Diet does have an impact
This one is definitely a neuro-hit: everyone agrees that in one way or another diet has an impact on children’s cognitive abilities. However, it’s also quite a broad statement, and although there are a lot of studies exploring links between diet and behaviour, there are also lots of holes in our knowledge, making it quite a controversial area. Here we will look at the short-term effects of what children eat then at the longer term influence of certain dietary choices.
Fourteen percent of UK school children skip breakfast, with this being more likely in the case of secondary school children and in areas of deprivation[i]. The studies looking at what happens to children’s performance in school when they miss breakfast show that performance is most clearly affected when tasks are more mentally demanding[ii], when they involve working memory (storing and manipulating information in the short term)[iii], and in the case of children who are poorly nourished in general[iv].
These studies primarily assume that breakfast is first and foremost a way of increasing levels of blood glucose, meaning that more energy is available to the energy-hungry brain throughout the morning. However, the short-term effects of eating on cognition could operate in at least two other ways[v]: consuming carbohydrates and consuming protein lead indirectly to the release of neurotransmitters in the brain[vi], which control communication between neurons and regulate mood, arousal and motivation, which in turn are likely to influence academic performance. At the moment there isn’t much work considering the type of breakfast that children consume and therefore the short-term influence of protein and carbohydrates on learning in school.
Another indirect effect of diet on cognition is via the negative impact of hyperactive behaviour. This has been a controversial issue for many years, but a recent comprehensive review[vii] found that artificial food colours have a small but measurable negative impact on children’s behaviour. Both school aged and pre-school children were found to be more hyperactive (over-active, inattentive and impulsive) after consuming artificial food colours, including but not limited to those with diagnosable ADHD. The authors suggest that there may be an impact on the classroom learning environment if the majority of children are experiencing these small behavioural changes; they also note that the magnitude of reported effect is reminiscent of the subclinical lead poisoning that resulted in the banning of leaded petrol. There is evidence that some children may be genetically at risk for being sensitive to artificial food colours [viii].
The most extreme long-term effect of diet on cognition is from malnutrition or undernutrition. According to the World Bank[ix], where undernutrition leads to stunted growth in the first two years of life, deficits in cognition are seen right through the teenage years, even after nutritional rehabilitation; and in various developing nations direct associations have been observed between the intake of calories, fat and protein and measures of cognitive development in pre-school children. Importantly, the benefits of supplementing the poor diet of children under two are known[x], but the benefits after two are not clear, highlighting the importance of good nutrition in the first years of life. It should be noted that undernutrition is usually accompanied by extreme poverty, which could impact on children’s mental development in many ways, making the unique contribution of diet difficult to untangle.
One area of diet receiving increased interest is the impact of micronutrients (vitamins and minerals), some of which are specifically linked to aspects of brain development and cognition in children[xi]. For example, an estimated 25% of the world population suffers from anaemia due to iron deficiency[xii], which, if it occurs in the first two years of life, is associated with short and long-term deficits in cognitive development and school achievement that stretch right through to the end of children’s school career[xiii]. Other micronutrients of note include iodine, with iodine insufficiency in children being associated with around a 13 point drop in IQ; and thiamine, an insufficiency of which can lead to aggressive behaviour in adolescents that can be reversed by providing thiamine supplements[xiv].
Micronutrient deficiencies are most usual in the context of general undernutrition, so evidence for the impact of deficiencies in the west is less persuasive. This is partly due to better diets in general and partly to access to fortified food (as initiated by the World Health Organisation[xv]), which includes the iodization of salt.
It is believed, however, that the western-style diet, which is high in saturated fat and added salt and sugar, comes with its own problems linked to cognition. Although the problems of the western-style diet are not well understood, again the manipulation of neurotransmitters is thought to be important here. In animal studies very clear results have emerged[xvi] showing deficits in learning and memory from about a week after switching to a diet high in fat, salt and sugar.
Children from deprived backgrounds are the ones at risk
The starkest effects of diet on learning are of course seen in those suffering from undernutrition, which is more common in developing nations. However, diet is related to socioeconomic status even in affluent countries, such that although children from more deprived backgrounds don’t tend to take in fewer calories, their diets do tend to be lower in quality, with fewer micronutrients and more added fat[xvii]. Children from deprived backgrounds in the UK are also more likely to skip breakfast, meaning that young children are at risk for poor school performance, in both the short and long-term, as a result of poor nutritional intake all over the world.
Notably, there is still a lot of work to be done on which aspects of diet influence what sort of thinking in which children. The research so far stresses the importance of good nutrition in the first two years of life, which really do seem to be crucial in this case. So the verdict here? Although it’s probably not as clear cut as most people think, (and see ‘Fish oils improve learning’), this is definitely a neuro-hit.
For a thorough and very readable summary from the WorldBank, see: http://www.cmamforum.org/Pool/Resources/Nutrition-brain-development-early-life-A-TTechnical-Brief-2012.pdf
For a report from 2008 for the Associate Parliamentary Food and Health Forum on the links between diet and behaviour see: http://www.foodforthebrain.org/media/229766/FHF.pdf
[iii] Mahoney C, Taylor H, & Kanarek R. The acute effects of meals on cognitive performance. In: Lieberman H, Kanarek R, Prasad C, eds. Nutritional Neuroscience. Boca Raton, FL: CRC Press; 2005, 73–91
[iv] Simeon, D., Grantham-McGregor, S. (1989). Effect of missing breakfast on cognitive functions of schoolchildren of differing nutritional status. American Journal of Clinical Nutrition, 49, 646–653.
[vi] Carbohydrates increase the ratio of the amino acid tryptophan in the blood relative to other amino acids, and as tryptophan is the precursor of serotonin, this results in an increase in serotonin levels. Tyrosine is an amino acid found in high-protein foods, and is converted in the brain to l-dopa, which in turn is converted to dopamine and subsequently norepinephrine and epinephrine.
[vii] Arnold, L. E., Lofthouse, N., & Hurt, E. (2012). Artificial food colors and Attention-Deficit/Hyperactivity symptoms: Conclusions to dye for. Neurotherapeutics, 9, 599–609 DOI 10.1007/s13311-012-0133-x
[viii] Stevenson, J., Sonuga-Barke, E., McCann, D., Grimshaw, K., Parker, K. M., Rose-Zerilli, M. J., Holloway, J. W., & Warner, J O. (2010). The role of histamine degradation gene polymorphisms in moderating the effects of food additives on children’s ADHD symptoms. American Journal of Psychiatry, 167, 1108-1115.doi: 10.1176/appi.ajp.2010.09101529
[x] See Grantham-McGregor, S., & Barker-Henningham, H. (2005). Review of the evidence linking protein and energy to mental development. Public Health Nutrition, 8 (7A), 1191–1201 DOI: 10.1079/PHN2005805
[xiii] Lozoff, B., Beard, J., Connor, J., Barbara, F., Georgieff, M., & Schallert, T. (2006). Long lasting neural and behavioral effects of iron deficiency in infancy. Nutrition Reviews, 64 (5;2), S34-S43.