What’s Diet Got to Do With It? Basic and Clinical Science Behind Diet and Acne

The current understanding of the pathogenesis of acne includes altered keratinization, follicular obstruction, overproduction of sebum, and microbial colonization ( Cutibacterium acnes ) of the pilosebaceous unit resulting in perifollicular inflammation. ¹ A deeper dive into the hormonal and molecular drivers of acne have implicated insulin, insulinlike growth factor 1 (IGF-1), corticotropin-releasing hormone, the phosphoinositide 3 -kinase/Akt pathway, mitogen-activated protein kinase pathway, and the nuclear factor κ B pathway. ^2-4 A Western diet comprised of high glycemic index foods, carbohydrates, and dairy enhances the production of insulin and IGF-1. A downstream effect of excess insulin and IGF-1 is overactivity of the mammalian target of rapamycin complex 1 (mTORC1), a major promoter of cellular growth and proliferation that primarily is regulated through nutrient availability. ⁵ This article will review our understanding of the impact of the Western diet on acne pathogenesis and highlight the existing evidence behind the contributions of the mTORC1 pathway in this process. Although quality randomized controlled trials analyzing these effects are limited, dermatologists should understand the existing evidence supporting the potential impacts of diet on acne.

The Western Diet

Glycemic Index—To assess the impact of a high glycemic index diet on acne, Kwon et al⁶ evaluated 32 patients with mild to moderate acne and placed them on a low or high glycemic index diet for 10 weeks. The low glycemic index diet group was found to have a 70% reduction in the mean number of inflammatory acne lesions from baseline (P<.05), while the high glycemic index diet group had no significant reduction. Noninflammatory lesion counts remained statistically unchanged.⁶ Smith et al⁷ studied 43 male patients with acne on either a low glycemic index diet or a self-directed high glycemic diet that was carbohydrate dense. The low glycemic index group showed greater improvement in lesion count as well as improved insulin sensitivity at 12 weeks. Specifically, the mean lesion count (SEM) decreased by 23.5 (3.9) in the low glycemic index group and by only 12.0 (3.5) in the control group (P=.03).⁷ Observational studies also have supported this hypothesis. After adjustment, an analysis of 24,452 participants in the NutriNet-Santé cohort found significant associations between current acne and the consumption of sugary beverages (adjusted OR, 1.18; 95% CI, 1.01-1.38) and the consumption of fatty and sugary products (adjusted OR, 1.54; 95% CI, 1.09-2.16).⁸ A Cochrane review that included only 2 studies (Kwon et al⁶ and Smith et al⁷) did not find evidence to suggest a low glycemic index diet for noninflammatory lesion count reduction but did note possible benefit for a reduction in inflammatory and total lesion counts; however, Kwon et al⁶ had incomplete data.⁹

Dairy—A large retrospective study including 47,355 nurses noted the frequency of milk intake was significantly associated with increased prevalence of acne in adolescence (prevalence ratio, 1.22; 95% CI, 1.03-1.44; P=.002).¹⁰ A 2019 meta-analysis further suggested a significant relationship between acne and milk in highest vs lowest intake groups (OR, 1.48; 95% CI, 1.31-1.66) with no significant heterogeneity between the studies (I²=23.6%, P=.24 for heterogeneity), as well as a positive relationship between the highest vs lowest intake of low-fat milk (OR, 1.25; 95% CI, 1.10-1.43) and skim milk (OR, 1.82; 95% CI, 1.34-2.47). In this meta-analysis, yogurt and cheese consumption were not significantly associated with acne (OR, 0.90; 95% CI, 0.73-1.11).¹¹ One non–evidence-based explanation for this may be that fermented dairy products have different biological actions. Pasteurized milk allows microRNAs that directly activate mTORC1 to persist, whereas the bacteria present in the fermentation process may augment this.¹² A separate meta-analysis from 2018 did find that yogurt consumption was positively associated with acne (OR, 1.36; 95% CI, 1.05-1.77; P=.022), highlighting the need for larger, more rigorous studies on this topic.¹³

Insulin and IGF-1—As reviewed above, acne has been considered a disease of Western society, with the Western diet at the center of this association.¹⁴ A typical Western diet consists of high glycemic index foods, carbohydrates, and dairy, all of which enhance the production of insulin and IGF-1. Insulin levels increase secondary to high blood glucose and to a lesser degree by protein intake.¹⁵ Insulinlike growth factor 1 production is most influenced by age and peaks during puberty; however, high protein diets also increase liver IGF-1 production and release.¹⁶ When present in excess, insulin can function as a growth factor. Insulin exerts its anabolic effects through the IGF-1 pathway; however, insulin and IGF-1 are produced in response to different signals.¹⁷ Endocrine production of IGF-1 represents 70% of blood levels, peaks at puberty, and rapidly declines in the third decade of life.¹⁸ Insulin is produced by the pancreas, and levels correspond to lifestyle and genetically induced insulin resistance.¹⁹

Adolescents have elevated levels of IGF-1 as a major driver of puberty-associated growth.²⁰ Despite the natural decrease in IGF-1 following puberty, acne persists in many patients and can even develop for the first time in adulthood in a subset of patients. A study of 40 acne patients and 20 controls found that patients with acne who consumed a high glycemic–load diet was significantly higher than the number of controls consuming a similar diet (P=.008). Additionally, significantly higher levels of mean (SD) serum IGF-1 on quantitative sandwich enzyme-linked immunosorbent assay in acne patients vs controls (543.2 [174.7] ng/mL vs 316.9 [95.7] ng/mL; P<.001) was identified, and these levels correlated significantly with high glycemic–load diet consumption.²¹ In another study, Kartal et al²² found that basal and fasting insulin levels and homeostasis model assessment scores evaluating for insulin resistance were significantly higher in 36 women compared with 24 age/sex-matched controls (P<.05). This finding remained significant even after excluding women with hyperandrogenemia (P<.05).²²

Highlighting the importance of IGF-1 in the pathogenesis of acne, patients with genetic disorders characterized by IGF-1 deficiency, such as Laron syndrome, do not develop acne despite having a functional androgen receptor. Treatment with IGF-1 in these patients induces acne, further supporting the role of IGF-1 in the pathogenesis of this condition.²³

The mTORC1 Pathway

Comprised of mTOR in addition to other proteins, mTORC1 is a nutrient-sensitive regulator of cellular growth, proliferation, lipid synthesis, and protein translation.⁵ Increased activity of mTORC1 has been described in diabetes, neurodegenerative disease, and cancer,^14,24 while decreased activity may promote longevity.²⁵ Regulation of mTORC1 occurs through several mechanisms. Growth factors such as insulin and IGF-1 promote mTORC1 activation through the PI3K/Akt pathway. Several amino acids—specifically branched chain amino acids such as alanine, arginine, asparagine, glutamine, histidine, leucine, methionine, serine, threonine, and valine—also can activate mTORC1 independently.²⁶ Excess glucose leads to decreased adenosine monophosphate–activated protein kinase and increased activity of mTORC1, which occurs separately from insulin or IGF-1.²⁷ Starvation blocks mTORC1 via increased adenosine monophosphate–activated protein kinase and starvation-induced hypoxia.^26,28 To activate mTORC1, both the IGF-1 or insulin signal and amino acid excess must be present.²⁹ Although not studied in acne, altering the dietary protein content in obese mice has been shown to perturb the mTORC1 pathway, leading to pathologic changes in the mTORC1-autophagy signaling axis, increased amino acid release into the blood, and an acute elevation in mTORC1 signaling.³⁰