Oddly

Skin, the largest organ in the human body, acts as a barrier to protect internal organs and cells from external elements. It also helps regulate body temperature, mediates sensations of touch, and produces vitamin D, a key regulator of bone, immune, and vascular health.3 Both intrinsic and extrinsic factors affect skin health and aging.4 An individual’s genetic background influences intrinsic factors such as skin pigmentation, skin thickness, microvasculature structure, and s*x hormones.4 Extrinsic factors such as smoking, diet, sleep, exercise, chronic diseases, and environmental factors including temperature, pollution, humidity, and UV radiation (UVR) can increase inflammation and oxidative stress that accelerate skin aging.4, 5, 6 Indeed, repeated UVR exposure can increase pro-inflammatory cytokines that contribute to the development of wrinkles and adverse pigmentation of the skin.7 Moreover, age- or obesity-related induction of protein glycation and inflammation can increase skin rigidity and impair skin repair.8

Suboptimal nutrition can adversely affect skin health, as evidenced in studies of micronutrient deficiencies. For example, deficiencies of vitamin A and vitamin C (VitC) can lead to thickening of the skin.9 Poor wound healing has been observed with deficiencies of VitC and essential fatty acids,9, 10, 11 and petechiae can be a result of vitamin E and vitamin K deficiencies.9 Inadequate intakes of riboflavin, niacin, pyridoxine, biotin, zinc, and essential fatty acids can lead to various forms of dermatitis.9,12 Classic studies on pellagra13 and acrodermatitis enteropathica14 identified niacin and zinc deficiencies, respectively, as causative factors. Although the data on micronutrient deficiencies are extensive, data on how specific foods or diets can influence skin health in well-nourished populations are limited.

Epidemiological studies suggest that abundant dietary intakes of specific plant-based foods are key in the maintenance of skin barrier health and function. A robust intake of vegetables, olive oil, and legumes was correlated with lower actinic skin damage caused by long-term UVR exposure among 2000 people aged 70 and older in Australia, Greece, China, Japan, and Sweden.1 Better adherence to the Dutch Healthy Diet Index guidelines that promote a diet rich in fruits and include yogurt, milk, and vegetables was significantly associated with fewer wrinkles in women.2 Among Japanese women, a significant inverse association has been observed between wrinkling and green and yellow vegetable intake.15 In contrast, diets consisting mainly of meat, refined grains, snacks, soft drinks, coffee, and alcoholic beverages were associated with more wrinkling in women.2

Plant-based foods are rich in polyphenols, carotenoids, and select vitamins typically not found in appreciable amounts in other food categories. However, each food has a unique nutrient profile that provides an array of bioactive compounds that either alone or synergistically may afford protection for the skin.

Given this information, we conducted a preliminary survey of recent literature on the potential effects of plant foods on skin barrier health and function. A majority of the trials discussed used study designs of dietary components and foods individually, at times, above dietary recommendations. However, the goal of this review is to spark interest in this field, as well as provide an overview of the available data for both the public and nutrition professionals who have an interest in the role of diet for the maintenance of skin barrier health and function. This review aims to provide more specificity in terms of the fruits and vegetables that may improve skin barrier function and meet the recommendations of the 2020-2025 Dietary Guidelines for Americans (DGAs).

Skin aging
Skin is often the first visible manifestation of the aging process. However, the effects of age on skin appearance are often similar to the effects associated with photodamage and environmental exposures. This makes the changes induced by chronological age on skin — often referred to as intrinsic aging — difficult to distinguish from other effects (27).

Instrinsic skin aging is characterized by decreasing support from the dermis to the epidermis. Ridges on the interface between the two layers are diminished, preventing the dermis from providing adequate mechanical support to the epidermis (27, 33). Collagen levels are lower and extracellular proteins in the dermis are more disorganized in skin of older individuals compared to younger adults (27, 33, 34). These changes result in increased skin fragility and laxity, as well as decreased size of the dermis and reduced vascularization, which reduces nutrient availability to the skin (27, 29). Aged skin keratinocytes are relatively slow to differentiate and shed, which alters their ability to maintain the stratum corneum. These changes may lead to an overall dull skin appearance and loss of protective ability of skin (27).

Antioxidant protection is often thought to diminish in the skin of older individuals, but the extent of these changes is unclear. The results of one human study suggest that catalase (an antioxidant enzyme) levels in the epidermis of elderly subjects were increased compared to young subjects independent of sun exposure (16). The capacity of aging skin to maintain levels of other antioxidant enzymes was unchanged. However, elderly subjects had lower levels of α-tocopherol in the epidermis and lower levels of ascorbic acid in both skin layers (16).

Most of intrinsic skin aging cannot be avoided. The cumulative effects of decades of skin wear cannot be reversed, although the effects on skin appearance may be temporarily masked. Aging skin has a diminished ability to respond to stress; therefore, skin damage from other sources can enhance the signs of skin aging. This is particularly true for ultraviolet (UV) exposures, because antioxidant capacity, immune function, and melanin production may all be impaired in aged skin (27). Therefore, treatments designed to attenuate damage from other sources are also important factors in limiting age-associated skin damage.