Studies of pre- or postnatal aflatoxin exposure and postnatal growth

Two studies were published involving a total of 680 children living in four agro-ecological zones of Benin and Togo in West Africa (Gong et al., 2002, 2004). In the cross-sectional study, height-for-age and weight-for-age were lower in a dose-dependent fashion for increasing aflatoxin exposures as measured by aflatoxin–albumin adducts (AF–alb) in serum (Gong et al., 2002). Separately, in a multivariate analysis controlling for these factors as well as age and sex, it was determined that AF–alb levels in children’s serum were significantly associated with weaning status: the earlier the weaning, the higher the aflatoxin exposure (Gong et al., 2003). In the longitudinal study, over a period of 8 months children with the highest aflatoxin exposures had the smallest gains in height (Gong et al., 2004). These results were also adjusted for weaning status, agro-ecological zone, and socioeconomic status. The important contribution of this body of work is that in both a cross-sectional and a longitudinal study, higher aflatoxin exposures were shown to be correlated with children’s height-for-age and also growth trajectories over a critical period of child development.

A study in The Gambia found a significant association between in utero aflatoxin exposure and growth faltering in infants (Turner et al., 2007). This longitudinal study of 138 pregnant women and their infants followed the infants for 1 year and controlled for season, sex, placental weight, maternal weight, and gestation time, with AF–alb measured by enzyme-linked immunosorbent assay (ELISA). AF–alb in maternal blood serum was a strong predictor of length/height gain and weight gain in the first year of life. It was predicted that if the maternal AF–alb levels dropped from 110 pg/mg to 10 pg/mg, the weights and heights of infants at age 1 year would increase by 0.8 kg and 2 cm, respectively (Turner et al., 2007).

In the United Republic of Tanzania, Shirima et al. (2015) studied a cohort of 166 infants aged 6–14 months at enrolment and followed them for 12 months. AF–alb was measured by ELISA at baseline and 6 and 12 months later. Anthropometric measurements were also taken at each time point. Aflatoxin levels in this study were lower than in the West African studies, rising from a geometric mean of 4.7 pg/mg at baseline to 23.5 pg/mg at the end of the study. The authors found no significant association between aflatoxin dose and stunting in this population.

No study has found an association between aflatoxin exposure and wasting, although wasting was not common in these populations.

Establishing causality of the association between aflatoxin exposure and growth faltering, as reported for studies in Benin and Togo, is uncertain due to the general difficulty of separating the effects of aflatoxin level from possible poor quality of the child’s diet. However, in the longitudinal study there was no association between AF–alb and micronutrient levels, suggesting that aflatoxin exposure was not accompanied by a general micronutrient deficiency (Gong et al., 2004). Furthermore, the infant diet in The Gambia includes groundnuts, as opposed to maize in Benin and Togo, and yet results were broadly consistent across these populations. The lack of an association between aflatoxin exposure and growth impairment in the Tanzanian study suggests that there may be a threshold effect. Generalizing the evidence from these four studies is difficult because of their limited geographical distribution (three sites in West Africa) and insufficient information on the links between aflatoxin level, dietary and other cofactors, and growth outcomes.

Studies of maternal aflatoxin exposure and birth outcomes

Shuaib et al. (2010) studied mothers’ AF–alb levels at delivery and birth outcomes (preterm birth, small-for-gestational-age, low birth weight, and stillbirth) in Kumasi, Ghana. In this study, AF–alb was measured using high-performance liquid chromatography (HPLC) in the blood of 785 mothers immediately after they had given birth. After adjusting for sociodemographic variables (age, education, socioeconomic status, residence, and type of toilet facilities), it was found that the mothers in the highest quartile of AF–alb levels were at significantly higher risk of having babies with low birth weight, defined as being below 2.5 kg (adjusted odds ratio, 2.09; 95% confidence interval, 1.19–3.68). None of the other birth outcomes were associated with aflatoxin measure.

For the postnatal growth outcome in the Turner et al. (2007) study in The Gambia described above, birth weight and length were measured but were not associated with maternal AF–alb concentrations in mid and late pregnancy.

The validity of the findings from these studies on aflatoxin and low birth weight is uncertain because they have small sample sizes for adverse birth outcomes, and thus may not be sufficiently powered to detect important outcomes. Furthermore, it is difficult in observational studies to separate the effects of aflatoxin dose from possible poor nutritional quality of the maternal diet (i.e. monotonous maize diet with little dietary diversity).

Studies of postnatal fumonisin exposure and infant growth

Two recent studies from the United Republic of Tanzania suggest that fumonisin exposure may also be associated with stunting in children. Kimanya et al. (2010) estimated fumonisin exposure in 215 infants by measuring fumonisin in maize flour and estimating the daily fumonisin intake of the infants based on mothers’ dietary recall. In this prospective cohort study, infants were enrolled at age 6 months and followed until age 12 months. Exposure was categorized as high or low using the Joint WHO/FAO Expert Committee on Food Additives (JECFA) provisional maximum tolerable dietary intake (PMTDI) of 2 µg/kg body weight/day as the cut-off. Even at baseline, the 26 infants in the high-exposure category were shorter than those with low exposure. By age 12 months, the highly exposed infants were significantly shorter (by 1.3 cm) and lighter (by 328 g) on average than the 105 infants with low exposure, after controlling for total energy and protein intake, sex, and village.

In the same study in the United Republic of Tanzania described above for aflatoxin exposure, Shirima et al. (2015) found that levels of urinary fumonisin B₁ (UFB₁) at recruitment were negatively associated with length-for-age Z-scores (LAZ) at both 6 months and 12 months after recruitment. Mean levels of UFB₁ from all three sampling times showed an inverse association with LAZ and length velocity at 12 months after recruitment. UFB₁ levels (averaged from two urine samples) at baseline and 6 months were associated with LAZ at 6 months and 12 months, respectively. Mean UFB₁ levels from all three time points were strongly inversely related to LAZ at 12 months.

These initial studies of fumonisin and infant growth are small and offer only limited evidence but do strongly suggest the need for further research on this relationship. The Shirima et al. (2015) study also demonstrates the co-occurrence of aflatoxin and fumonisin in maize-based diets and emphasizes the need for multiple mycotoxin assessments to make clear inferences about causal factors.

Uniting aflatoxin and fumonisin in a single framework is critical because dietary co-exposure is common in Africa and parts of Latin America (see Chapter 1). Smith et al. (2012) suggested possible mechanisms by which foodborne mycotoxin exposure, singly or in combination, may contribute to impaired growth by compromising gut health. Gut enteropathy has been associated with chronic immune stimulation, which is inversely correlated with growth during infancy (Campbell et al., 2003). Increased intestinal permeability may allow translocation of microbial products, which can stimulate a systemic inflammatory response. Smith et al. (2012) described two main pathways by which environmental enteropathy may cause growth retardation: malabsorption of nutrients in the small intestine and systemic immune activation, resulting in suppression of the insulin-like growth factor 1 (IGF-1) axis, which is strongly associated with stunting in African infants (Prendergast et al., 2014). In older children (6–17 years), there is evidence that aflatoxin modulates IGF-1 (Castelino et al., 2015).

Scientific gaps and future research needs

Taken together, the studies described above suggest that mycotoxin exposure contributes to child growth impairment independent of, and together with, other risk factors that may cause stunting.

Among the multiple potential causes of growth faltering in young children globally, dietary mycotoxin exposure emerges as a potentially important factor. The weight of evidence linking aflatoxin with growth impairment has increased over the past five decades of research – first, primarily in animal studies and, in the past decade, in the epidemiological studies reviewed above.

One critical knowledge gap is the mechanism or mechanisms by which mycotoxins may cause child growth impairment. Nor, indeed, is it known whether all mycotoxins use the same mechanism of toxicity that leads to growth impairment (and this should not be assumed). As such mechanisms are elucidated, the weight of evidence linking mycotoxins with growth impairment would become stronger. Several possible mechanisms have been proposed; certainly, one or more may be relevant to the role of mycotoxins in growth impairment.

Immune system dysfunction mediated by mycotoxin exposure (Bondy and Pestka, 2000; Turner et al., 2003) could increase risk of infections in children, which can lead to growth impairment from energy losses (e.g. diarrhoea or vomiting) and/or energy expended on recovery from illness. Also, aflatoxin/fumonisin-mediated changes in intestinal integrity could make hosts more vulnerable to intestinal pathogens (Gong et al., 2008b; Smith et al., 2012).

The IGF-1 axis may represent a common causal pathway in mycotoxin effects on hepatocellular carcinoma as well as growth retardation. Deregulation of the IGF axis has been identified in the development of hepatocellular carcinoma. An increased expression of IGF-2 and the IGF-1 receptor (IGF-1R) and associated binding proteins with degrading receptors have emerged as crucial events in malignant transformation and tumour growth, in altering cell proliferation, and in deactivation of apoptotic pathways. Aflatoxin B₁ (AFB₁) was shown to induce phosphorylation of IGF-1R and activation of the signalling cascade involving Akt (also known as protein kinase B) and Erk1/2 (extracellular signal-regulated protein kinases) in hepatoma cell lines (Ma et al., 2012). AFB₁ was also found to downregulate insulin receptor substrate 1 (IRS-1) while upregulating IRS-2 through preventing proteasomal degradation. Of interest is that the p53 mutant p53-mt249 increases IGF-2 transcription, suggesting that p53 mutation may be a link between AFB₁ and IGF-2.

Given the widespread global prevalence of aflatoxin and fumonisin exposures and the large associations observed with stunting in the seminal studies from West and East Africa, additional prospective studies are needed in a wider variety of contexts. If the associations reviewed in this chapter are established, then the global burden of disease associated with mycotoxin exposure may be far greater than that based on mycotoxin links to cancer. Future prospective studies must be designed with adequate sample size to elucidate thresholds in dose–response and rigorously control for other known causes of growth faltering, such as low nutrient intake and diarrhoea prevalence. Studies of wasting as an outcome (in addition to stunting) would be informative. Intervention studies in humans are ultimately needed to disentangle effects of toxins from effects of monotonous maize diets and associated poverty.