Lipids are the scientific name for fats, oils, and a few related compounds. They are one of the three main energy nutrients, alongside carbohydrate and protein, but they also build cell membranes, cushion organs, and carry certain vitamins. This guide explains what lipids are, the different kinds of dietary fat, what they do in your body, and which fats support your health.
What lipids are
Like carbohydrates, lipids are built mainly from carbon, hydrogen, and oxygen. Because they carry far more hydrogen relative to oxygen, they pack more energy per gram than carbohydrate does. Their defining feature is that they do not dissolve in water. The group also includes the fat-soluble vitamins (A, D, E, and K) and cholesterol.
In everyday terms, "fats" are lipids that are solid at room temperature, like butter, while "oils" are lipids that stay liquid, like olive oil. Both are concentrated energy: fat provides about 9 calories per gram, more than double carbohydrate or protein.
Where lipids come from in food
Dietary fat comes from both animal and plant foods. Animal sources include meat, milk, and dairy; plant sources include nuts, seeds, avocados, and the oils pressed from them. Some fats also come from processed foods, butter, and margarine. The type and amount of fat you eat influence your risk of heart and metabolic disease, which is why not all fats are equal.
The main classes of lipids
- Fatty acids are chains of carbon with an acid group at one end. They are the building blocks of most other fats.
- Triglycerides are the main fat in food and the body's main storage fat. Each one is a glycerol backbone holding three fatty acids.
- Phospholipids form the membranes around every cell. They are like triglycerides but with one fatty acid swapped for a phosphorus-containing group.
- Sterols have a ringed structure. The best-known is cholesterol, which your cells need for their membranes and to make certain hormones.
The four types of dietary fat
- Saturated fat has no double bonds in its carbon chain, so it packs tightly and is usually solid at room temperature. Found in fatty meat, butter, cheese, and tropical oils like coconut and palm.
- Monounsaturated fat has one double bond, which kinks the chain and keeps it liquid. Found in olive oil, canola oil, avocados, and nuts.
- Polyunsaturated fat has two or more double bonds. Found in oily fish (salmon, mackerel), flaxseed, walnuts, and many vegetable oils. This group includes the essential omega-3 and omega-6 fats your body cannot make.
- Trans fat is made by adding hydrogen to liquid oils to harden them. Found in partially hydrogenated oils and some processed and fried foods. This is the one type to avoid.
What lipids do in the body
- Energy. Triglycerides store energy efficiently in fat tissue and release it when needed, at about 9 calories per gram.
- Insulation. A layer of fat under the skin helps hold body heat.
- Protection. Fat around organs like the kidneys cushions them against impact.
- Cell structure. Phospholipids and cholesterol build the membranes that hold cells together and let them communicate.
Which fats are healthy?
This is where the practical guidance matters most, and the major health bodies largely agree.
Replace saturated fat with unsaturated fat. Swapping some saturated fat for unsaturated fat lowers blood cholesterol and reduces cardiovascular risk, which is the core advice of the American Heart Association and American College of Cardiology lifestyle guideline (Eckel et al., 2013). A World Health Organization review reached the same conclusion: replacing saturated fat with polyunsaturated fat improves blood-lipid profiles (Mensink, 2016). In practice, that means leaning on olive oil, nuts, seeds, avocado, and oily fish, and going easier on butter, fatty cuts of meat, and full-fat dairy. The American Heart Association's dietary fats guidance lays this out simply.
Avoid trans fat. Trans fat is the clearest villain. A major review concluded that trans fat raises the risk of heart disease more, per calorie, than any other nutrient (Mozaffarian et al., 2006). Many countries have now restricted it, but it can still appear in some processed and fried foods, so check labels for "partially hydrogenated oil."
Keep cholesterol in perspective. Dietary cholesterol matters less than once thought for most people; the bigger lever is the type of fat you eat (NHLBI). For the wider eating pattern that supports healthy fats, see our guide to anti-inflammatory foods.
The bottom line
Lipids are not the enemy. They fuel your body, build your cells, and carry key vitamins. What matters is the kind of fat: favor unsaturated fats from plants and fish, limit saturated fat, and avoid trans fat. Get that balance right and fat becomes one of the most useful parts of your diet. For related reading, see how fat fits into boosting metabolism naturally and eating avocado during intermittent fasting, or browse our nutrition guides.
Frequently Asked Questions
What are lipids? Lipids are a family of compounds that include fats, oils, certain vitamins, and cholesterol. They do not dissolve in water, store energy efficiently, and form part of every cell membrane.
What is the difference between fats and oils? Both are lipids. Fats are solid at room temperature and tend to be more saturated (like butter); oils are liquid and tend to be more unsaturated (like olive oil).
Which fats are healthiest? Unsaturated fats from olive oil, nuts, seeds, avocado, and oily fish. Health authorities recommend replacing saturated fat with these and avoiding trans fats.
Are all saturated fats bad? Saturated fat is not poison, but replacing some of it with unsaturated fat lowers cardiovascular risk. The bigger problem is trans fat, which should be avoided.
How much energy do lipids provide? About 9 calories per gram, more than double what carbohydrate or protein provides, which is why fat is the body's most energy-dense fuel.
Medical disclaimer: This article is for informational purposes only. Always consult a qualified healthcare provider before making changes to your health routine.
Sources
- Eckel RH, et al. 2013 AHA/ACC guideline on lifestyle management to reduce cardiovascular risk. Journal of the American College of Cardiology, 2014 — PubMed. https://pubmed.ncbi.nlm.nih.gov/24239922/
- Mensink RP. Effects of saturated fatty acids on serum lipids and lipoproteins: a systematic review and regression analysis. World Health Organization, 2016. https://www.who.int/publications/i/item/9789241565349
- Mozaffarian D, et al. Trans fatty acids and cardiovascular disease. New England Journal of Medicine, 2006 — PubMed. https://pubmed.ncbi.nlm.nih.gov/16611951/
- American Heart Association. Dietary Fats. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/dietary-fats
- National Heart, Lung, and Blood Institute. Blood Cholesterol. https://www.nhlbi.nih.gov/health/blood-cholesterol
All sources accessed 31 May 2026.
