Fiber is primarily from plant sources and provides plants with their shape and structure. Dietary fiber refers to a group of complex carbohydrates that humans can not breakdown to use for calories as we lack the digestive enzymes. In the past because of increasing public preferences for softer white breads, grains were increasingly refined to make flours, but in the processing the flours lost fiber as well as other vitamins (iron, riboflavin, niacin and thiamin). Flours were then enriched to replace lost nutrients e.g. fiber, as well as fortified with added nutrients e.g. folate(1). The 2008 American Dietetic Association (ADA) evidence analysis review of fiber studies(2) indicate that there is some evidence that adequate dietary fiber intakes from foods or supplements may provide some protection from cardiovascular disease by lowering blood pressure, improving serum lipid levels, and reducing inflammation. But more studies are needed to clarify if fiber provides health benefits towards promoting weight loss; lowering blood sugar levels for people living with diabetes; and if fiber is helpful in colon cancer prevention; and prevention of gastrointestinal disorders such as chronic constipation, irritable bowel syndrome and diverticulitis.

Most Americans obtain their dietary fiber from white flour food products and potatoes, and the usual intake of fiber in the US is only 15 grams/day. The Food and Nutrition Board of the Institutes of Medicine recommends that an adequate fiber intake for adult women is 25 grams per day, for adult men is 38 grams per day or to aim for 14 grams of dietary fiber for every 1,000 calories consumed. Adequate fiber intake for infants aged 0 to 12 months old, the very old, and critically ill people is not determined3.  The ADA’s 2008 nutrition recommendations now include encouraging ‘the public to consume adequate amounts of fiber from a variety of plant foods’2 such as whole grains, legumes (beans), fruits and vegetables.  So how does this apply to people living with organic acidemias (OAs)?

Most people living with OAs are following a medically monitored and prescribed protein restricted diet. Some OA patients must only limit their daily amount of protein intake in foods; while others must also consume an individualized, OA disorder-specific metabolic formula (medical food).  Table 1 lists selected foods and the fiber and protein content for a given serving size, data gathered from the USDA National Nutrient Database(4).  The table shows that most high fiber-rich foods such as whole grains and legumes are also higher in protein content, so that these high fiber foods are often avoided or limited in people following OA protein restricted diets.  But many lower protein foods such as fruits and vegetables can also be a significant source of dietary fiber for people living with OAs. There is also a low protein food company currently enriching their low protein flour with added fiber, and many low protein bread recipes include the addition of psyllium fiber to improve texture of the low protein bread slices.

In the past, fibers were most often defined by their chemical properties; soluble fibers e.g. pectins, gums, mucilages that partially dissolve in water or insoluble fibers e.g. cellulose, hemicellulose, lignin that do not dissolve in water. Insoluble fibers may help with intestinal regularity, while soluble fibers may help prevent heart disease. Lower protein foods containing insoluble fibers include; raspberries, banana, apple, kiwi, tomato, cucumber, potato and lower protein foods with soluble fibers include apple, grapefruit, pears, broccoli, and potato(1). Most foods contain both soluble and insoluble fibers. Therefore in 2002 the Institute of Medicine recommended we use the terms; dietary fiber, functional fiber, and total fiber to help future fiber research. Dietary fiber is the nondigestible carbohydrates and lignin that are intrinsic and intact in plants.  Functional fibers are isolated nondisgestible carbohydrates that have beneficial physiological effects in human beings. Total fiber is the sum of dietary fiber and functional fiber(3).

Regardless of fiber definitions, many people living with OA disorders often go through stages of eating limited amounts of foods by mouth, requiring additional metabolic formula tube feedings to meet their daily nutritional needs.  People living with OAs who eat small amounts of food often have particular preferences for favorite foods not necessarily including fiber rich or nutritive food choices, just like most Americans.  Formula manufacturers currently do not add fiber supplements to their OA disorder specific metabolic formulas. Therefore to provide a significant daily source of dietary fiber for people living with OAs who eat limited amounts of food by mouth, parents and clinicians may need to consider the addition of a fiber supplement.

Formula manufacturers first started to add fiber supplements to their non-metabolic formulas assuming that it would help normalize bowel function and prevent constipation and diarrhea. The 2008 ADA evidence analysis of the health implications of dietary fiber(2) concluded that there is a lack of studies to prove the health benefits of taking fiber supplements. There have also been a few case reports that excessive fiber intake may be harmful for some people resulting in excessive diarrhea or bowel obstructions. Excessive dietary fiber intake may also interfere with the absorption of some medications as well as nutrients such as iron, calcium and zinc. There is also the logistical possibility of some fiber supplements (e.g. gums) when added to formulas, thickening formulas then blocking enteral formula feeding tubes. Anne Pylkas and others in a 2005 fiber study(5) compared various dietary fibers for short chain fatty acid (e.g. acetate, propionate, and butyrate) production. The researchers found that hydrolyzed guar gum and galactomannan produced the greatest amount of total short chain fatty acids; methylcellulose and arabinogalactan resulted in the most propionate production. It is important to note that this study was done in test tubes not with actual human participants living with OAs. But the results of this study suggest that there may be particular types of fiber supplements that are contraindicated for people living with propionic acidemia and short chain fatty acid metabolism disorders.  Also if short chain fatty acid intestinal production is theorized to be helpful in intestinal function and health; the avoidance of these scfa promoting fibers may negate part of the health benefits of taking fiber in the first place.

There are more studies and public awareness about the probable health benefits of consuming an adequate amount of fiber in foods that may also apply to people living with OAs. But further studies are needed to prove the health benefits, clarify the amount of daily fiber and the type of fibers most appropriate for people living with OAs.  It’s known that anyone trying to increase dietary fiber intake should do so slowly over time with adequate fluids to lessen the acute symptoms of abdominal distention, bloating and flatulence.  People living with OAs should always first consult their metabolic clinic physician and dietitian before adding fiber supplements and/or trying to increase dietary fiber intake in foods to best weigh the health risks vs. health benefits for their individual situations. 

1. Roberta Duyff, MS, RD, FADA, CFCS. ‘Fiber Your Body’s Broom’. American Dietetic Association Complete Food and Nutrition Guide, 2nd Ed. John Wiley & Sons, Inc. Hoboken, NJ 2002
2. Joanne Slavin et al. ‘Position of the American Dietetic Association: Health Implications of Dietary Fiber’ Journal of the American Dietetic Association, Oct  2008;108:1716-1731
3. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. 2005. Washington, D.C.: The National Academies Press. Website: http://books.nap.edu/openbook.php?isbn=0309085373
4. USDA National Nutrient Database for Standard Reference website:http://www.nal.usda.gov/fnic/foodcomp/search/
5. Anne Pylkas, Lekh Juneja, and Joanne Slavin. “Comparison of different fibers for in vitro production of short chain fatty acids by intestinal microflora” Journal of Medicinal Food,  March 2005; 8(1):113-116

Written for the 2009 Winter OAA Newsletter