One of the most common claims I see on the internet with regards to infant feeding is that the human body cannot digest grains until the age of one (or 2, or 4, or any other random age). This could not be further from the truth! Not only are grains easily digestible by the infant body, but they are a staple part of the diet, providing energy as well as many essential micro-nutrients.
What are grains?
When one thinks of grains they usually think of things such as wheat, barley, rice, corn, and oats. In reality, a grain is a type of fruit with a fused seed, and encompasses even things like legumes and beans, and other vegetables (USDA 2013). For the sake of this post, all discussion on grains will be restricted to "cereal grains", such as wheat.
A cereal grain is a type of grass that produces an edible seed with endosperm, bran, and germ (USDA 2013). The primary macro-molecular component of grains is polysaccharide chains of glucose known as "starch" (Raven et al. 2013). So when we talk about the ability to digest grains, what we're really looking at is the body's ability to break apart the glycosidic linkages of these starch chains.
How does starch digestion work?
1. Starch digestion begins in the mouth with salivary enzymes (Wardlaw et al. 2013). Enzymes are catalysts. That means they assist in chemical reactions but are not part of the reaction. An enzyme provides a binding site for larger molecules and, once joined with the molecules, will alter its shape, exposing the chemical bonds (Raven et al. 2013). Once exposed, these chemical bonds are broken in a process called hydrolysis. A water molecule is added to the bond site, splitting the 2 molecules apart. In starch digestion, the enzyme "alpha-amylase" binds to the starch, exposing the glycosidic linkages holding the glucose molecules together (Raven et al. 2013). The starch is then broken in to different disaccharides (maltose and isomaltose) and some oligosaccharides (alpha-dextrins, trisaccharides) (Lieberman, Marks 2012).
2. The second stage of starch digestion occurs in the stomach. There is very little enzyme activity in the stomach, however alpha-amylase retains its biological activity while in the fundus portion of the stomach, where it remains for about an hour before it mixes with gastric acid (Vance 1999; Matthews 1916) This means that the hydrolysis of starches by salivary amylase continues even after the food leaves the mouth.
3. After being broken down by the stomach, the shorter starch chains then enter the upper portion of the small intestine (duodenum). The pancreas secretes digestive enzymes, one of which is also alpha-amylase. This alpha-amylase is identical to the alpha-amylase in the saliva, acting as a catalyst in the further hydrolysis of alpha-dextrins in to maltose, isomaltose, and maltotriose. (Lieberman, Marks 2012).
The activity of alpha-amylase is limited to certain glucose polymers with glycosidic linkages at specific prime points. Therefore, alpha-amylase is not the only enzyme responsible for starch digestion. The maltose, isomaltose, maltotriose, and other oligosaccharides produced through amylase hydrolysis require additional enzymes to further break them down. This is where the brush-border region of the small intestine comes in to play. (Lieberman, Marks 2012).
4. The brush border region of the small intestine (lower portion of the duodenum as well as the jujunum) contains millions of microvilli attached to a single-layer of secretory and absorptive cells. The cells of the brush border region secrete more enzymes, such as glucoamylase, sucrase-isomaltase complex, lactase, and some others (Bowen 2006; Lieberman, Marks 2012). These enzymes are called "brush border enzymes". It is here that the products of amylase hydrolysis are further hydrolyzed to yield glucose monomers, which are then absorbed in to the bloodstream.
5. Any remaining starch that is not digested at this point will continue to the large intestine. Not all starches will be digested. Cellulose is a structural form of starch that comprises the cell walls of most plants (Raven et al. 2013). In cereal grains, cellulose makes up a portion of the germ and the bran. The glycosidic linkages that hold the glucose monomers together in cellulose cannot be broken down by simple hydrolysis, and thus cellulose makes up most of our dietary fiber (Raven et al. 2013; Wardlaw et al. 2013). In the large intestine, microflora aid in further starch digestion, breaking down the resistant starch polymers in to short chain fatty acids and gases (Lieberman, Marks 2012). Some recycling of nutrients occurs here, and the fatty acids are used in metabolic processes by the colon's own cells, but most of the biological activity in the large intestine serves to bulk and form the feces.
How do infants digest starches?
Much of the confusion surrounding starch digestion in infants stems from our understanding of infant pancreatic function. The pancreas is the organ responsible for secreting most of the digestive enzymes in to the small intestine. Because pancreatic function in infants is immature, their pancreas does not secrete large amounts of alpha-amylase (Karn, Merritt 1976). Salivary amylase begins being produced at about 18 weeks gestation, and has been found to be at full adult levels by around 5 months of age (Karn, Merritt 1976; Christian et al. 1999; Blackburn 2007). Pancreatic amylase production begins a few weeks to a few months after birth and slowly increases, approaching adult levels by about 16 months of age, though it may take years for full maturity to be reached in some children (Lebenthal 1987).
Erroneous application of that knowledge has resulted in claims that infants "cannot" digest starches. In fact, what is understood to be "adult levels" of alpha-amylase is actually an abundance of alpha-amylase (Lieberman, Marks 2012). Both the pancreas and the salivary glands secrete amylase in quantities that exceed biological need. So what is considered an "adult level" is far more than is actually used to digest starch. Adults can efficiently digest starches with as little as 10% of the alpha-amylase normally secreted (Lieberman, Marks 2012). Furthermore, the intestinal epithelial cells that secrete the brush border enzymes are fully mature and secretion of brush border enzymes is at 50-100% adult value at the time of birth, increasing even more rapidly in the next few days (Blackburn 2007).
To further determine the role of pancreatic amylase in starch digestion, we look at infants and children who have Cystic Fibrosis (CF). Individuals with CF have an inborn error that alters the way their cells diffuse water and salts across the cell membrane. This results in excess mucus being produced, which inhibits the secretion of many pancreatic enzymes. However, in infants and children with CF, starch digestion is one of the better digestive processes. Lipid and protein digestion is inhibited in much greater amounts than is starch digestion. Infants and children with CF are recommended to maintain a high carbohydrate diet, specifically because of the efficiency of starch digestion in the absence of full pancreatic function (Goodin, 2005).
There is no scientific evidence to suggest that lowered production of pancreatic alpha-amylase is in any way detrimental to infants. It is hypothesized that the lower production of pancreatic enzymes in infants is a protective method to avoid degradation of the intestinal epithelial lining. But other compensatory digestive mechanisms more than make up for the lack of pancreatic amylase (Blackburn 2007). In fact, infants also do not produce "adult" quantities of most other pancreatic enzymes either, which begs the question that if we cannot give our infants grains, what do we give them? Furthermore, do ALL components of a diet need to be fully digested? The answer is no. Human breast milk contains several types of indigestible oligosaccharides (pre-biotics) that survive digestion intact and feed the microflora of the colon. Insoluble fiber is also a necessary part of the human diet (Barile, Rastall 2013).
Complementary solid food feeding of infants should begin at 6 months (WHO 2009; AAP 2012), and the most suitable complimentary food is actually starch. While most arguments against grain introduction focus exclusively on cereal grains, the same polysaccharide starch chains that are found in grains are also found in many other vegetables. Carrots contain the same amylose and amylopectin starch chains that are found in grains, and yet I have not heard a single convincing argument with regards to why infants cannot digest carrots.
Starch digestion is a complex process that involves much more than the presence or absence of one singular enzyme. Meanwhile, whole grains continue to satisfy a majority of the human body's energy needs, as well as being a rich source of the essential nutrients folate, selenium, magnesium, and manganese, as well as many B vitamins.
References:
1. United Stated Department of Agriculture. MyPlate Recommendations. 2013
2. Biology (10th Edition). Raven, Mason, Johnson, Losos, Singer. C. 2014 Mcgraw Hill
3. Contemporary Nutrition: A Functional Approach. Wardlaw, Smith, Collene. C. 2013 Mcgraw Hill
4. Marks' Basic Medical Biochemistry (4th Edition). Lieberman, Marks. C. 2012 Lippincott Williams & Wilkins
5. Physiological Chemistry. Mathews. C. 1916
6. "Effects of Salivary Amylase". Vance. C. 1999 University of California, Clermont
http://biology.clc.uc.edu/students/114-Fall99/Amylase.htm
http://biology.clc.uc.edu/students/114-Fall99/Amylase.htm
7. "Small Intestine: Brush Border Enzymes". Bowen. C. 2006 Colorado State University
8. "Differential Expression of Salivary and Pancreatic Human Amylase Loci in Prenatal and Postnatal Development". Journal of Medical Genetics. Vol 13 p. 96-102 Tyre, Karn, Merritt. C. 1976
http://jmg.bmj.com/content/13/2/96.full.pdf
http://jmg.bmj.com/content/13/2/96.full.pdf
9. "Starch Digestion in Infancy". Journal of Pediatric Gastroenterology and Nutrition. Vol 29 Issue 2 p. 116-124. Christian, Edwards, Weaver. C. 1999
http://journals.lww.com/jpgn/Fulltext/1999/08000/Starch_Digestion_in_Infancy.4.aspx
http://journals.lww.com/jpgn/Fulltext/1999/08000/Starch_Digestion_in_Infancy.4.aspx
10. "Role of Salivary Amylase in Gastric and Intestinal Digestion of Starch". Journal of Digestive Diseases and Science. Vol 32 Issue 10 p. 1155-1157. Lebenthal. C. 1987
11. "Nutrition Issues in Cystic Fibrosis". Practical Gastroenterology. Goodin. C. 2005
12. "Human Milk and Related Oligosaccharides as Prebiotics". Current Opinion in Biotechnology. Vol 24 Issue 2 p. 214-219. Barile, Rastall. C. 2013
http://www.ncbi.nlm.nih.gov/pubmed/23434179
http://www.ncbi.nlm.nih.gov/pubmed/23434179
13. American Academy of Pediatrics. 2012. "AAP Reaffirms Breastfeeding Guidelines"
14. World Health Organization. 2009. "Infant and Young Child Feeding"
http://whqlibdoc.who.int/publications/2009/9789241597494_eng.pdf
15. Maternal, Fetal, and Neonatal Physiology: A Clinical Perspective. Blackburn. C. 2007 Elsevier Health Sciences
