Skin aging: what the evidence supports, and what it does not

Compare the skin on the inside of your upper arm with the skin on the back of your hand. Same person, same age, same genes. The gap between them is a rough picture of the two processes dermatologists separate when they talk about skin aging, and it is the best starting point for judging any product or habit that claims to slow it down.

What follows is general information about what the research shows, not advice for any one person's skin.

Two kinds of aging

Intrinsic aging is the part driven by time and genetics. Cell turnover in the epidermis slows. The dermis, the thick layer underneath, gets thinner and holds fewer active fibroblasts, the cells that make collagen and elastin. The wavy junction between the two layers flattens, which makes skin more fragile, and oil production falls. The visible result is fine wrinkling, dryness, laxity and slower healing. Hormonal change, including the fall in estrogen at menopause, adds to it.

Extrinsic aging comes from the environment, and ultraviolet light dominates it, which is why it is often called photoaging. Photoaged skin shows coarse wrinkles, uneven pigment, sun spots, small visible blood vessels and a rough, leathery texture. Under the microscope it shows clumps of abnormal elastic material, a change called solar elastosis. The inner arm shows mostly intrinsic aging. The back of the hand shows both.

Collagen and elastin, and how they wear out

Collagen, mainly types I and III in adult skin, makes up most of the dry weight of the dermis and gives it strength. Elastin forms a much smaller network that lets skin spring back after it is stretched. Both turn over slowly, and elastin made in youth has to last for decades.

Research led from the University of Michigan in the 1990s, summarized in a 1997 review in the New England Journal of Medicine, showed how UV attacks this structure. Even doses too small to cause sunburn switch on enzymes called matrix metalloproteinases that break collagen down, while also cutting new collagen production. Repeated over years, this leaves fragmented collagen that fibroblasts cannot rebuild well. The same researchers reported that pretreating skin with tretinoin blunted some of these changes, which helps explain why retinoids sit near the top of the table below.

The outside factors, one by one

Ultraviolet light

UVB acts mainly on the epidermis and causes sunburn. UVA reaches deeper into the dermis, contributes heavily to photoaging and passes through ordinary window glass. The case that UV drives most visible skin aging is strong. It rests on decades of laboratory, microscope and population research, and on the plain contrast between covered and exposed skin.

Smoking

Smoking reduces blood flow to the skin and is linked to higher levels of collagen-degrading enzymes. In a widely cited study of 186 pairs of identical twins photographed at the Twins Days festival in Ohio (Plastic and Reconstructive Surgery, 2009), twins with more years of smoking or more sun exposure looked older than their siblings. A randomized trial of smoking is impossible, but findings this consistent, with a clear mechanism, count as strong evidence.

Sleep

Here the evidence is limited. A small 2015 study of 60 women in Clinical and Experimental Dermatology found that poor sleepers had more signs of intrinsic aging and slower barrier recovery than good sleepers. A study like this cannot separate sleep from the stress, diet and other habits that travel with it.

Diet

Also limited. An analysis of more than 4,000 American women in the first National Health and Nutrition Examination Survey (American Journal of Clinical Nutrition, 2007) linked higher intakes of vitamin C and linoleic acid with skin that looked less wrinkled and less dry. That is an association in observational data. When a randomized trial in Australia gave some participants beta-carotene supplements, it found no effect on skin aging.

Air pollution

A German study of older women (Journal of Investigative Dermatology, 2010) linked exposure to traffic-related particles with more pigment spots on the face. Later studies point the same way, but the research base is still small.

Topical ingredients with good evidence

Sunscreen

The key trial comes from Nambour in Queensland, Australia. Researchers randomly assigned 903 adults under 55 to daily broad-spectrum sunscreen or to their usual, occasional use, and followed them for four and a half years. In their 2013 report in Annals of Internal Medicine, the daily group showed no detectable increase in skin aging over the study, and aging was about 24 percent less than in the occasional-use group.

Regulators reflect this. Under FDA labeling rules, sunscreens labeled "Broad Spectrum" with SPF 15 or higher may say they help reduce the risk of skin cancer and early skin aging when used with other sun protection measures, while products that do not meet that standard must warn that they only help prevent sunburn. The FDA's sunscreen page explains the labels.

Prescription retinoids

Tretinoin, an acid form of vitamin A, is the best studied anti-aging topical. Double-blind trials since the late 1980s found that it modestly improves fine wrinkles, roughness and mottled pigmentation in sun-damaged skin, and in the mid-1990s the FDA approved a tretinoin cream for those signs. Tazarotene, another prescription retinoid, later gained approval for facial sun damage. Results take months, irritation and dryness are common early on, retinoids increase sun sensitivity, and prescription retinoids carry pregnancy warnings.

Over-the-counter retinol is converted to retinoic acid in the skin. It has some trial support, but the effects are smaller and the studies fewer.

The weaker tiers

Alpha hydroxy acids such as glycolic and lactic acid smooth texture by loosening the outer layer of dead cells. The FDA notes that they increase sensitivity to sunburn and recommends a sunburn alert on labels.

Topical vitamin C is an antioxidant and is needed for collagen synthesis, so the idea is sound. The trials, though, are small, short and often industry funded, and ascorbic acid breaks down quickly in many formulas, so the evidence is weaker than for retinoids. Niacinamide and cosmetic peptides sit in a similar place. Oral collagen supplements have a growing number of small trials, most funded by manufacturers, testing many different products.

Evidence at a glance

Procedures, briefly

All of these are medical procedures with their own risks, and results depend heavily on who performs them.

• Chemical peels apply acids that remove outer layers in a controlled way, from light to deep.
• Laser resurfacing either removes thin layers (ablative) or heats the dermis to prompt collagen remodeling with less downtime (non-ablative).
• Intense pulsed light targets brown spots and small blood vessels.
• Microneedling makes tiny punctures that trigger a wound-healing response.
• Botulinum toxin injections relax the muscles behind expression lines. The FDA first approved a botulinum toxin for frown lines between the brows in 2002.
• Dermal fillers, most often hyaluronic acid gels regulated by the FDA as medical devices, restore lost volume.
• Platelet-rich plasma and exosome treatments are sometimes added after microneedling. Evidence for both is early.

Where "stem cell" and growth factor creams sit

Three quite different things are sold under this banner.

1. Plant "stem cell" extracts, from cultured apple, grape or other plant cells. Plant cells cannot act as stem cells in human skin, and they are not alive in the jar. Any effect comes from the extract's chemistry, as with other botanical antioxidants.
2. Growth factor and conditioned-medium ("secretome") serums, made from the fluid in which human cells such as skin fibroblasts or stromal cells were grown. Small studies, often without a control group and often run by the seller, report modest improvements. Growth factors are large proteins that do not cross intact skin easily.
3. Exosome products. The FDA has stated that there are no FDA-approved exosome products, and its 2019 safety notification followed serious adverse events in patients who received them.

There is also a legal line. Under US law a cosmetic is meant to cleanse or beautify, while a product intended to change the structure or function of the body is a drug, and the FDA lists claims such as "regenerate cells" among those that make a product a drug. That is why many of these labels choose their verbs so carefully. Our page on stem cell skincare and secretome labels shows how to read them, and our explainer on what makes a stem cell a stem cell covers the biology. If the pitch moves from a cream to an injection, the warning signs of unproven stem cell clinics apply.

A practical yardstick: the items rated strong above were tested against a placebo or an untreated comparison group over months or years. A product whose evidence is a four-week before-and-after photo series run by its seller belongs in the limited row, whatever it costs.