Nutrition Across the Life Course and Risk of Young-Onset Breast Cancer: Mechanisms, Evidence, and Prevention Opportunities

The incidence of cancer in young adults has risen worldwide. Women comprise a disproportionate share of young-onset cases, among whom breast cancer predominates. This shift parallels globalization and urbanization, including the wider adoption of Western-pattern diets. Although hereditary syndromes explain a minority of cases, the secular rise underscores the impact of modifiable exposures, particularly diet. Prenatal life, neonatal life, childhood, adolescence, and early adulthood are critical periods during which dietary exposures may shape long-term mammary development. Mammary tissue undergoes rapid proliferation and differentiation during development, creating windows of heightened susceptibility to carcinogenic insults. However, most existing studies emphasize dietary exposures during a single developmental period; the entire span of critical developmental windows plays a formative role in shaping young-onset breast cancer (YoBC) risk, and the mechanisms underlying this life-course shaping remain insufficiently characterized. This review comprehensively synthesizes evidence on how nutrition across sensitive developmental windows shapes the risk of YoBC. We evaluate protective and adverse dietary factors within these stages and examine mechanistic pathways linking early-life nutrition to carcinogenesis, focusing on hormonal regulation, epigenetic programming, chronic inflammation, and the gut microbiome. A structured literature search was conducted in PubMed, Embase, and Web of Science for English-language articles published from 1990 through May 2026, supplemented by hand-searching of relevant reviews and key primary studies. By framing nutrition and breast cancer through a life-course lens, this review provides an integrated foundation for stage-specific prevention strategies and identifies priority directions for future research on early-life dietary determinants of YoBC.