Why does alcohol have calories?

You down a shot of vodka and then after you start to wonder how many calories it has. You pull out your smartphone and check…100? What, it’s just clear liquid? How is that possible? It doesn’t have any sugar, fat, or protein and isn’t that where calories come from?

To understand this, let’s start by making sure we understand what calories are, and why high-calorie items get turned into fat. We know that foods with lots of sugar, fat, or protein have lots of calories. Calories are a unit of energy. Molecules we eat that provide more calories give the body more ability to do work, in this case, to run its cells.

Your cells need a LOT of energy; they’re constantly making and breaking down proteins, reading their genes, and fighting off invasions. Zooming out from cells to organs, two major hotspots for energy use are your muscles and brain. Both use tons of energy! And that’s just under normal conditions; when something goes wrong in your body, your cells go into overdrive trying to get back to status quo.

So how does food give us energy? It all comes down to a molecule called ATP. (Scientists just love their acronyms, don’t they?) ATP stands for adenosine triphosphate but it could also stand for ALL THE POWER. The bonds in ATP store lots of energy. You can think of ATP like a rechargeable battery: you use up the energy by breaking bonds and recharge them by rebuilding those bonds.

The fat you eat is broken down into fatty acids and glycerol. Fatty acids are turned into water and carbon dioxide. (Yep, the same carbon dioxide that is messing with our ozone.) This happens by processes called beta-oxidation and the Krebs Cycle, both processes that make that powerful molecule ATP. The other molecule I mentioned that’s made when you break down fat—glycerol—also feeds into energy-generating processes that make ATP.

Now, how are carbohydrates used for energy? Carbohydrates (or carbs as we tend to call them) are big molecules that are made up of smaller molecules of glucose and other sugars. Carbs are broken down into glucose molecules, which are used in a pathway called glycolysis to make ATP.

So, how is the ALL THE POWER—I mean adenosine triphosphate—used? I’ll give you just a few examples. Our cells use ATP to move molecules across their membranes and to assemble and disassemble the cell’s skeleton. ATP is also used as a signal within and between cells. The last uses I’ll mention are that ATP is used to make DNA and proteins. As you can see, ATP is pretty important.

So, what happens when you have plenty of energy in the form of ATP? This high energy state promotes pathways that make the fat molecules in your fat cells.
Your fat cells get bigger, and so do you. “Not that there’s anything wrong with that!” Ok I couldn’t help but throw in that old Seinfeld reference. Whether there’s something wrong with being fat is a way complicated medical and societal issue that we’ll come back to on another day.

Ok so now you have a bit of a sense of how the energy from food can get stored as fat. But we talked about how you can get energy from foods with sugar or fat, but alcohol doesn’t have those things.

So what’s up? Even though alcohol doesn’t have sugar, fat, or protein, your body turns the alcohol into something that gets broken down and produces ATP in the process. These ATP molecules are why alcohol has calories. The alcohol that’s in your beer and cocktails is ethanol, which has 2 carbons, 5 hydrogens attached to those carbons, and a hydroxyl group. A hydroxyl is an oxygen and a hydrogen bonded together, and it’s this hydroxyl group that makes ethanol an alcohol.

As you sip on your vodka cocktail, your body is ingesting ethanol, which gets turned into acetic acid in a process that makes ATP. (Fun fact: acetic acid is the main ingredient in vinegar!) The acetic acid is then broken down by the Krebs Cycle, yielding even more ATP molecules.

Another way of looking at energy production is how many kilojoules a process makes. You’re familiar with kilojoules, even if you don’t know it. That’s because a kilojoule is about a ¼ of a dietary calorie. To understand how we count the calories we get from a shot of vodka, you need to know that chemists measure quantities in amounts called “moles.” It’s really similar to how we measure eggs in dozens, except that a mole of something is way way way more than a dozen. If you want to know how much bigger, if you divide a mole by a dozen, the number you get has 22 zeros!

A shot has 1.5 oz, and there’s about a 1/3 of a mole of ethanol in a shot of 40% vodka. The process of metabolizing a mol of ethanol makes 1,325kJ of energy for our bodies. So a shot of 40% vodka has about 400kJ or 100 dietary calories.

What else has 100 calories? 2 cups of strawberries, 1 medium sweet potato, 1 slice of cheese, and 1 slice of bread each has 100 calories. But today we’re talking about beverages. You can find 100 calories in 4 oz. of wine, 7 oz. of beer, 8 oz. of soda if you’re going the non-alcoholic route, or of course, 1 shot of vodka.

Now you understand why this clear colorless beverage can have so many calories—it’s because it provides the body with molecules that feed into energy-producing pathways. So, if calories are something that matter to you, maybe pass up the soda mixer and use seltzer water instead.

And if you had trouble following this math, don’t feel bad. I skipped lots of steps in my simplified explanation, and this stuff is way complicated!

Thanks to Dr. Phil Kyriakakis, Ray Mak, and Alina Sokolskaya for their helpful comments in the writing process.

Still confused about something? Have more questions? Or do you want to add any info, notice any errors, or just want to give me some feedback or ideas for future podcast episodes or blog? Let me know!

National Institute of Alcohol Abuse and Alcoholism “Alcohol Alert”
“Overview: How Is Alcohol Metabolized by the Body?
Principles of Biochemistry, 4th Edition


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I've been writing about my weight loss (-100lbs, now just -80 bc of regain) for years. Now I also write about science. Find me at thegeneticsgal.com and say hi.

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