B cells face multiple restrictions on glucose and energy metabolism. Their lineage-determining transcription factors repress glucose uptake and pentose phosphate pathway activity, while their low numbers of mitochondria and small cytoplasmic volume set narrow limits for mitochondrial ATP production and autophagy as alternative energy sources. During activation, B cells can balance temporary increases of energy expenditure. However, permanent hyperactivation of kinases, for instance, downstream of an autoreactive B cell receptor (BCR) or a transforming oncogene, can cause energy stress and cell death. Here, I propose that B cell-intrinsic restriction of ATP represents a safeguard to eliminate autoreactive or pre-malignant B cells. If the metabolic gatekeepers are compromised, influx of additional glucose may fuel permanent increases in metabolic demands and pathological B cell proliferation, driven by an autoreactive BCR or a transforming oncogene.