I thoroughly enjoy fatty foods, especially steak. Last weekend, I ate four steaks in less than 36 hours. And I’m not talking extra-lean sirloin, I’m talking the fancy stuff: ribeye and New York strip.
High-fat diets rich in saturated fat from delicious foods like steak, eggs and bacon have surged in popularity over the past decade. The prevalence of nutritional approaches like Keto and Carnivore have brought mainstream media attention to low-carb, high-fat diets. While this opened up a welcome discussion of the fact that dietary fat and cholesterol aren’t the heart disease inducing devils we once thought they were, it did force another false narrative: that a low-carbohydrate diet is the best option for health, weight loss and body composition.
This simply isn’t true. In fact, there’s no “best” diet for weight loss because when it comes down to it, caloric balance is by far the most important factor. Whether you follow a high-fat or low-carb diet, as long as you eat fewer calories than you burn, you’ll lose weight.
Yet people often assume that if they want to lose weight, they need to eat fewer carbs. Foods like bread, pasta, rice and even fruit are suddenly off limits, and fatty alternatives like avocado, butter and almonds are fair game. And while a high-fat diet often helps people lose weight (as long as it results in a caloric deficit), there are some drawbacks to a low-carbohydrate diet, especially when it comes to performance. Basically, if you like training hard, you should think twice before adopting a low-carbohydrate diet.
Keep in mind that I’m not a doctor, dietician or nutritionist. I can’t tell you exactly what to eat, and you shouldn’t take my advice in lieu of what your doctor or dietician is telling you to do. However, I can present you with some scientifically-proven facts that should cause you to consider the drawbacks of a low-carbohydrate diet, especially if you care about getting stronger, building muscle or excelling at the sport you play.
Reduced Athletic Performance
Carbs are the body’s favorite fuel for high intensity activity. While fat is burned primary at rest and during prolonged low-intensity activity (i.e., walking or jogging for more than 30 minutes or so), as soon as you start to put forth significant effort, your body switches to burning primarily carbs because of how efficient they are at supplying ATP (your body’s energy currency).
I snagged this graphic from my old Exercise Physiology textbook and it illustrates the shift between fat and carbohydrate metabolism based on exercise intensity.
While this chart uses VO2max, which indicates aerobic activity such as running or biking, you can bet your ass that lifting weights utilizes carbs almost exclusively for fuel.
A low-carbohydrate diet denies your body of it’s most convenient fuel source. Carbs are readily available from multiple depots in the body, including:
- Muscle glycogen
- Liver glycogen
- Blood sugar
Depending on what activity you’re doing and how recently you ate a carb-rich meal, your body can utilize carbs from these different sources. Simple carbs like sugary sports drinks and gels can be put to good use almost immediately. In fact, carbohydrate digestion begins while they’re still in your mouth, well before they reach the stomach, making them a favorite of endurance athletes and team sport athletes like soccer, football and field hockey players.
Fat, on the other hand, is stored in adipose tissue and must be broken down into free fatty acids (FFA) to be used for energy by a process called lipolysis. Lipolysis is generally a slow process, especially compared to the breakdown of carbs for energy (called glycolysis). Athletes who consume a low-carb, high-fat diet will often experience a lack of energy and decrease in strength, speed and power due to the inefficient means by which they must fuel their activity.
Trying to use fat to fuel intense exercise like sprinting, jumping and lifting weights is like trying to put diesel in a car that takes gasoline. It’s simply the wrong fuel for the job.
Inefficient Fat Loss
OK, but what if you don’t care about athletic performance and just want to get shredded? You’re not trying to win a championship game or set any lifting records. All you want is six-pack abs, so a low-carbohydrate diet would make sense, right?
No matter the fuel source, a process called the Krebs Cycle generates energy for exercise. As muscle glycogen is depleted (via intense exercise, a low-carb diet, or both), the Krebs Cycle slows down. Advocates of high-fat diets often say that once muscle glycogen is gone, the body HAS to use fat for fuel, which is great because that helps you burn more fat. What they forget to mention is that ATP production from fat oxidation ALSO slows down when muscle glycogen gets depleted.
There’s a saying that “fat burns in a carbohydrate flame” because glycolysis (the oxidation of carbs) produces pyruvic acid, which increased Kreb-cycle intermediates, specifically oxaloacetate (OAA). Without OAA, the Krebs cycle slows down, which reduces ATP production, limits muscular performance and reduces energy expenditure overall.
In plain English, burning fat for fuel might be tapping directly into the stuff you want to get rid of, but it’s a much slower process overall. If you eat plenty of carbs and train hard, you might be burning a higher ratio of carbs to fats in the moment, but you’ll end up burning more fat overall due to increased overall calorie expenditure.
What’s more, it’s much harder to build muscle on a low-carbohydrate diet. First of all, lifting weights builds muscles, and lifting performance will suffer if you don’t eat many carbs. Next, carbs refuel muscle glycogen stores so you can RECOVER from lifting and train more often, leading to faster muscle growth. Finally, if you have a large amount of muscle mass, you’ll have a higher resting metabolism because muscle is “expensive” for your body to hold on to. And what’s your body’s favorite fuel at rest? You got it, FAT.
So if you eat more carbs and train hard, you’ll build more muscle and ultimately burn more fat when you’re sitting around doing nothing.
Easy to Overshoot Caloric Intake
You can’t cheat this physiological fact: in order to lose fat, you must eat fewer calories than you burn. It doesn’t matter if you eat low fat, low carb, vegan, carnivore, etc. If you don’t strike the right caloric balance, the weight won’t come off. That said, a high-fat diet makes it harder to stay in a caloric deficit because fatty foods are more calorically dense than foods rich in carbs or protein.
Every gram of fat you eat contains 9 calories, compared to just 4 calories for every gram of carbs or protein. This is why you can eat a whole head of broccoli (yes, broccoli is a carb!) and a couple handfuls of almonds (mostly fat and a little protein) and end up eating the same number of calories.
Because fatty foods are so dense, this drastically reduces the food volume (i.e., the actual physical amount of food) that you can eat. While some people can feel full and satisfied on a quarter of an avocado and a few slices of bacon, many people are more successful with their weight loss efforts when they can fill up on less calorically-dense food and eat to their heart’s (or in this case, stomach’s) content.
I generally hate fitness-related infographics, but this image from Precision Nutrition gets the point across. High-fat foods don’t take up much room in your stomach, making you want to eat more and making it more likely you’ll overshoot your calories.
Another hidden benefit of carb-rich foods like fruits and veggies: they’re high in both fiber and water content. This combination helps keep you feeling full for longer, which can reduce food cravings and ultimately help you eat fewer calories. So if anyone tells you fruit will make you fat, challenge them to a strawberry eating contest and see if either of you turns into Jabba the Hutt (spoiler alert: with strawberries clocking in at a whopping 40 calories per cup, you won’t).
As long as you account for total calories, you can crush carbs from all your favorite sources: veggies, fruits, pasta, rice, potatoes, etc. And remember, the carbs that are often vilified as unhealthy – things like potato chips, donuts, candy bars, etc. – are actually really high in fat too. So it’s not the carbs that are screwing you over, it’s the combination of foods that are high in carbs AND fat packaged in a way that makes them easy to overeat while you’re binge-watching The Office on Netflix for the hundredth time.
Decreased Mental Performance
Turns out your brain loves carbs just as much as your muscles. In fact, glucose is your brain’s preferred fuel. Remember, glucose can be derived from any of the macronutrients (carbs, fat or protein), but it’s most efficiently obtained from carbs.
In cases of prolonged carb deprivation, the body can enter ketosis, where fat is used for fuel because of the lack of available glucose. The body then increases production of ketones, which serve as an alternate energy source for the brain. This is the basis for the popular Keto diet.
Many low-carb advocates report “improved mental clarity” when eating a ketogenic diet. While ketogenic diets have been shown to have a positive effect on neurological disorders like epilepsy, there’s little evidence to show that ketosis actually improves cognitive function in humans.
Anecdotal mental enhancement can likely be chalked up to the placebo effect. Logically, less glucose available for the brain wouldn’t lead to improved brain function, but perhaps a more “even keel” feeling that could be interpreted as mental clarity. Or perhaps eating less processed shitty foods on a ketogenic diet just makes people think more clearly. This is 100 percent my own non-scientific opinion, but the physiology just doesn’t add up.
Eat Carbs, Kick Ass
There is nothing inherently magical about a low-carbohydrate diet. In fact, there are plenty of reasons to eat lots of carbs, especially if you care about your performance in athletics or in the gym. And ultimately, if you’re trying to lose weight, is a caloric deficit – NOT a lack of carbs – that’s going to help you lose weight. So before you go low carb, weigh the pros and cons.
