People with acne make high amounts of sebum and the composition of the sebum is different, with high amounts of squalene and low amounts of linoleic acid. When free radicals react with sebum, it causes inflammation in a process known as lipid peroxidation. Also, linoleic acid is a building block for ceramides, which contributes to low levels of ceramides in the skin.

References:
Pappas A. Dermatoendocrinol. 2009 May-Jun; 1( 3): 157–161.

In acne, skin cells inside your pores stick together, known as follicular hyperkeratosis. This acts like a bottleneck, trapping oil in the follicle. As oil builds up in the pores, acne causing bacteria grow to high levels. This is thought to be directly caused by low linoleic acid levels in the sebum and low ceramide levels in the skin.

References:
Thiboutot D et al. J Clin Aesthet Dermatol. 2013 Feb; 6( 2): 18–24.

C. acnes bacteria lives within the follicles on everyone's skin. People with acne may have different strains on the skin and react to it more, leading to inflammation. Even though C. acnes bacteria is involved in the development of acne, acne is not an infection. You cannot catch or spread acne.

References:
Fitz-Gibbon S, et al. J Invest Dermatol. 2013. 133 ( 9): 2152-2160
Dessinioti C, et al. Clin Dermatol . Jan-Feb 2010;28( 1):2-7. 

Sebaceous glands (aka your oil glands) are more than just oil factories. They react to hormones and acne causing bacteria, leading to inflammation that cause pimples. In fact, this inflammation has been shown to occur even in normal appearing skin in people with acne. Under the microscope, inflammation can be seen around the oil glands even before any visible pimples form.

References:
Jeremy AHT, et al. J Invest Dermatol. 2003. 121 ( 1): 20-27

The term "hormonal acne" is commonly used to refer to acne breakouts in adult women, associated with the menstrual period. However, the truth is that the most basic level, all acne is hormonal. Androgen hormones like testosterone and its more active form dihydrotestosterone (DHT) bind to receptors on the oil glands. This increases oil production and drives inflammation. These hormones are present in both men and women. That being said, women often experience acne flares around the time of the menstrual period because of increases in hormones levels which trigger oil gland activity. In some cases, Dermatologists may prescribe hormonal therapies by mouth to treat acne by regulating the levels and activity of these hormones.

References:
Zaenglein A et al. J Am Acad Dermatol . 2016 May;74( 5):945-73.

When your mother told you not to eat too many sweets on Halloween, she was right. Foods with a high glycemic index, namely foods high in refined sugars and starch, raise blood sugar and promote inflammation in the skin. High sugar concentrations in the blood increase levels of insulin like growth factor-1 (IG-1), which is known to activate oil glands through a messenger called SREBP. This means more oil production and inflammation in the skin, with worsening of acne. Cow's milk, particularly skim milk, has also been associated with acne. Interesting, yogurt and cheese are not associated. Whey protein and Vitamin B12 supplements may also contribute to acne breakouts. The original research into the association between diet and acne came from studies of hunter-gatherer societies that eat fresh fruits, vegetables, and whole grains. In these societies, acne is almost non-existent, which is why many believe that the Western diet is a major contributing factor for the development of acne.

References:
Pappas A. Dermatoendocrinol. 2009 Sep-Oct; 1( 5): 262–267.
Baldwin H, et al. Am J Clin Dermatol. 2021; 22( 1): 55–65.

The same hormones that prepare our bodies to seal with stress events can actually lead to pimples. As par of our bodies' fight or flight response, our bodies increase production of cortisol. Our brains make a hormone called corticotropin releasing hormone (CRH) which directly increases cortisol levels from the adrenal glands. One other effect of CRH is that it actually activates the oil glands. An enzyme called 5-LOX is also involved in oil gland activation. This means more oil production as well as inflammation that causes acne. Stress related acne is well-documented. For example, students have been shown to break out during testing periods significantly more than during regular times at school.

References:
Ganceviciene, R. Br J Dermatol . 2009 Feb;160( 2):345-52.
Chiu A, et al. Arch Dermatol . 2003 Jul;139( 7):897-900.

The wrong cleanser, cream, or makeup may contribute to acne. Thick, heavy, or occlusive products have been shown to physically block pores and lead to breakouts. This was originally described as "pomade acne," where pimples developed along the hair line after hair pomades got on the skin, blocked the pores and contributed to pimples. In some cases, heavy moisturizers or even liquid foundation makeups may have a similar effect. It is usually recommended that people with oily or acne prone skin use oil-free, non-comedogenic skincare products. This means that they have been shown not to cause breakouts. Besides cosmetic products, other external factors like face masks have have also been shown to worsen acne likely because they trap dirt and oil on the skin and cause inflammation because of direct friction against the skin's surface.

References:
Kligman AM, et al. Arch Dermatol. 1972 Dec;106( 6):843-50.
Plewig G. Arch Dermatol . 1970 May;101( 5):580-4.