When it comes to dieters and health trendsters, carbs and fats have had avid fans and haters alike, thanks to a very polarised portrayal in the media. As such, we’ve seen both extremes, with low-fat diets minimising fat and oil intake, and low-carb diets minimising all carb-rich foods. Low-fat diets have been around for ages, based on the premise that the fat you eat is the fat you carry. On the other hand, low-carb diets have more recently surged in popularity, promising big results. Each extreme makes use of different assumptions about how these macros are used in our bodies.
- So what’s the difference between carbs and fats?
- Which do our muscles prefer to use as fuel?
- How does fat get stored in our bodies?
- What happens if we go to the extreme end of a low-fat diet, or a low-carb diet?
- What do the studies say?
These are all questions that I’ve been asking myself, and I’d love to share my learning journey with you. I’m no expert: not a dietician, not a biochemist. But hopefully my lay understanding of these ideas after extensive reading can translate into some explanations that make sense to other non-experts 🙂
I’m dividing this topic into three parts. In this first part of my Carbs vs Fats series, I’ll talk about what carbohydrates and fats are, and what happens in our bodies when we eat them. In other words, how our bodies metabolise (break down) or store fats and carbs, and how they use them as an energy source.
So what is a carbohydrate?
A carbohydrate is a molecule that appears in many different forms, from simple sugars (such as in candy) to complex starches (such as in vegetables). In food science, a ‘carbohydrate’ refers to any food that is rich in complex carbohydrate starches or simple carbohydrate sugars. Even fibre is a carbohydrate!
What happens when we eat carbs?
- You eat a baked potato or drink a fruit smoothie—loaded with carbs.
- The carbohydrates are broken down and converted to glucose (a simple sugar) in your bloodstream.
- This blood glucose (or blood sugar) is the main source of fuel for all the cells in your body, including your brain (which uses about half a cup of sugar’s worth of calories in a day) and your muscles.
- How do your muscles use this blood glucose?
- If you were to go for a walk or a run shortly after the meal, your muscles would use that blood glucose as fuel.
- If, instead, you decide to chill out and take a nap, that blood glucose gets converted to a more complex sugar called glycogen, and stored in your muscle tissue.
- If you go to the gym a bit later for a workout, your blood sugar levels will be lower, and your muscles will use its glycogen stores as fuel instead.
- (You’ll see later what happens when you’ve depleted your muscles’ glycogen stores.)
How do carbs end up as fat?
- Let’s say you eat a low-fat (but high sugar) vanilla pudding and wash it down with a cup of sweet hot chocolate: way more sugar than your body can use that day, and you end up with a large excess of blood glucose in your system.
- That excess glucose gets converted into fatty molecules that are stored in your fat cells (i.e. the fat cells get bigger).
- (Some day when you eat less than you use, your body will be able to use that stored fat as fuel.)
- Here’s the thing, though: the process of converting sugars to stored fat actually uses quite a lot of energy: about 23% of the calories ingested as carbs are used up by the time the fatty molecules are formed. You’ll soon see why this is important.
In short: your muscles (and brain) use carbohydrates as their main fuel source. Excess carbs are converted to fat, but that process uses a lot of energy.
And what is a fat?
Using the broadest biochemical definition, dietary fat (e.g. from oils, nuts and seeds, avocados, meat, dairy, eggs) is a molecule called a triglyceride, basically a group of fatty acids chained together. Fats are insoluble in water—as you’ll see later, something special needs to happen in order for fats to get through our bloodstream.
What about ingested fat?
- Maybe you had some scrambled eggs with butter-fried mushrooms and a sizzling vegan sausage for breakfast—a pretty high-fat meal, even though you’re watching your calorie intake (fats contain 9 calories per gram, carbs and protein contain 4 calories per gram).
- The food gets all the way to your intestines before the fat content is registered: your intestines signal to your body that you don’t need any more food. (Unfortunately, that’s usually too late: you’ll have finished your meal long before, and maybe had a second serving.)
- From your intestines, the large fat molecules (triglycerides) are broken down into smaller fat molecules so that they can cross the intestinal membrane, after which they’re transformed back into triglycerides.
- These fat molecules are packaged with cholesterol molecules (which were produced by your body, or from those scrambled eggs) by special things called chylomicrons, which enable the groups of fatty molecules to float through your circulatory system, which is aqueous (water-based).
- From there, the fat molecules get deposited and stored in the liver and in the body’s fat tissues.
- This whole process is lengthy and complicated, but remarkably, only about 3% of the calories ingested as fat are used up in the process of storing the fat in the liver and fatty tissues (a lot less than the 23% used up in the process of converting carbs to fat). If you’re wanting to reduce your body fat, the message is clear: limit your fat intake.
Ok, how do we burn fat?
- So you ate a few hours ago, now you’re at the gym and you’ve been sweating for 30 minutes or so.
- After all that work, your muscles’ glycogen stores are depleted and they need to find another source of fuel.
- Now your body starts using your fat stores as a fuel source, “burning” fat to produce the energy your muscles need.
- Remember that fat produces about twice the amount of energy as carbohydrates (per gram).
It takes considerable work to burn fat, and yet the fat we consume gets deposited very quickly and efficiently to be stored in our body’s fatty tissues and liver. If we want to limit or reduce the fat we store on our bodies, it makes sense to focus on carbohydrates as a fuel source, and limit our intake of fat.
In fact, studies have shown that when comparing the effects of sugar and fat side-by-side on our appetites later in the day, added sugar does a better job of suppressing appetite later, than added fat. In other words, our bodies are better able to sense extra calories from carbohydrates (whether simple or complex) than from fats, to regulate our appetite.
In the next parts of this series on Carbs vs Fats, I’ll dive into other questions, such as the role of fibre in carbohydrate-rich foods, diabetes and carbs, and maybe even keto (and ultra-low-carb diets). If you have any comments or questions on this topic, I’d love to hear from you. Email me, or leave a comment 🙂
By the way, my main source of information for this piece was this module of a course from Lumen Learning. Check it out if you want the nitty-gritty.
Part 2: Diabetes, insulin resistance & diet
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