Dietary Factors Affecting Learning Behavior
Types of diet that can impact learning.
By Margaret Lahey and Shari Rosen
Diet can influence cognition and behavior in a number of ways. These include both the obvious condition of not enough food, and the less obvious problems associated with adequate caloric intake but insufficient intake of particular nutrients. This section includes these factors whereas later sections concern other factors including digestion of foods and adverse reactions to particular foods.
Few would doubt that an insufficiency of food could influence learning and behavior (Smart, 1993). Food deprivation is known to influence brain neurochemistry and the development of the nervous system (Wiggins, Fuller, & Enna, 1984). Children from families reporting food insufficiency are more likely to have problems with health, psychosocial behavior, and academic learning than are children from similar income levels where the families do not report insufficient supply of food (Alaimo, Olson & Frongillo, 2001; Alaimo, Olson, Frongillo & Briefel, 2001; Kleinmann, Murphy, Little et al., 1998). According to estimates by the Third National Health and Nutrition Examination Survey, a little over four percent of the population in the United States suffers from food insufficiency (Alaimo, Briefel, Frongillo et al., 1998).
But having enough calories to consume is not the only way in which diet could affect learning and behavior. Malnutrition can co-exist with sufficient caloric input and if experienced early in life can lead to long-lasting changes in brain neural- receptor function, particularly those functions that are related to emotional responses to stressful events (Strupp & Levitsky, 1995). Animal experiments indicate that nutrition in fetal and early postnatal stages can influence factors such as metabolism, behavior, and learning (Lucas, Morley, & Issacs, 2001; Strupp & Levitsky, 1995). In some cases animal performance improves when deficient nutrients are added to the diet. In human infants, low birth weight is associated with risk for learning disabilities as well as schizophrenia and one factor influencing low birth weight is sub-optimal maternal nutrition (e.g., Johnson, & Breslau, 2000; Suagstad, 1999; Wahlbeck, Forsen, Osmond, et al., 2001; Worthington-Roberts, 1985). For human infants, deficiency in intake of certain nutrients such as iron, zinc, and long chain fatty acids have been associated with lower performance on cognitive tasks (Lucas et al., 2001; & for review of the importance of long chain fatty acids see Lipids paper in this series, Lahey, 2002). For example, preterm infants fed breast milk score higher on cognitive tasks than those fed un-enriched formula (e.g., Birch, Garfield, Hoffman, et al., 2000; Lucas et al., 2001). In their review of the influence of nutrition on mental development, Lucas and colleagues (2001) conclude that failure to meet overall nutrient needs during critical periods of brain development has significant consequences for cognitive development and that human milk contains factors that promote neurodevelopment and can lessen the effects of poor nutrition.
The effects of malnutrition continue beyond infancy. Malnourished children in India were found to have delays in vision, fine motor skills, language skills and personal-social skills (Vazir, Naidu, & Vidyasagar, 1998). Malnutrition is generally thought by many in the United States to be a problem for so called developing nations. However, it is clear that even when food supply is sufficient, nutrition is often less than optimal in the United States. For example, inadequate fruit and vegetable intake was reported by approximately one-third of Minnesota adolescents who completed a questionnaire on food habits; poor food habits were, in turn associated with chronic illness and poor school achievement (Neumakr-Sztainer, Story, Resnick & Blum, 1998). Over 80 percent of the adults in a county in Minnesota also failed to meet standard dietary recommendations for fruits and/or vegetables and for dietary fat per day (DeBoer, Thomas, Brekke, et. al., 2003). Moreover, one in six children in the United States lives in poverty (Reuters Health News, 2001) and poverty is associated with poor nutrition even when insufficiency is not a problem (Nelson, 2000). Fortunately, there is some evidence that improving nutritional intake positively affects cognitive development (e.g., Freeman, Klein, Townsend, & Lechtig, 1980).
Given adequate supply of both macro- and micro-nutrients, dietary factors can still have an effect on learning and behavior. Even in healthy individuals, feelings of fatigue and distress decline thirty minutes after eating a meal regardless of whether the meal was primarily carbohydrate or protein (Christensen & Redig, 1993). After fasting overnight, ingestion of a particular macronutrient (i.e., fat, carbohydrate, or protein) differentially improved performance in comparison to placebo on a series of tasks; all nutrients influenced delayed paragraph recall but only fat improved attention and only protein improved rate of forgetting (Kaplan, Greenwood, Winocur, & Wolever, 2001). In addition to the overall nutrient content, diet contains substances that can affect mood, behavior, and cognition either directly or through metabolic conversion (Groff & Gropper, 1999). In a prior paper posted in this series, "Lipids: their possible role in developmental disorders,” we reviewed data on how the inclusion of foods high in the omega 3 essential fatty acids might have a positive effect on cognition, mood and behavior. Some foods contain pharmacologically active substances that have the potential to affect behavior (Warner, 1995). The metabolites of some foods have the potential to influence neurotransmission; some amino acids available from foods are precursors for neurotransmitters within the body. For example, carbohydrate meals not only elevate blood glucose but also the plasma ratios of amino acids that influence neurotransmission (Christensen & Redig, 1993). Moreover, certain peptides derived from digestion of proteins in milk and wheat have narcotic like qualities that have the potential to affect neurotransmitters if they get into the central nervous system. Foods, or their additives, may even be able to prevent cognitive degeneration. Recent evidence indicated that the spice curcumin reduced Alzheimer type brain pathology in mice, which may explain the finding of low prevalence of Alzheimer's disease in India where the spice is widely used in food preparation (Chandra, Pandav, Dodge, et al, 2001; Lim, Chu, Yang, et al., 2001). It is also possible that diet influences the expression of certain genes (Simopoulos, 1994). Thus, as noted in Hypothesis A above, the foods we eat do have the potential to negatively or positively affect cognition and behavior and this potential appears to vary with individuals.