeding maize (corn) flour naturally enriched in the stable isotope 13C to infants (n = 10; mean age 11 months, range 7–16), Weaver et al (25) demonstrated that the addition of a small quantity of amylase-rich flour resulted in a 33% greater recovery of 13C in breath compared with that of corn flour alone, implying better digestion and absorption. Importantly, the recovery of 13C with the amylase-rich flour was inversely correlated with age suggesting that younger infants received an advantage related to lower pancreatic amylase levels (25). Christian et al (26) studied infants (n = 13; mean age 12 months, range 8–23) who were fed 13C-enriched wheat flour. Using mathematical modeling, they attempted to separate the 13CO2 excreted in breath from digestion and absorption of the wheat starch from that produced from the metabolism of the short-chain fatty acids produced by colonic microbial fermentation (26). They estimated that up to 20% of the starch ingested was digested in the colon (26).

A more direct and quantitative method to assess the digestibility of cereal, both in terms of the starch component and the minor protein component, is through use of fecal measurement. The recovery of 13C in stool following the feeding of 13C-enriched cereal quantitatively measures cereal digestibility (27). In a proof of concept study, it was shown that 1-month-old infants (n = 4) could digest and absorb 3.7% to 13.1% of a 1 g/kg feeding of corn cereal (27). Importantly, there were no differences in the recovery of 13CO2 in breath following feeding of the cereal and an equal dose of glucose polymers or glucose (n = 16), demonstrating the difficulty of quantifying cereal digestion through breath testing (27).

Using fecal 13C measurements, the digestibility of rice cereal was quantitated in 1-month-old infants (28). Infants (n = 8) were fed 4 g of rice cereal in formula (an amount commonly used to thicken feedings for infants with gastroesophageal reflux) (28,29). As would be expected, energy and nitrogen intakes were increased whenever rice cereal was added to the formula. Despite this, however, net retention of energy and nitrogen did not increase following cereal feeding (28). Rather, fecal bacterial mass increased almost 7-fold, suggesting that the cereal was nutritionally unavailable to the infant, but was avidly utilized by the colonic microbiota (28). It was estimated that 30% of the cereal reached the colon,12% was recovered in stool, and 18% was fermented by colonic bacteria (28). In contrast to the findings in these 1-month-old infants, older infants (3–5 months of age; n = 9) were able to digest 95% of the nitrogen in the rice cereal (vs 79% in the 1-month-old infants) with no increase in fecal mass, thus, implying improved digestion and absorption of the cereal starch, as well, compared with that in 1-month-old infants (30).

These studies underscore the limited capacity of young infants to digest and absorb cereal (starch), related to the immaturity of pancreatic (amylase) function. Small intestinal mucosal maltase-glucoamylase, however, is capable of hydrolyzing the nonreducing terminal α-1,4 glucose bonds of nonbranched starches, but is most active against short-chain polymers (<10 glucose units) (31–33). Limited data suggest that young infants appear to have measurable maltase-glucoamylase activity (34–36). The digestibility of glucose polymers was measured in 1-month-old infants (n = 12) who were fed 13C-enriched glucose polymers (degree of polymerization [DP] = 3–8 and average DP = 43) (37). The digestion of the short-chain polymers was complete, but 5 of 12 infants had measurable malabsorption (4–19%) of the long-chain polymers (37). No correlation was found between digestibility of the polymers and 13CO2 excretion in breath (37). A study in preterm infants (n = 21, mean weight 2051 g) compared the digestion and absorption of short-chain glucose polymers with that of lactose, using the triple lumen intestinal perfusion method (38). The digestion and absorption of the polymers was greater than that of lactose, underscoring the importance of maltase-glucoamylase in the carbohydrate-related nutrition of preterm infants, in addition to that of older infants (38). Given the limited ability of young infants to digest starch, the argument could be raised that some degree of starch malabsorption could have a prebiotic effect. Data suggest that Bifidobacteria do contain the genes encoding for enzymes capable of degrading starch (39).

In summary, recommendations are underscored for the timing of the introduction of complementary cereal feeding, not only by recognizing the physical ability of the infant to swallow solids, and the possibility of allergy issues, but also by the limited starch digestive capacity of the young infant. The studies reviewed emphasize the great variability among infants in the rate at which pancreatic amylase function develops, and, therefore, the ability of infants to digest starch. One must also consider the potential of cereal feeding to cause poor weight gain and/or diarrhea (40). Conversely, evidence exists suggesting that the feeding of starch can enhance pancreatic amylase activity (16).

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Keywords:

developmental α-amylase deficiency; maltase-glucoamylase; maltodextrins; starch digestion; sucrase-isomaltase

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