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Childhood energy intake and adult mortality from cancer

Editor—Frankel et al suggest that they have shown that restriction of nutrients in childhood protects against cancers not related to smoking.1 Unfortunately, while working ingeniously with the data, they seem not to appreciate their more fundamental failings. Individual dietary assessment as a research tool is notoriously unreliable,2 and when individual intake is estimated from family intake the potential for bias and error multiplies.3 The authors acknowledge this in their discussion, but they still present a misleadingly precise looking table of age specific mean intakes, which must actually have been estimated from the family data. 
 
An association was found despite the imprecision of the survey instrument, but the authors have not adjusted for all confounders. The two main determinants of energy intake are lean body mass and activity levels. Activity levels are not likely to be associated with risk of cancer, but lean body mass is strongly correlated with height. The data thus probably simply show that tall children eat more and also have a higher risk of cancer. The authors also treat their subjects as individuals, when the risk factor of interest has been measured at a family level, inflating the sample size threefold or fourfold. They should have used the family as the unit of analysis; if they had done so they might not have shown a significant association. 
 
Energy intake throughout childhood seems unlikely to explain the relation between height and risk of cancer since it is predominantly in infancy that growth depends on energy; requirements for growth thereafter represent a tiny proportion of total intake.4 Finally, despite the increase in the risk of cancer, all cause mortality was unrelated to energy intake, suggesting that it also protects against other causes of death. Throughout history energy restriction has led to increased death rates, which have plummeted in the developed world as the food supply has improved. I would therefore suggest that much more than replication of these findings will be needed before there can be any implications for public health.