A critical appraisal of the efficacy of dietary supplements on skin ageing

The aim of this thesis is to provide a comprehensive and detailed understanding of an aspect of nutrition and dermatology. In particular, in investigating the evidence for the anti-ageing effects of oral supplements and topical applications on the skin by assessing the measurement parameters and methods used. The thesis broadly examines research related to the use of either topical or oral nutrient supplements for reducing skin aging. Chapter 2 focuses on a scoping review which assesses the literature base for human intervention trials that have examined the efficacy of dietary supplements by examining their effects on parameters of skin aging and the commonly used skin parameters, and reporting the equipment and methodologies used. Human studies addressing skin anti-ageing effects of dietary supplements were systematically searched from February 2021 to April 2022 from four databases: CINAHL, Embase, PubMed, and Scopus. The scoping review protocol was submitted to the Open Science Framework with the DOI 10.17605/OSF.IO/7A8JB as a prospective registration. The Cochrane Risk of Bias (version 2) assessment tool was used to assess the quality of the randomised controlled trials (RCTs) included in the review. The selection criteria used resulted in 87 eligible studies of which, 30 human studies were suitable for detailed analysis which included 22 randomised controlled trials and, 10 intervention studies with two studies contributing to both. The majority of the studies yielded positive outcomes, with dietary supplements enhancing a wide range of skin parameters used to assess indications of ageing. However, 7 of the 22 RCTs examined had a high risk of bias, whereas 10 had a low risk of bias and 5 at some concern level. Moreover, 18 representative skin parameters were identified and reviewed, which are commonly used in the review studies which assessed either appearance, physical qualities, or biochemical parameters of the skin. The most widely measured skin parameters are skin wrinkles, hydration and elasticity with the corresponding measured methodologies or equipment are skin replica analysis, corneometer and cutometer. A meta-analysis was not possible since the included studies employed a range of dietary supplements and employed a diversity of different methodologies. To our knowledge, this is the first review that has classified and summarised the skin parameters, methods and equipment in skin ageing and food supplementation. In conclusion, these results support the claims that topical applications can slow down the progress of skin ageing, but also, this is enhanced if combined with oral supplements. Based on the scoping review, the most effective oral supplements are those with combination formulations including micronutrients, macronutrients and anti-oxidants or plant extracts.