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Dietary science is complicated-- one day something is good for you and the next it's not. Learn what we DO know about fat chemistry in this episode of SciShow.

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Where Does Some of that Carbon Go?:

Hosted by: Hank Green
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Trans fats are bad
Fat Nutrition Overview:
Monounsaturated vs Polyunsaturated
Essential Fatty Acids:
Low fat diets don’t help:
Reduction in saturated fat has a small but measurable affect on cardiovascular health:
Hank: We don’t talk a lot about dietary science here on SciShow because, frankly, it can seem like a hype factory designed more to generate clicks than to generate understanding. People want to hear what to eat and what not to eat in order to be healthy. The problem with that... is that it’s complicated. It’s too complicated to fit into a headline. Eating lots of fat will make you unhealthy, but so will eating lots of anything.

Studies show that cutting fat intake is useless when attempting to help people lose weight if those calories are replaced by something else, which they tend to be. Now fat is relatively straightforward chemically, but it’s boggled dietary scientists and, especially, journalists writing about it for decades. But the basics of what we understand aren’t that complicated, so let’s get to the heart of fat.

[SciShow Intro plays]

Fats are molecules that organisms use to store energy, they are very good at that and so they can be converted into lots and lots of energy for running an organism, whether it’s a seed growing into a plant or a neuron firing in your brain. But, basically, the first thing to know about fat is that it is energy dense. So per gram, fat has more than twice the energy of protein or carbohydrates.

Fats, like everything we eat, are made of chemicals. And those chemicals are primarily composed of large chains of carbon atoms. Each of those carbon-carbon bonds can be broken by your body and converted into usable energy. The energy density of fat combined with the fact that people who ate high fat diets tended to be less healthy was the original cause of the anti-fat craze that started in the 60s and continues at least to some extent today.

But it turns out that fats are not inherently bad for you, indeed, they are an absolutely necessary part of the diet. But there are definitely some fats that promote health more than others, and some that are downright dangerous. You’ve heard of the main categories: You got your trans, saturated, unsaturated, hydrogenated, polyunsaturated, omega 3... all of these words are chemistry terms that get talked about a lot without discussing what they mean, like, at all. But this is SciShow, so let’s go there.

Carbon has four unpaired electrons, so each carbon atom likes to bond to four other things. This is methane: carbon bonded to four hydrogens. And yes, I am using snatoms-- Veritasium’s molecular modeling kit which I’m very excited about. If the carbon is in a saturated carbon chain, two of those things will be other carbons-- unless it’s the end of the chain. And two of those things are going to be hydrogen. If this is what the carbon chain looks like with a bunch of carbon atoms, each bonded to two carbons and two hydrogens, that is a saturated fat, which tend to be solid and are mostly made by animals...though also coconuts. The thing that saturated fats are saturated with is hydrogen. Every carbon is bonded to two of them, except the last, which is bonded to three.

And unsaturated fats are not saturated. They have fewer hydrogens. How? Well they’ve got at least one double bond between two carbons in the chain. Each double bond means two fewer hydrogens. So unsaturated fats don’t have as many hydrogens as they could have per carbon atom. And this is not as minor a change as it might sound. Single bonds can rotate around their axis no problem, but double bonds lock a structure into place. And that double bond can either lock the chain into a kink or lock it straight.

In nature, a double bond in a fat chain almost always locks it into a kink. This prevents the fat molecules from stacking into lattices making them more likely to be liquid at room temperature. So unsaturated fats tend to be liquid, making them oils, and come mostly from plants and fish. The kinked configuration is called the “cis isomer” and the straight one is the trans isomer. The word ‘isomer’ just means that the molecules have the same chemical formula, but are different shapes. And the different shapes matter... a lot.

Saturated fats are less healthy than unsaturated fats because they pack together easily and can form plaques in your arteries and can also interact with cholesterol making it build up in your bloodstream. Cis unsaturated fats (which the rest of the world just calls unsaturated fats) don’t stack well and so don’t form plaques. So that difference exists, but it isn’t as big as we used to think it was. It turns out our bodies are pretty good at dealing with whatever we throw at it, as long as it’s fairly familiar.

Worse by far are the trans unsaturated fats. These are almost all manufactured by adding hydrogens or “partially hydrogenating” some unsaturated fat like soybean oil. So trans fat and hydrogenated fats are the same thing: one refers to the process through which they are created, the other their chemical structure. These are fats that have double bonds AND stack fact, they stack even better than saturated fats AND our biochemical systems aren’t designed to break them down AND they interact badly with cholesterol.

A two percent increase in daily intake of trans fat corresponds with a 23% increase in the chance of heart disease. Amazingly they were initially seen as a healthy alternative to saturated fats and are now in the process of being completely banned in America. Now, even in these categories there’s variation, and inside that variation comes more active areas of research where less is known for sure. Polyunsaturated fats, or fats with more than one double bond might be more or less healthy than monounsaturated fats. No one really agrees because they both have benefits and drawbacks, and it matters where in the chain the double bonds are. You should probably have some of both.

And If the double bond is just before the third carbon from the end, we call that an omega 3 fatty acid and they seem to have a wide range of benefits. And they’re also one of the two essential fatty acids (the other being omega 6. Essential compounds are chemicals that our bodies need to function correctly but can’t manufacture on their own, so we literally need to eat those kinds of fats in order to survive. And indeed, we need to eat fat in general to survive. It’s good for you... in the correct quantities.

So those are the basics of fat you guys! They have TONS of energy in them, so they’re great when you need calories. But they also make it super easy to eat too many calories. They’re absolutely necessary for life, they taste good and are good for you, but unsaturated fats are better and trans fats might very well kill you.

Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to And if you’re interested in Snatoms-- I’m not sure if the Kickstarter is still going on-- but they’re basically these little magnetic modeling kits. They’re very cool, we’ll put a link in the description.