Everyone reading this is probably familiar with the food pyramid—the one that has grain and potatoes on the bottom, meat close to the top, and added fat and oil at the very top. Historically, doctors and the USDA have generally recommended a high-carb, low-fat diet, in line with the food pyramid. A low-carb diet, with high-fat intake, was not something you'd normally expect to hear your MD recommend. However, the food pyramid never had a very solid basis in science—it primarily reflected which industries spent the most money on lobbyists. And in the last few decades, several movements have arisen to push back against the high-carb dogma. In most cases they’ve ever tried to turn it on its head, advocating instead for a high-fat, low-carb approach to eating. Neither group is entirely correct, but there is an answer to the question of how much carbohydrates you should consume—it’s just a little more complicated than “a lot” or “very little.” Here’s what the research says about low-carb vs. high-carb diets.
The (Flawed) Basis for the Low-Carb vs. High-Carb Diet War
First, a quick primer on why this carb fight is happening in the first place. The rationale behind low-fat, high-carb eating is thus: fat has nine calories per gram, while carbohydrates and protein have four calories per gram. Also, dietary fat can be transported directly to your adipose (fatty) tissue and turned into body fat. Therefore, eating fat will make you fat much more easily than eating carbs.
This argument has a few flaws. While fat has more calories than carbohydrates, it’s also more satiating on a gram-for-gram basis. This is one of the virtues discussed by advocates of the low-carb diet. Plus, your body needs a fair amount of fat for both structural and hormonal purposes. And of course, while fat can be deposited directly to your adipose tissue, it’s still going to be burned for energy if you’re in a caloric deficit.
As for low-carb diets, the rationale behind them is that eating carbs causes your body to produce insulin. Insulin is a storage hormone—it causes your body to store calories as fat. Carbs, the argument goes, also raise your blood sugar, and they are absorbed faster than fat, so a high-carb diet will lead to chronically elevated blood sugar and insulin levels, wild swings in blood sugar, and concomitant swings in energy level as well as excessive hunger.
This has its own problems. Namely, protein also raises insulin. And insulin actually suppresses appetite via its effects on the brain. And while eating carbs acutely raises blood sugar and insulin levels, a high-carb diet doesn’t necessarily lead to chronically elevated blood sugar and insulin. Most importantly, insulin isn’t bad. While it is a storage hormone, it causes all body tissues—including muscle and other lean tissue, not just fat—to store energy. And it only signals your body to store energy—there still needs to be energy for your body to store.
The Truth: It’s Mostly about the Calories
The thing is, regardless of the hormonal nuances of the two dietary approaches, your body still has to obey the laws of physics—specifically, conservation of mass-energy. Studies have consistently shown that, on average, when calorie and protein intake are equated, low-fat and low-carb diets produce similar amounts of weight loss.
Now, in practice, calories and protein aren’t always equated. In studies that don’t carefully equate calorie and protein intake between the two groups, the low-carb group sometimes gets better results due to higher protein intake. On the other hand, either diet can outperform the other as long as you eat less—groundbreaking, we know. But there are a couple of other big, big caveats here.
First, the thermic effect of food is highest in meals that contain a mix of protein, fat, and carbohydrates. That is, mixed meals boost your metabolism more than meals that leave out one of the three macronutrients. This doesn’t mean you need to eat a “balanced” diet—5–10 grams of fat and 10–20 grams of carbs are enough to more or less maximize the thermic effect of food—but it is a strong argument against completely avoiding either fat or carbohydrates.
Second—and here’s the big one—low fat compared to a low-carb diet are only equally effective on average. That doesn’t mean that everyone will respond equally well to both diets—they won’t.
The Catch: It’s Also about Your Insulin Sensitivity
Obesity is often—though not always—accompanied by metabolic syndrome—a set of symptoms including insulin resistance, chronically elevated blood sugar, and high cholesterol. Several studies have shown that people with metabolic syndrome do better on low-carb diets, while those without metabolic syndrome lose more weight on low-fat diets.
Other studies have found insulin sensitivity to be a strong predictor of which diet people will do best with. Insulin-sensitive subjects lose more weight, and feel better, on high-carb diets, while insulin-resistant subjects get better results from carbohydrate-restricted diets.
How to Eat Based on Your Carb Tolerance
This isn’t an excuse to just pick whichever diet you prefer—unfortunately, your subjective taste in food is not a good predictor of which diet will give you better results, and people who follow a prescribed diet get better results than people who eat whatever they want.
Instead, you should eat the foods your body responds to best. You can determine your insulin sensitivity via blood tests, but this can be expensive and time-consuming, as it requires multiple tests to get a good read. Luckily, there’s an easier way.
Whether it's a low-carb diet or a high-carb diet or something in between, the best diet for you is the one that makes you feel best after a meal (not necessarily during it). It will give you more energy, and more stable energy, and will leave you feeling fuller for longer. So you can figure out your ideal diet with a simple self-experiment. Over the course of two weeks, cycle through eating high-carb, low-fat; low-carb, high-fat; and moderate-carb, moderate-fat meals. Following each meal, monitor your energy level, mood, and hunger every hour. The category of meal that leaves you feeling energized and satiated for hours afterward is the kind of meal you should be eating.
Most people fall somewhere in the middle, and will find no clear advantage to either approach—they do best with moderate-carb, moderate-fat meals. But a large minority will feel better at one extreme or the other. To truly optimize your diet, you need to spend some time experimenting on yourself.