Why trans fats became a food villain | Science News for Students

Why trans fats became a food villain

Food science produced a new fat — and then discovered it wasn’t all that good for us
Sep 29, 2016 — 7:00 am EST
Doughnut sprinkles

Some sweet foods, such as the sprinkles on this doughnut, get their texture and bland, sweet taste from trans fats — a type of fat made from vegetable oil. But emerging research is showing that these fats aren’t the healthier alternative to butter that they were once made out to be.


On a recent road trip, we stopped at a gas station to fill up our truck’s tank and pick up snacks. Inside the dingy, stale-smelling roadside store, I picked up a packet of thick cookies. The package proclaimed “zero grams trans fat!” Throughout the shop, more products echoed the first. “No trans fats here!” “Now trans fat free!” It’s the same story in stores across the United States. Many baked goods and prepared foods, from doughnuts to burritos, now declare themselves “trans fat free.”

Eating too much fat can cause health problems. But even in moderation, some fats are healthier than others. Now studies suggest that synthetic trans fats — a type made by adding extra hydrogen atoms to the chain-like molecules of vegetable oil — can act like a dietary villain.

Doctors once thought trans fats might be better for health than those found in meats and dairy. And because trans fats are cheap, food companies were eager to substitute them in foods. 

Og trans fat label
This food package advertises zero grams trans fat. Such advertisements are becoming increasingly popular with food companies.
John Kannenberg/Flickr (CC BY-NC-ND 2.0)

But over time, scientists slowly began to realize that artificial trans fats were no healthier than butter or other animal fats. Indeed, they might actually be worse for health. Now, the U.S. Food and Drug Administration — which helps to regulate the foods that go on our grocery store shelves — has declared that trans fats are not “generally recognized as safe,” or GRAS. This means food companies now have less than two years to get rid of them for good.

This series of events might seem like a failure of science. But the rise and fall of trans fats is actually an example of the scientific process at work. Science can yield new and seemingly useful inventions, such as trans fats. But sometimes those things may have unintended side effects.

flaky crust
For a perfect, flaky pie crust, bakers need the right types of fats.

For a perfect crust, just add fat

Baked goods such as pie crusts and doughnuts need fats. Fats help create a fluffy pastry and moist taste. Until the early 1900s, those fats usually came from animals. Butter comes from cow’s milk. Lard is pork fat. Tallow is fat from beef cattle or sheep. Though they come from different animals, these fats all have two things in common. First, they are solid at room temperature. Second, they don’t quickly become rancid — that is, develop a nasty taste as they undergo chemical reactions with the oxygen in air.

All fats are made of chemicals known as fatty acids. These are long chains of carbon atoms strung together. To be stable, a carbon atom needs to have four other atoms bonded to it. When one carbon atom connects to two other carbons (the ones above and below it in the chain), two bonds are used in those connections. That leaves two free spots on the outside of the chain. Hydrogen atoms can bond at these sites.

In a saturated fat, two hydrogen atoms bond to each carbon atom in the chain, with three hydrogens on the last carbon dangling at the end of the line. Because this fat molecule is soaked in hydrogen, scientists call these fats saturated or fully hydrogenated. These chains of fatty acids are long and straight. That allows them to line up nicely with each other. Their straight-chain structure helps saturated fats stay solid at room temperature. Animal fats such as butter, lard and tallow are saturated fats.

saturated model
This is a model of a saturated fat. The carbons (black) have all of their available bonds saturated with hydrogen (white).
Benjah-bmm27/Wikimedia Commons

Vegetable fats are unsaturated. That means some of their carbon chains have bonding sites free of hydrogen atoms. In these fats, some of the carbons in the chain use two of their bonds on only one connection. These double bonds leave less room for hydrogen.

Most vegetable fats are also cis-oriented molecules. That means the hydrogen atoms all bind on the same side of the carbon chain. When the hydrogen atoms are evenly distributed on both sides of the chain and there are no double bonds, the chain is straight. But when there are double bonds, the hydrogen atoms line up on only one side of the chain. That makes the chain of atoms bend or kink. These unsaturated fatty acids do not line up well with each other. The end result is that such fats tend to be liquid — oils — at room temperature.

unsaturated fat
This is a model of an unsaturated fat. The carbons (black) have a double bond, where they bond to each other in place of two hydrogens (white). The result is a kinked molecule.
Edgar181/Wikimedia Commons

Saturated fats from animals are best for baking. Their stable structure means they can be easily whipped up to hold air — making a frosting frothy and a cake fluffy. The fats also add flavor. But it costs more to raise animals than to grow plants, so animal fats are more expensive than vegetable oils. And that explains what drove scientists to engineer solid fats from vegetable oils.

Playing chemical musical chairs

More than a century ago, the French chemist Paul Sabatier discovered a way to turn unsaturated fats into something that resembled saturated fats. He bubbled hydrogen gas through vegetable oil. But first he added a catalyst — a substance that triggers a chemical reaction. Doing this broke down some double bonds. That allowed more hydrogen atoms to bond with the chain of carbon atoms.

This industrial process is now known as hydrogenation. It replaces double bonds between carbon atoms with hydrogens. If all of the double bonds are replaced this way, the fat will be fully hydrogenated — making a saturated fat. But if the process is stopped early, some double bonds will remain. Food chemists refer to this as partial hydrogenation.

During hydrogenation, something else also happens: The orientation of the bonds changes.

This is how it happens: When hydrogen gas is bubbled up through the fats, “it’s kind of like musical chairs,” explains Sarah Ash. She studies nutrition at North Carolina State University in Raleigh. The hydrogen atoms on the fat chains start in their normal positions, on their carbon “chairs.” Then the bubbling — the “music” — starts and the hydrogens get off of their carbon chairs and run around. Some of the carbon double bonds break. That leaves free spots for more hydrogen atoms to slip into the molecule. When the gas stops bubbling, the hydrogen atoms have to find “chairs,” so that they can quickly bind to available carbons again.

cis trans
The cis fatty acid (bottom) has a bend in the chain from the double bond. In contrast, the trans fatty acid (top) has hydrogens on opposite sides of the chain. The resulting molecule is a flat line.
Wikimedia/Public domain

But as in musical chairs, the hydrogens don’t always end up bonded to the same carbon atom onto which they originally had been seated. Instead, they may land on the opposite side of a double bond. Those opposite-side positions are known as trans. Trans-oriented chains are partially hydrogenated, which means they still have some double bonds. But the trans position balances out the carbon chain, making it a straight line, similar to a saturated fat. These “trans fats” line up nicely with each other, just as saturated fats do. And that means trans fats, too, are solid at room temperature.

With trans fats, the food industry had a cheaper solid fat to make pie crusts and to fry up potatoes. Soon, these fats began appearing on grocery shelves in products such as Crisco, which used trans fats produced from cottonseed oil. “It was marketed as a better alternative to lard,” Ash says. As doctors began to worry about the effect of animal fats on heart health, artificial trans fats — which were always from plant sources — seemed like a better option.

Trans fats also were less expensive than animal fats. They could sit on a shelf for months without going rancid. And they were seen as being “cleaner,” somehow, than fats from animals. Crisco, after all, was produced in a laboratory — no animals there.

And unlike lard, trans fats didn’t taste like they came from animals. “They were nice, bland white fats,” explains Tom Brenna. He studies food science at Cornell University in Ithaca, N.Y. “If you use lard or butter to make a frosting, it will taste like lard or butter. But partially hydrogenated vegetable oil doesn’t taste like anything.” These new fats were smooth and creamy and lasted as long as animal fats.

Crisco can
In its original form, Crisco shortening was made entirely of trans fats. Now, the product has changed its formula and is trans fat-free.
Chemical Heritage Foundation/Wikimedia Commons (CC BY-3.0) Permissions: CC BY-3.0

Crisco arrived on the market in 1911. In a 1996 national dietary survey, people in the United States were eating almost 6 grams (0.2 ounce) of trans fats per day. That’s almost 10 percent of the recommended daily limit of fat.

The trouble with trans fats

In the 1980s, doctors began to worry about saturated fats, especially ones from animals. There was evidence that people who ate a lot of saturated fats appeared to be more likely to get heart disease. These cardiovascular ailments are conditions that affect the heart and blood vessels.

Concerns about saturated fats arose from their effects on cholesterol (Koh-LES-tur-awl). This is a fatty material that helps to make up the membranes of our cells. The body makes cholesterol. Many foods also supply cholesterol to the body. To move within the body, cholesterol is carried through the blood by packages” known as lipoproteins (LIP-oh-PRO-teens).

One form, known as high-density-lipoprotein (or HDL) cholesterol is considered a “good” type. HDL shuttles its cholesterol to the liver where it will be prepared for excretion from the body. It also can go to organs such as the testes to be made into hormones.

Doctors tend to refer to low-density-lipoprotein (or LDL) cholesterol as the “bad” type. It can build up in arteries and eventually block the flow of blood. Too much LDL cholesterol in the blood is considered a sign that a person might be at risk for heart disease.

Doctors found that people who ate more saturated fats had higher levels of LDL cholesterol and total cholesterol. So doctors began to recommend that people replace more of the saturated fats in their diet with unsaturated ones. Food companies responded to these worries by using more trans fats. These fats were not saturated. They weren’t from animals. And as far as anyone knew at the time, they were safe.

But soon, people began to worry about trans fats, too. In 1990, Ronald Mensink and Martijn Katan, two scientists then at Wageningen Agricultural University in the Netherlands, published a study in the New England Journal of Medicine. It showed that trans fats might elevate LDL cholesterol, just as saturated fats do. “It spurred a tremendous amount of research,” says Alice Lichtenstein. She works at Tufts University in Boston, Mass. As a nutritional biochemist, she studies the chemicals that make up our food and how they interact with our bodies.

baked cookies
Now, people prefer to bake cookies without trans fats. But Ash and Lichtenstein say the most important advice isn’t to give up ingredients, but to eat everything in moderation.
timiewisnm/Flickr (CC-BY-SA 2.0)

After the paper by Mensink and Katan came out, other scientists — including Lichtenstein — combed through data from old studies on trans fats and also launched new studies.

Soon, many different laboratories showed that trans fats raised blood levels of the bad cholesterol. Also worrisome: Unlike saturated fatty acids, trans fats don’t raise levels of that “good” — HDL — cholesterol. Together, high levels of LDL cholesterol and low levels of HDL cholesterol tend to boost the risk of heart disease.

Scientists also found that saturated fats appeared less harmful than they had first thought. In 2015, researchers compared the results of 41 different studies. Saturated fats might not be associated with cardiovascular disease after all, they found. And many of the ways that people used to avoid saturated fats turned out to be counterproductive. Replacing saturated fats with sugar isn’t any better for health. Neither is avoiding all fats.

sugar replacement
Companies may advertise low fat foods, but may replace some of that fat with sugar.

Doctors now recommend limiting saturated fat to less than 10 percent of all calories eaten. And as much as possible, people are asked to replace saturated fats with unsaturated ones, such as many of the fats in olive and canola oils. The goal is not  to replace saturated fats with sugar, and definitely not with trans fats.

Even though scientists continue to debate the health effects of eating saturated fats, all generally agree that manufactured trans fats are harmful. Scientists still are not sure how trans fats affect cholesterol, though. One possibility is that trans fats slow the breakdown of LDL particles, says Lichtenstein. This might make them build up in the blood. But how that might happen remains a mystery.

But the FDA doesn’t have to know exactly how something works to regulate it. In 2003, it issued a ruling that trans fats would get a separate line on nutrition labels, starting in 2006. Getting an extra line on a label doesn’t seem like it should be a big deal. But food companies were not eager to advertise how much of these trans fats a food had. So they quickly worked to make sure they brought it down to zero. “I don’t think customers used the information,” Lichtenstein says. “But I think the companies realized it looked better for their products to be listed as zero trans fat."

Butter is back

Over time, evidence continued to build that artificial trans fats are not heart friendly. In June 2015, FDA finally declared that partially hydrogenated oils — the artificial trans fats — are not GRAS. By June 18, 2018, companies may no longer sell foods in the United States that contain these fats. Many companies already have removed them.

C is for cookie
Now, people prefer to bake cookies without trans fats. But Ash and Lichtenstein say the most important advice isn’t to give up ingredients, but to eat everything in moderation. The obsessed Cookie Monster shouldn’t have the same food all the time.
HappyLogolover2011/Wikimedia Commons

To do this, though, companies have had to change their recipes.

“They will reformulate with other kinds of vegetable fats, palm oil, coconut oil,” explains Brenna, at Cornell. “These days we … can create plants that will give oil profiles of anything we like.” Just as scientific research discovered trans fats, science is now finding ways to replace them.

Manufactured trans fats may fade from U.S diets, but that won’t automatically make these foods healthy. “Just because a food has zero grams of trans fat doesn’t mean it’s good for you,” notes Lichtenstein. “Trans fat is just one component [of our diet]. Cardiovascular disease is a slowly progressing disease and it starts young.” Lowering risk doesn’t end with ditching trans fats from the diet, she warns. It means eating a healthier diet overall and getting plenty of exercise. 

If you eat everything in moderation, Ash says, you may need to worry less about what fat or ingredient is the next diet villain. Cookies or doughnuts — the trans fat-free kind — still can be, as Cookie Monster says, “a sometimes food.”

Editor’s note: This article was updated on November 23, 2016 to clarify the definition of cholesterol.

Power Words

(for more about Power Words, click here)

atom    The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

biochemistry    A field that marries biology and chemistry to investigate the reactions that underpin how cells and organs function. People who work in this field are known as biochemists.

carbon    The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cardiovascular    An adjective that refers to things that affect or are part of the heart and the system of vessels and arteries that move blood through the heart and tissues of the body.

catalyst    A substance that triggers or speeds up a chemical reaction without itself being affected.

cell    The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.

chemical    A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

chemical bonds    Attractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

chemical reaction    A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chemistry    The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.

cholesterol    A fatty material in animals that forms a part of cell walls. In vertebrate animals, it travels through the blood in little vessels known as lipoproteins. Excessive levels in the blood can signal risks to the blood vessels and heart.

cis    (in chemistry) This refers to the position of atoms on a molecule, around a double bond. When two molecules are on the same side of the bond, they are in the cis conformation. When they are on opposite sides, they are in the trans conformation.

dairy    Containing milk or having to do with milk. Or a building or company in which milk is prepared for distribution and sale.

double bond    A type of bond between two atoms within a molecule. In a single bond, atoms share two electrons. In a double bond, they share four. This bond is slightly less stable than a single bond.

fatty acid    Large molecules made of up chains of carbon and hydrogen atoms linked together. Fatty acids are chemical building blocks of fats in foods and the body.

Food and Drug Administration    (or FDA) A part of the U.S. Department of Health and Human Services, FDA is charged with overseeing the safety of many products. For instance, it is responsible for making sure drugs are properly labeled, safe and effective; that cosmetics and food supplements are safe and properly labeled; and that tobacco products are regulated.

electron    A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

fat    A natural oily or greasy substance occurring in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat is also a vital nutrient, though it can be harmful to one’s health if over consumed in excess amounts.

fatty acid    A large molecule made of up chains of carbon and hydrogen atoms linked together. Fatty acids are chemical building blocks of fats in foods and the body.

hormone    (in zoology and medicine)  A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

hydrogen    The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

hydrogenation    An industrial process used to add hydrogen atoms to a molecule. When applied to oils, it makes them solid at room temperature.

lard    Pork fat, used for cooking and baking.

lipoprotein     A structure that the body makes to shuttle cholesterol around the body via the bloodstream. Lipoproteins tend to be known by how densely the cholesterol is packed within them. High density lipoproteins (or HDLs) are usually considered good for health. Low density lipoproteins are considered generally harmful. Their size is not the issue so much as where they release the cholesterol that they're carrying.

liquid    A material that flows freely but keeps a constant volume, like water or oil.

molecule    An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nutritional biochemistry     The study of the chemicals that make up our food and how those nutrients interact with the cells and tissues of the body.

oxidation    A process that involves one molecule’s theft of an electron from another. The victim of that reaction is said to have been “oxidized,” and the oxidizing agent (the thief) is “reduced.” The oxidized molecule makes itself whole again by robbing an electron from another molecule. Oxidation reactions with molecules in living cells are so violent that they can cause cell death. Oxidation often involves oxygen atoms — but not always.

partial hydrogenation    An industrial process that adds hydrogen atoms to a molecule, and stops before all of the bonds have been filled. This may leave a molecule with double bonds. In food processing, partial hydrogenation can create trans fats.

rancid    Having a rank, unpleasant smell or taste. Some food products become rancid as they decompose or oxidize.

saturated fat    A fat molecule made from chains of carbon atoms, where each carbon has at least two hydrogen atoms attached to it (those on the ends have three). These chains have no double bonds. Saturated fats are found in animal fats (such as butter and lard), as well as in vegetable fats (such as coconut oil). Their long, straight chains allow them to become solid at room temperature.

solid    Firm and stable in shape; not liquid or gaseous.

tallow    A form of beef or mutton (sheep) fat.

testes   (singular: testis) Organs in the males of many animal species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.     

trans    (in chemistry) This refers to the position of atoms on a molecule, around a double bond. When two molecules are on opposite sides of the bond, they are in the trans conformation. When they are on the same side, they are in the cis conformation.

trans fat      Short for trans-unsaturated fatty acids. Trans fats most often are made when scientists bubble hydrogen through an unsaturated vegetable fat. As the hydrogen atoms rearrange, some come down on opposite sides of a double bond, in the trans position. This creates a straight fatty acid, which is a solid at room temperature and can substitute for animal fats in processed food. Recently, such trans fats have been associated with increased risk of heart disease and are being removed from processed foods. Another type of trans fats occur naturally. They are chemically different. And in contrast to the industrially manufactured types, these trans fats have a number of health benefits.

unsaturated fat    A fat molecule made of chains where some carbon atoms are not fully saturated with hydrogen atoms. These carbons form double bonds with the carbons next to them. Unsaturated fats are found in vegetable fats such as canola, olive and corn oil. Their double bonds create kinks in the molecules, which results in the fats often being liquid at room temperature.


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Journal: P.C. Calder. Functional roles of fatty acids and their effects on human health. Journal of Parenteral and Enteral Nutrition. Vol. 39, September 2015, p. 18S. doi: 10.1177/0148607115595980.

Journal: R.J. de Souza et al. Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies. BMJ. Vol. 351, August 12, 2015, p. 351. doi: http://dx.doi.org/10.1136/bmj.h3978.

Journal: E.M.M. Ooi et al. Effect of dietary fatty acids on human lipoprotein metabolism: A comprehensive update. Nutrients. Vol. 7, June 2, 2015, p. 4416. doi: 10.3390/nu7064416.

Journal: A.H. Lichtenstein. Dietary trans fatty acids and cardiovascular disease risk: Past and present. Current Atherosclerosis Reports. Vol. 16, June 8, 2014, p. 433. doi: 10.1007/s11883-014-0433-1.

Journal: R.P. Mensink and M.B. Katan. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects. The New England Journal of Medicine. Vol. 323, August 16, 1990, p. 439. doi: 10.1056/NEJM199008163230703.