- Introduction: Obesity compromises survival in children with cancer in high-income countries (HICs) and is accompanied often by sarcopenia. In low and middle-income countries (LMICs), where the great majority of children live, the prevalence of under-nutrition is as high as 95% in those with cancer. Nutritional support improves clinical outcomes, including survival.Areas covered: This narrative review describes the evolution of attention to nutrition in children with cancer and the increasing understanding of this relationship. An initial focus on obesity in children with acute leukemias in HICs has been matched more recently by a recognition of the negative effect of under-nutrition on survival in children with cancer in LMICs. These observations have stimulated explorations of underlying mechanisms, including dysbiosis of the gut microbiome, and structured nutritional interventions to redress adverse outcomes.Expert opinion: Studies of the gut microbiome and metabolome have yielded important information on the pathogenesis of malnutrition in children, providing new avenues for interventions. Combinations of plant products that are inexpensive and readily available in LMICs have been shown to 'mature' the microbiome and the corresponding plasma proteome in children with acute malnutrition, offering the prospect of cost-effective remedies that are tested in children with cancer.