Research over the past 100 years (1) has repeatedly indicated caloric restriction protects against tumor development and increases longevity. However, more recent research, specifically two studies in monkeys (2), suggests these benefits may only be seen when caloric restriction is compared to an unhealthy diet or unhealthy controls (3).
The majority of older research on caloric restriction tested the effects of continuous energy restriction (CER) — that is, reducing the number of calories consumed every day for weeks, months, or years. Recent research has begun to focus on intermittent energy restriction (IER), which refers to any time-restricted form of calorie restriction, ranging from daily or extended fasts to alternate-day restricted feeding. This 2016 paper reviews research suggesting IER may reduce cancer risk, cancer progression, or cancer biomarkers.
At the time of this review’s publication, the majority of the research testing the impact of IER on cancer biomarkers or progression had been conducted in rat studies. The majority of these studies (4), but not all (5), indicated IER reduces rates of tumor development. IER more consistently prevented the growth and development of mammary tumors than prostate tumors, which may be explained in part by the interruptions in menstrual cycles induced by IER in rats (6).
Two other trials provide a potential explanation for these inconsistencies (7). In these studies, IER regimens involving complete or near-complete caloric restriction (>75%) on fasting days suppressed IGF-1 (insulin-like growth factor) levels and reduced tumor cell proliferation while less intensive fasts did not. It is worth noting, however, that rodents and humans respond differently to some fasting regimens, with rodents transitioning into ketosis more quickly and with less caloric restriction than fasting.
At the time of this paper’s publication, the authors did not find any research specifically testing the effects of IER on tumor development and/or progression in humans. They did note, however, that IER (often an alternate-day fast or similar program) rapidly reduces serum insulin levels and insulin receptor activity in humans, which is hypothesized to reduce cancer risk (8).
These conclusions are consistent with those featured previously on CrossFit.com. Valter Longo has argued the metabolic benefits of fasting are primarily due to fasting-induced suppression of IGF-1. Others similarly argue fasting triggers a “metabolic switch” that transitions the body from a state of high insulin, high glucose, and carbohydrate dependence to one of low insulin and glucose, reduced carbohydrate dependence, and increased fat burning; they also argue this metabolic transition reduces progression of metabolic disease and cancer risk factors.
The takeaway: Preliminary evidence, primarily in rodents, suggests fasting reduces cancer risk biomarkers (including IGF-1 and insulin levels) and may reduce the development and progression of certain tumors. More intensive fasts, such as complete fasting (100% caloric restriction) on alternating days, show more consistent benefits; more moderate fasts show equivocal clinical impact.