Ask any skin expert about ceramides, and they’ll sing the praises of the ingredient and the products that contain it for maintaining healthy skin. In other words, adding ceramides into your skincare routine as you age is necessary for maintaining skin that looks and feels healthy. Ceramides are produced from glucosylceramide and sphingomyelin by β-glucocerebrosidase and sphingomyelinases, respectively. “The skin cells are essentially the brick, and the lipids or the fats are in between, and that’s what we call the mortar,” Levin explains. Apoptosis, or Type I programmed cell death, is essential for the maintenance of normal cellular homeostasis and is an important physiological response to many forms of cellular stress. Currently, the means by which ceramide acts as a signaling molecule are not clear. Levin says you’ll find them in liquid form and most often in moisturizers, but they’re also used in cleansers, toners, and serums with active ingredients like retinoids and glycolic acid, which allow for deeper penetration of the ceramides. [33] Additionally, ceramides are being explored as a potential therapeutic in cancer.[34]. They’re most effective in vacuumed packaging, like containers with pump dispensers, which prevents air from entering and keeps the formula stable. The truth is, ceramides are actually lipids, or in other words, fats! Everything You've Wanted to Know (And More), Isopropyl Myristate Is the Moisture-Locking Ingredient You've Never Heard Of, This Is Why Emollients Are the Secret to Getting Rid of Dry Skin, Trust Us: These 21 Lotions Will Heal Dry Skin in No Time, The 12 Derm-Approved Body Lotions That Protect Your Skin With SPF. Ceramide 3 = Ceramide NP = N-stearoyl phytosphingosine. She says even though the structure of ceramides can be dif… This treatment is also formulated with a … According to Jaliman, there are nine different ceramides found in the skin and the difference is based on how long the carbon chains are. Ceramides also help the skin protect against environmental aggressors like irritants, and pollution. “It’s a retinol, glycolic, peptide, and antioxidant all-in-one repair serum that has ceramides, which allows for tolerability alongside these really active, effective ingredients.”. Ceramides can appear on a product label under many different names (for example, ceramide AP, ceramide EOP, or ceramide NP), which understandably causes confusion when shopping for products. “In terms of a ceramide with something active, I really like this one,” Levin says. Ceramide is rarely found in bacteria. The following is a list of ceramides found in human skin and also made synthetically for use in skincare products: 1. ceramide AP 2. ceramide EOP 3. ceramide NG 4. ceramide NP 5. ceramide NS According to Melissa Kanchanapoomi Levin, MD, dermatologist, these different ceramides vary in terms of the length of their carbon chains. There are three major pathways of ceramide generation. She has contributed to Byrdie, as well as Harpers Bazaar, Marie Claire, Allure, Vogue, and the New York Times, and more, Skin Relief Intense Moisture Repair Cream, A guide to cocktailing skincare ingredients for your skin type. Ceramide is the core molecule of sphingolipids. Even though the SC is very thin—10-30 millimeters or one-tenth the thickness of a piece of paper—it is the main barrier of the skin. It has been shown that ceramide generated from sphingomyelin by sphingomyelinase activation in response to stimuli serves as a lipid modulator. Ceramides are naturally part of our body, found at the highest concentrations in the uppermost layer of our skin (the stratum corneum). Several studies have attempted to define further the specific role of ceramide in the events of cell death and some evidence suggests ceramide functions upstream of the mitochondria in inducing apoptosis. [32] They are also used in cosmetic products such as some soaps, shampoos, skin creams, and sunscreens. Usually, you’ll find the word ‘ceramide’ in the ingredient list (ceramide AP, EOP, NG, NP, of NS), but not always. The role of epidermal lipids in cutaneous permeability barrier homeostasis", "Loss of ceramide synthase 3 causes lethal skin barrier disruption", "The moisturizing effect of a wheat extract food supplement on women's skin: a randomized, double-blind placebo-controlled trial", "Central role of ceramide biosynthesis in body weight regulation, energy metabolism, and the metabolic syndrome", "Ceramide: a common pathway for atherosclerosis? [24] An elevated level of ceramide can cause insulin resistance by inhibiting the ability of insulin to activate the insulin signal transduction pathway and/or via the activation of JNK.[25]. They are found naturally … Besides keeping things from coming in, ceramides also prevent things from leaving the skin, such as moisture. Once you have a better grasp of this skincare ingredient (we promise you will by the end), keep scrolling to find out both dermatologists’ top product recommendations for replenishing your ceramide levels. All the above precede the onset of full-blown type 2 diabetes. According to Jaliman, there are nine different ceramides found in the skin and the difference is based on how long the carbon chains are. Compounds containing dihydroxy bases (sphingosine, dihydrosphingosine, sphingadienine) can be converted to ceramides by the procedure proposed by Carter et al (J Lipid Res 1961, 2, 227). We always say 1:2:1 (one cholesterol, two ceramides, and one fatty acids), which is the ratio of these lipids for the natural skin barrier.". Strong water-retention function. As a moisturizer, ceramides work best when applied immediately following a shower to lock-in extra moisture, and Jaliman suggests using the product before bed as well. It also comes packaged in an airtight bottle with a pump applicator to keep the formula protected from light, air, and outside contaminants. If ceramides are listed at the bottom of the ingredient list, then the product is going to have a very small concentration of them. One hypothesis is that ceramide generated in the plasma membrane enhances membrane rigidity and stabilizes smaller lipid platforms known as lipid rafts, allowing them to serve as platforms for signalling molecules. [8] Aggregation of LDL cholesterol by ceramide causes LDL retention in arterial walls, leading to atherosclerosis. Ceramides are lipids that help form the skin’s barrier and help skin retain moisture. Contrary to previous assumptions that ceramides and other sphingolipids found in cell membrane were purely supporting structural elements, ceramide can participate in a variety of cellular signaling: examples incl… This means that ceramides keep these compounds out. “Ceramides are known to be part of this complex family of fats or lipids called a sphingolipid,” Levin says. A ceramide is composed of sphingosine and a fatty acid. How Much Do You Really Know About Sodium Hyaluronate? In turn, 3-keto-dihydrosphingosine is reduced to dihydrosphingosine, which is then followed by acylation by the enzyme (dihydro)ceramide synthase to produce dihydroceramide. There are different ceramide types (conveniently numbered – i.e., ceramide-3) and six are commonly found in skin. Because of its apoptosis-inducing effects in cancer cells, ceramide has been termed the "tumor suppressor lipid". 59 Ceramidase hydrolyzes ceramide to sphingosine base and FA. Naturally found in the skin, Ceramide is the glue that binds skin cells together, helping the skin’s barrier feel rehydrated, supple and protected. However, you can avoid allergic reactions and product sensitivities by testing a small amount on your skin first. Ceramides are a type of naturally occurring, long-chain fatty acid (i.e., fat or lipid) that makes up about 50 percent of the skin’s outer layer (the epidermis).. This moisturizer is another favorite of both dermatologists (and many others, Levin adds) for its combination of cholesterol, hyaluronic acid, five different ceramides, and enzymes to help soften fine lines and wrinkles for an overall smoother, healthier complexion. More and more products like The Inkey List Ceramide Night Treatment are released, claiming to repair … Resistance to insulin is a pathophysiological state related to the decreased response of peripheral tissues to the insulin action, hyperinsulinemia and raised blood glucose levels caused by increased hepatic glucose outflow. Hello! [16][17] Together with cholesterol and saturated fatty acids, ceramide creates a water-impermeable, protective organ to prevent excessive water loss due to evaporation as well as a barrier against the entry of microorganisms. “It’s incredibly important for the structure of your stratum corneum to remain intact because it serves as a barrier to protect the rest of your skin and keeps all that hydration in,” she says. Previously, she was the senior beauty editor at Cosmopolitan.com. And with the added benefit of being fragrance-free, it's also a safe choice for those with sensitive skin. Main benefits: Rebuild and restore the protective barrier of the skin to retain moisture, improve visible signs of aging, and block environmental damage. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer. It may sound familiar – maybe you have seen it somewhere in an ingredient list of your new moisturiser, but never really knew what it is, or what it does for your skin. For example, there's The Inkey List's Ceramide Night Treatment - an effective product that combines ceramides with skin-plumping hyaluronic acid for ultra soft, ultra plump skin - or Sarah Chapman Comfort Cream D-stress which targets the main signs of sensitivity, calming irritation and reducing redness. [7] Ceramides induce insulin resistance in many tissues by inhibition of Akt/PKB signaling. This suggests a role for ceramide in the biological responses of all these agents. Ceramides have no known side effects. Ceramide has also been shown to form organized large channels traversing the mitochondrial outer membrane. [15], Ceramide is the main component of the stratum corneum of the epidermis layer of human skin. Brooke Shunatona is a freelance beauty writer. [17] In the hyperplastic disorder psoriasis the water permeability barrier is compromised. This liquid can be used as a lightweight moisturizer or worn as a toner and followed with a thicker moisturizer. Ceramides are a component of our skin's natural protective barrier, so applying them topically is a great way to keep skin moisturized, strong, and protected from potential irritants. Ko says “any applied ceramide may have a benefit, even if the amount is slight,” and a Japanese study showed an increase in skin moisture content of more than 100 percent with ceramide use. Below, Debra Jaliman, MD, board-certified dermatologist in NYC and Melissa Kanchanapoomi Levin, MD, dermatologist and founder of Entière Dermatology in NYC explain what exactly ceramides are, how they function, why you need them, and more. If our bodies already have ceramides, why we would need to add them into our skincare routines as well? However, owing to the conflicting and variable nature of studies into the role of ceramide in apoptosis, the mechanism by which this lipid regulates apoptosis remains elusive. Moreover, as rafts on one leaflet of the membrane can induce localized changes in the other leaflet of the bilayer, they can potentially serve as the link between signals from outside the cell to signals to be generated within the cell. The combination of these with the cholesterol and fatty acids greatly improves the condition of the skin barrier and the appearance of your skin. If ceramides are listed at the top of the ingredient list, then the product will contain a higher concentration of them. Other articles where Ceramide is discussed: sphingolipid: …most simple sphingolipids are the ceramides (sphingosine plus a fatty acid), widely distributed in small amounts in plant and animal tissues. Because there are 9 types of ceramides (numbered 1-9), you will usually find a number following the name ceramide on an ingredient list. Frederic Charpentier / Getty Images Where Ceramides Are Located . Use your cleansers first, followed by toners and serums, and save the moisturizer for the final step at night or right before your SPF if it’s during the day. Because sphingomyelin is one of the four common phospholipids found in the plasma membrane of cells, the implications of this method of generating ceramide is that the cellular membrane is the target of extracellular signals leading to programmed cell death. Ceramide is subsequently transported to the Golgi apparatus by either vesicular trafficking or the ceramide transfer protein CERT. Please read disclaimer. If youve ever struggled to control eczema, rashes, or dry skin, you may have come across ceramide moisturizers in your hunt for the perfect lotion. A ceramide is composed of sphingosine and a fatty acid. If you're someone who's tried retinol before and given it up because of the irritation it caused, you might want to a try a formula like this one, which contains ceramides to prevent the irritation. [29][30][31], Ceramides may be found as ingredients of some topical skin medications used to complement treatment for skin conditions such as eczema. [18] Key components of the extracellular lipid lamellae of the stratum corneum are ultra long chain (C28-C36) ceramides. One of the most important factors in shopping for skin care products is taking the time to research the ingredients they contain; and if you have dry, flaky, or itchy skin, then you may have already heard that ceramides skin care items can be an essential part of your skin care kit. Ceramide is a component of vernix caseosa, the waxy or cheese-like white substance found coating the skin of newborn human infants. [7] Unsaturated fat does not have this effect. “All very good ingredients which will help with dark spots, fine lines, and wrinkles.”. Once in the Golgi apparatus, ceramide can be further metabolized to other sphingolipids, such as sphingomyelin and the complex glycosphingolipids. Ceramides are a class of fatty acids called lipids. First, the sphingomyelinase pathway uses an enzyme to break down sphingomyelin in the cell membrane and release ceramide. They’ll probably also fill you in on the fact that ceramides exist in your body naturally and make up half of your skin barrier—which is where the confusion lies for many of us. [3], Constitutive degradation of sphingolipids and glycosphingolipids takes place in the acidic subcellular compartments, the late endosomes and the lysosomes, with the end goal of producing sphingosine. Rachel is a board-certified dermatologist and Assistant Clinical Professor at Mount Sinai Hospital Department of Dermatology. The top layer of the skin is called the epidermis, and the top layer of the epidermis is called the stratum corneum (SC). We found out the answer to that question as well as a number of other inquiries we all have about the highly praised but not fully understood beauty ingredient, straight from the skin experts themselves. Who should use it: In general, anyone with dry skin or anyone interested in anti-aging skincare. The ceramides used in skincare products typically are derived from plants (wheat germ is a common source) or are synthetic; there’s no research showing that either form is preferred over the other. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer. Ceramides are fat molecules in your skins top layer that help keep your skin moist and soft. De novo synthesis of ceramide occurs in the endoplasmic reticulum. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Third, in the "salvage" pathway, sphingolipids that are broken down into sphingosine are reused by reacylation to form ceramide. [5][6], Ceramides induce skeletal muscle insulin resistance when synthesized as a result of saturated fat activation of TLR4 receptors. Without the proper ratio of ceramides, the skin’s barrier can become compromised, leading to dryness, itching and irritation. Five ceramidase isoforms, which show different pH … [2], De novo synthesis of ceramide begins with the condensation of palmitate and serine to form 3-keto-dihydrosphingosine. [1] Contrary to previous assumptions that ceramides and other sphingolipids found in cell membrane were purely supporting structural elements, ceramide can participate in a variety of cellular signaling: examples include regulating differentiation, proliferation, and programmed cell death (PCD) of cells. Both dermatologists recommend applying your ceramides twice daily, but during which step in your routine depends on your product of choice. Works well with: Cholesterol and fatty acids. Ceramides can appear on a product label under many different names (for example, ceramide AP, ceramide EOP, or ceramide NP), which understandably causes confusion when shopping for products. This class of ingredients is usually contained in cosmetic products such as facial moisturizer, eye cream, anti-aging serum, foundation, sunscreen shampoo/conditioner, lip gloss/lipstick and body firming lotion. Thank you for watching. All four layers of the epidermis contain ceramides, and they play an important role by creating a barrier that reduces infection and helps to retain the skin’s moisture. This leads to the egress of proteins from the intermembrane space. Up next: A guide to cocktailing skincare ingredients for your skin type. The synthetic ceramides in this Byrdie editor-approved formula are bioidentical to the ceramides that naturally exist in your skin. Ceramides are a family of waxy lipid molecules. Skin naturally loses ceramides with age, becoming drier and less supple. “There are three big classes of lipids: ceramides, cholesterol, and fatty acids,” Levin says. [21] With aging there is a decline in ceramide and cholesterol in the stratum corneum of humans. Substances known to induce ceramide generation, Mechanism by which ceramide signaling occurs, "Orm/ORMDL proteins: Gate guardians and master regulators", "Ionizing radiation acts on cellular membranes to generate ceramide and initiate apoptosis", "The sphingolipid salvage pathway in ceramide metabolism and signaling", "Aging up-regulates expression of inflammatory mediators in mouse adipose tissue", "Lipid-induced insulin resistance mediated by the proinflammatory receptor TLR4 requires saturated fatty acid-induced ceramide biosynthesis in mice", "Ceramides and glucosylceramides are independent antagonists of insulin signaling", "Ceramide-activated phosphatase mediates fatty acid-induced endothelial VEGF resistance and impaired angiogenesis", "p53-dependent ceramide response to genotoxic stress", "Caspase-dependent and -independent activation of acid sphingomyelinase signaling", "A house divided: ceramide, sphingosine, and sphingosine-1-phosphate in programmed cell death", "Thematic review series: skin lipids. Here's the Deal, This Unheard of Ingredient is a Major Skin-Saver, Everything You Need to Know About Glycolic Acid Peels, What Is Ferulic Acid? People with skin conditions such as eczema and psoriasis generally produce fewer ceramides. "There are certain ingredients that work better when they’re paired with ceramides, and that’s really that trifecta of your natural skin barrier. Ingredients like phytosphingosine and sphingosine are not instantly recognisable as ceramides, but are ceramide precursors which stimulate skin to produce ceramides. [23], Inhibition of ceramide synthesis with myriocin in obese mice may lead to both improved leptin signaling and decreased insulin resistance by decreasing SOCS-3 expression. Ceramides are a family of waxy lipid molecules. Hydrolysis of sphingomyelin is catalyzed by the enzyme sphingomyelinase. [4], As a bioactive lipid, ceramide has been implicated in a variety of physiological functions including apoptosis, cell growth arrest, differentiation, cell senescence, cell migration and adhesion. Levin recommends this formula to her patients who are interested in anti-aging products. Here, dermatologists … This reaction is catalyzed by the enzyme serine palmitoyl transferase and is the rate-limiting step of the pathway. Up to nine different ceramides have been identified in the skin and within the ceramides there are two types: Sphingosine and Phytosphingosine. Ceramides are lipids that are naturally found in the skin which help maintain the skin’s natural barrier and prevent moisture loss. The salvage pathway has been estimated to contribute from 50% to 90% of sphingolipid biosynthesis. [3] Roles for ceramide and its downstream metabolites have also been suggested in a number of pathological states including cancer, neurodegeneration, diabetes, microbial pathogenesis, obesity, and inflammation. Second, the de novo pathway creates ceramide from less complex molecules. Don’t use with: Ceramides are safe to use in conjunction with most if not all ingredients. The long-chain sphingoid bases released from the lysosome may then re-enter pathways for synthesis of ceramide and/or sphingosine-1-phosphate. Advertisement info: This blog post uses affiliate links. It is suspected that ceramide levels in the skin decline with age. Ceramide 2 = Cermamide NS = N-stearoyl sphinganine. [11], One of the most studied roles of ceramide pertains to its function as a proapoptotic molecule. Ceramides work best for skin when they’re combined with … [9] Ceramides cause endothelial dysfunction by activating protein phosphatase 2 (PP2A). But although the structure of ceramides may differ, she says their function is pretty much the same and nothing to stress over. Ceramides are lipids (aka fat molecules) that help the skin retain moisture and allow for proper function in the skin barrier. To distinguish between the types of Ceramides: Ceramide 1 = Ceramide EOS. Age and ceramides. Ceramide accumulation has been found following treatment of cells with a number of apoptotic agents including ionizing radiation,[2][12] UV light,[13] TNF-alpha,[14] and chemotherapeutic agents. Helps skin retain water and fade small wrinkles. Ceramides - I bet you have heard about them. Have been trying to understand ceramides lately, done a bit of research but as there are a lot of detailed posts on … The final reaction to produce ceramide is catalyzed by dihydroceramide desaturase. Ceramides and Their Importance to Skin Care. Ceramide links with other molecules to yield derivatives, such as sulfatides, globosides, glucosylceramide, sphingomyelin, and gangliosides. People with eczema dont have enough ceramides in their skin, and people with psoriasis have different ratios of certain ceramides causing eczemas trademark itchy bumps and psori… The word ceramide comes from the Latin cera (wax) and amide. [10] In mitochondria, ceramide suppresses the electron transport chain and induces production of reactive oxygen species. The professionals seem to agree that ceramides are pretty much all upside, no matter your routine. Ceramide plays an important role in signal transduction in programmed cell death (apoptosis), the cell cycle, and cell differentiation and senescence. They’re naturally found in skin cells and make up about 50 percent of the outer layer of skin (epidermis).. Thus, ceramide synthase family members probably trap free sphingosine released from the lysosome at the surface of the endoplasmic reticulum or in endoplasmic reticulum-associated membranes. What it is: An overnight ceramide formula to help skin feel rehydrated, supple, and protected. [19][20], The stratum corneum is composed of 50% ceramides, 25% cholesterol, and 15% free fatty acids. These lamellar sheets are thought to provide the barrier property of the epidermis. The other sphingolipids are derivatives of ceramides. There are nine natural ceramides which have been identified in human skin. This 3% blend of Ceramides works overnight at various levels of the skin to target dehydrated skin. Ceramide can be further hydrolyzed by acid ceramidase to form sphingosine and a free fatty acid, both of which are able to leave the lysosome, unlike ceramide. [22] A clinical trial using ceramide-rich wheat extract showed increased skin hydration in those taking the extract rather than the placebo. 118 votes, 57 comments. I received no money to write it. Ceramides are naturally derived from our skin but can also be made synthetically for use in skincare products to replenish what you’ve lost. Basically, because ceramides are made of fats, they can create a protective barrier against invaders that bind to water. Helps skin retain water and protects it from environmental factors. Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. There has been research suggesting that when ionizing radiation causes apoptosis in some cells, the radiation leads to the activation of sphingomyelinase in the cell membrane and ultimately, to ceramide generation. “It has ceramides and a unique peptide, which are basically small little protein molecules that signal the skin to do very specific things.” The blend of peptides, ceramides, and antioxidants in this formula work to plump and protect the skin, even skin tone, and smooth texture from fine lines and wrinkles. Ceramides are among the most worthy of opponents when it comes to battling dry skin, which is why we rounded up 13 best ceramide skin-care products to slather on this winter. ", "Ceramide Channels Increase the Permeability of the Mitochondrial Outer Membrane to Small Proteins", "Ceramide forms channels in mitochondrial outer membranes at physiologically relevant concentrations", "Ceramides - Skin Lipids That Keep Skin Moisturized", "Safety Assessment of Ceramides as Used in Cosmetics", "Apoptotic Sphingolipid Ceramide in Cancer Therapy", https://en.wikipedia.org/w/index.php?title=Ceramide&oldid=984345852, Creative Commons Attribution-ShareAlike License, This page was last edited on 19 October 2020, at 16:52. It is the type of ceramide and its structure that determines the function of the ceramides, but their overall function is to help maintain the … [18] Ceramide VI is the most abundant ceramide of the skin, along with ceramide II, and has been exploited to model the organization of the stratum corneum lipidic network. Dermatologists Say These Are the Absolute Best Lotions for Eczema, 13 Face Creams That Will Quench Even the Driest of Complexions, Byrdie uses cookies to provide you with a great user experience and for our. Ceramide 2, 3, 6II Lamellar Liquid Crystal Emulsion is a stable pre emulsion base containing ceramide 2, ceramide 3, ceramide 6II, and phytosteryl isostearate. "Slugging" Is the K-Beauty Skincare Trend Going Viral on Reddit, If You Haven't Tried These Paula's Choice Products Yet, You're Missing Out, The Little-Known Vitamin Your Skin Needs to Really Glow, Can Giving Up Moisturizer Save Your Skin? “This moisturizer is great because it has three types of ceramides, niacinamide, and hyaluronic acid,” Jaliman says. Ceramides play an incredibly crucial role in the barrier function of the skin. In the case of glycosphingolipids, exohydrolases acting at acidic pH optima cause the stepwise release of monosaccharide units from the end of the oligosaccharide chains, leaving just the sphingosine portion of the molecule, which may then contribute to the generation of ceramides. Please click here https://helpmeotc.com/learn_more to visit our site. Jaliman is also a fan of this formula, which is full of five ceramides, panthenol, and sodium hyaluronate to make the skin plump, smooth, and supple. So which one to choose in a skincare product? 3 – Preparation of ceramides from glycosphingolipids Ceramides are less easily prepared from glycosphingolipids than from sphingomyelin. “By the time you’re in your 30s, you’ve lost about 40% of your skin’s ceramides, and by the time you’re 40, you’ve lost about 60%,” Levin explains. Similar to how a stack of bricks wouldn’t be as effective at protecting without the mortar sealing everything in place, neither are your skin cells without ceramides. One of both Levin and Jaliman’s favorites, this oil- and fragrance-free face and body moisturizer soothes dryness and itchiness and restores hydration in even the driest skin. For those with very dry skin, Jaliman recommends this drugstore formula for the body that contains ceramides and oat flour to hydrate and soothe the skin. "I call it a smart moisturizer,” Levin says. One of the best ways to understand how ceramides function is to imagine your stratum corneum (the outermost layer of your epidermis) to be made up of bricks and mortar. Sign up to talk with a nutrition counselor. How often can you use it: It’s safe for frequent use and is recommended to be applied twice daily. The salvage pathway re-utilizes long-chain sphingoid bases to form ceramide through the action of ceramide synthase. Remember, water and fat do not like each other! [35] Bacteria of family Sphingomonadaceae, however, contain it.
2020 list of ceramides