Does the fasting-mimicking diet such as Prolon actually work?
For healthy people with risk factors such as elevated blood pressure or blood lipids, there are indications that the diet offers short-term benefits, but the long-term effects and applications in diseases such as diabetes or cancer have not yet been proven in humans.
A randomised trial with 100 participants showed that three cycles of five consecutive fasting days per month produced measurable improvements: lower body weight, less abdominal fat, lower blood pressure, and a lower level of a growth-hormone-like substance linked to ageing and cancer. No serious side effects were reported. The effect was greatest in people who already had elevated values for blood pressure, blood sugar, or blood lipids.
In that same group, the diet also improved fasting blood sugar, blood lipids, and a measure of inflammatory activity in the blood. The last finding came from a post-hoc analysis of the trial, meaning it was not the primary aim of the study. The finding is therefore interesting but less definitive than the main result.
For people with excess weight, a review article concludes that the diet can promote weight loss, but that hardly any studies have followed its effect over the longer term. Whether the weight stays off after a year or two has not been properly investigated.
For more serious conditions such as MS, inflammatory bowel diseases, breast cancer, and diabetes, expectations should be tempered. For MS and bowel diseases, the positive results come largely from mouse studies, supplemented by very small pilot studies in humans. The same applies to breast cancer: promising mouse data and comparable metabolic changes in patients, but evidence that tumours actually shrink in humans is still lacking. For type 2 diabetes, a randomised trial has only just begun; no results are yet available.
A not unimportant caveat across several studies: lead researcher Longo has a financial interest in L-Nutra, the company behind the commercial FMD product Prolon. This does not mean the results are incorrect, but keep it in mind when evaluating the more optimistic claims. Furthermore, the authors of a review article warn that prolonged or repeated dietary restriction may adversely affect the immune system and wound healing.
Two randomised studies (one with human data, one conducted primarily in mice), four non-indexed publications (including a study protocol and two mouse studies), and two review articles (narrative, not meta-analyses). Total human randomised evidence is limited to essentially one RCT (n=100) for metabolic outcomes.