The skin barrier is one of the body’s first lines of protection against environmental insults, including allergens, chemical irritants, microbial insults, and UV radiation. Disruption of this natural line of defense can result in xerosis, irritation, chronic dermatitis, and other cutaneous effects [ 1 ] . Given the skin barrier’s major role in providing protection from multiple threats, there has been much interest in strategies to improve and “strengthen” the skin barrier. A number of research studies have now examined the role of specific dietary interventions in enhancing skin barrier function. We review this research and provide an overview of the effects of prebiotics, probiotics, fatty acids, and emerging research on other substances.

Many methods of measuring skin barrier function exist. Despite the many methods available, the optimal method of measurement is not well characterized, and conflicting results have been noted when evaluating the utility and accuracy of some methods. These uncertainties must be considered when evaluating the results of any dietary interventions on skin barrier function.

The most commonly used methods measure the physical layer of the skin barrier. The parameters targeted most often are stratum corneum hydration (SCH) and transepidermal water loss (TEWL). SCH measures the static water-holding capacity of the skin, while TEWL measures dynamic water losses [ 13 ] . Research indicates that both of these measures may be necessary to fully characterize the functioning of the skin barrier, and they are often used together. SCH is the amount of water contained in the stratum corneum. A few different methods are used to measure this parameter. These include electrical measurements such as capacitance, electrical impedance, and conductance [ 14 ] . Capacitance in particular measures surface hydration, but does not give information about the actual water content or thickness of the skin [ 5 ] . A newer method is Raman spectroscopy paired with multiphoton microscopy, which can measure full-thickness hydration of the stratum corneum [ 5 , 15 ] . Studies on evaluation of SCH reveal limitations of these measurement methods. Another measure of skin barrier function is that of transepidermal water loss (TEWL), which measures the insensible water loss through the skin and provides information on the lipid properties of the skin [ 14 ] . This method has been validated for in vivo study of skin permeability [ 16 ] although the utility of this method for in vitro testing is questionable.

Synbiotics are a combination of probiotics and prebiotics, and their benefits are theorized to be due to effects on the gut microbiome. Among other effects, beneficial gut microbes produce short chain fatty acids, which help improve epithelial barrier function and modulate the inflammatory response [ 17 , 18 ] . Phenols, in contrast, are considered markers of a disturbed gut microbiome and have been demonstrated to disrupt keratinocyte differentiation in mice [ 19 ] .

Probiotics are live microorganisms that are thought to have health benefits [ 20 ] . Lactobacillus , Bifidobacterium , and Saccaromyces species are among the most common microbes found in probiotics, and many but not all are found in human flora [ 21 , 22 ] . Prebiotics such as galactooligosaccharides (GOS) are substances which, when ingested, preferentially improve the growth of beneficial gut microbes.

Clinical studies of synbiotics in the treatment of AD have been promising, but little information is available on their skin barrier effects. In a meta-analysis of 6 studies, synbiotics with mixed bacterial strains were helpful for the treatment of AD in adults and children over the age of 1 year [ 23 ] . In a separate study, skin barrier function was measured. In this randomized controlled trial (RCT), consumption of the prebiotic GOS along with a probiotic milk for 4 weeks led to improved skin hydration. Serum phenol levels were also restored [ 24 ] .

Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Derivatives of omega-3 fatty acids are thought to influence the skin barrier by acting as transcription factors and decreasing inflammation as immune modulators [ 35 ] .

Studies in skin models and cultured human keratinocytes suggest that DHA can increase filaggrin expression, attenuate inflammation, and improve epidermal keratinocyte differentiation [ 36 – 38 ] . An animal study demonstrated decreased TEWL, increased skin hydration, and decreased skin barrier alteration by acetone [ 39 ] . EPA and the ester of EPA have been shown in animal studies to influence ceramides in skin [ 40 , 41 ] . However, studies have utilized different methodologies and have shown varying improvement [ 36 – 38 ] .

Several small human trials have examined the effects of oils rich in fatty acids. Flaxseed oil, high in ALA, was shown to improve TEWL, skin hydration, skin scaling, and roughness in female study subjects after 12 weeks of daily consumption [ 42 ] . In another study, improvements in TEWL, skin hydration, and skin sensitivity were noted [ 9 ] . In both studies, the proportion of plasma ALA in those consuming flaxseed oil increased.

Another RCT evaluated hempseed oil ingestion in those with AD. Hempseed oil contains both omega-6 and omega-3 fatty acids, in a ratio of about 2:1. Subjects reported improved skin dryness and less use of dermal medications with the hempseed oil. Although there was a trend to improved TEWL, this was not significant [ 43 ] .

Limited human trials have studied the effects of fish oils specifically on TEWL. One RCT found that use of DHA for 8 weeks led to significant improvement in clinical findings of AD and an increase in plasma levels, but did not evaluate TEWL [ 44 ] . Another RCT found that a 4-month supplementation with fish oils led to an overall 30% improvement in clinical AD scores, although this was similar to a comparison group given corn oil. Of note, both groups had a baseline of significantly lower levels of serum fatty acids compared to patients with psoriasis [ 45 ] .

While these studies are small, they suggest that consumption of oils rich in omega-3 fatty acids have the potential to improve skin barrier function, possibly in those with lower serum levels at baseline. Larger studies in humans are needed to assess the magnitude of effect of these treatments.

Evening primrose oil (EPO) and borage oil (BO) have been studied for the treatment of AD, with overall disappointing results [ 46 ] . However, while many AD studies have focused on measures of clinical disease severity, few have investigated their effects on skin barrier function directly. Of these, some have reported promising results.

Gamma-linolenic acid (GLA) is an omega-6 fatty acid that has exhibited beneficial properties. EPO contains 8%–10% GLA and BO contains up to 25% GLA. [ 47 , 48 ] . One study described a negative correlation between TEWL and serum levels of GLA in children with AD. The study also noted a correlation between barrier dysfunction severity and deficiencies in essential omega-6 fatty acids such as GLA [ 49 , 50 ] .

Human skin lacks the enzyme delta 6-desaturase, which converts linoleic acid (LA) to GLA [ 51 – 53 ] . As such, the skin depends on GLA created in the liver [ 53 ] . Oral supplementation of EPO and BO supplies GLA directly [ 47 , 48 ] .

It is believed that the benefits of EPO and BO may be due to the influence of GLA and its metabolites on regulatory mechanisms of barrier function [ 53 ] . Metabolites of GLA exhibit anti-inflammatory properties and are linked to increased ceramide synthesis and improved skin barrier function [ 47 , 49 , 54 , 55 ] . They also inhibit the formation of leukotriene (LT)B ₄ [ 49 , 56 – 58 ] .

Clinical studies of EPO and BO have varied widely, with differing populations and doses. However, multiple studies have demonstrated significant beneficial effects on TEWL [ 42 , 53 , 55 , 59 ] . Stratum corneum hydration, overall, has not shown significant increases [ 53 , 55 , 60 ] .

Negative results have been reported as well, as 2 placebo-controlled trials found no differences in skin barrier function after GLA supplementation [ 49 , 60 ] . One of these, however, did find significant improvement in TEWL in a subgroup. These patients at baseline had low serum levels of a GLA metabolite, suggesting the importance of baseline status in determining who might benefit from supplementation [ 49 ] .

Overall, while EPO and BO supplementation has not shown benefit for the treatment of AD, they have shown some promise in enhancing skin barrier function. Further research should examine baseline levels of GLA that may benefit from supplementation, as well as comparing dosing regimens.

Several dietary interventions have been studied for their potential impact on the skin barrier, primarily in the form of dietary supplements. Laboratory and animal studies have described potential mechanisms of action. While some benefits have been seen in clinical studies, a number of questions remain. Clinical trials to date have utilized small sample sizes, and many have used varying dosing regimens. Some studies have evaluated multiple active ingredients, making it difficult to isolate the effects of a single ingredient. In addition, several interventions have been studied in disease states, and therefore may not be applicable to those without skin disease. Some studies have found that a baseline status of nutrients impacts clinical outcomes and is an important variable that should be studied further in future clinical trials. It is notable that few studies have investigated the role of dietary patterns on skin barrier function. This is an important area for future study. Research studies have indicated, for example, that changes in diet can impact the gut microbiome within 1 day [ 65 ] , with potential implications for the skin barrier. Current research has been focused on interventions with dietary supplements and a few foods, but further research on the effects of foods and dietary patterns on the skin barrier is warranted. Although questions remain about dosing, duration, and which populations may benefit, results to date for some interventions have been promising and further research is appropriate.