Over the past decade, carbs have received a lot of blame as the driver of America’s obesity epidemic. Why? In 1977, the US government issued “Low Fat Guidelines” to combat the rise in obesity. (1) Obesity rates went on to more than double over the next twenty years, proving that fat wasn't the culprit. (2)

That left the other two macronutrients - protein and carbs. Since higher protein intake is associated with weight loss, that left only carbs. (3) Now, carbs need a new PR agent because their reputation has taken a hit in recent years. Is that hit deserved? Yes and no, here's why.

Weight is ultimately determined by the number of calories you consume. Calories are a unit of energy and carbs contain 4 calories per gram, which means consuming 25 grams of carbs is consuming 100 calories.

A 2018 Stanford study confirmed that the number of calories you consume is more important than whether a diet is low-carb or low-fat. (4) Researchers randomly divided 600 people into two groups: "healthy low-carb" or "healthy low-fat". Over the next 12 months, participants ate according to their group's diet with both groups consuming a similar amount of calories. What were the results? There was no significant difference in weight loss between the two groups. Meaning the people on the low-fat diet lost just as much weight as the people on the low-carb diet.

Many times academic studies have conflicts of interest with how they're funded, particularly studies that determine consumer spending habits. For example, the Twinkie diet made headlines when a researcher lost 27 pounds eating only Twinkies, Doritos, and other refined snacks to prove that sugar doesn't cause obesity. (5) It was revealed later that he had been funded by Coca-Cola. (6)

The Stanford study has no such funding conflicts. It is only a single study but it is a thorough, recent, and well-designed study that had statistically significant results. Those results challenge the idea that low-carb diets are superior and conclude that calories matter most. But, it's not that simple.  

TL;DR

Weight loss or gain is a product of calories consumed. A thorough 2018 study showed no difference in weight loss between low-carb and low-fat groups.

Not All Carbs Are Created Equal

Looking at the results from the Stanford study, it's reasonable to conclude that low-fat and low-carb diets are equally effective. That's why the below chart makes things a lot more confusing.

Each set of bars is a study - those blue bars are weight that was lost on a low-carb diet and the gray bars are for low-fat diets. The blue bars are longer in every study except one. That means twenty out of twenty-one studies showed greater weight loss for low-carb diets compared to low-fat diets. (7) How can this be true when the Stanford study concluded that weight loss is the same for low-carb and low-fat diets? We have an answer.

In the Stanford study, both groups were instructed to do something unique. Both groups were told to "(1) maximize vegetable intake; 2) minimize intake of added sugars, refined flours, and trans fats; and (3) focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible." (8) In other words - they didn't eat refined carbs.

Those twenty-one studies in the chart did not restrict refined carbs. Why does that matter? For two reasons.

First, refined carbs are the least satiating food you can eat. Satiety is how full a food makes you feel after eating it. Researchers examined a number of foods to determine how satiating they were and found the least satiating foods to be croissants, cakes, donuts, and candy bars. (9) Notice anything? They’re all calorie-dense, refined carbs. Carbs from whole-food sources are the opposite. They have more nutrition, fiber, and are not hyperpalatable (very easy to eat), all which makes them much more filling than their refined counterparts. (10) In fact, the most satiating food on the planet is the white potato. Take a look at the chart below (blue bars are what you want to look at).

The second reason it's important to differentiate between whole and refined carbs is because refined carbs have high Glycemic Loads. Glycemic Load (GL) measures the impact that eating a certain food will have on your blood sugar and is more accurate than Glycemic Index because it accounts for the quantity of food. (11) The lower the number, the lower the impact. Low is less than 10, medium is 10-20, and high is anything over 20. Almost all refined carbs have a GL of 15 or higher, which results in blood sugar spikes. (12) When blood sugar spikes, the body responds by secreting insulin - the “fat-storing hormone” - which opens the doors to your cells and stores the sugar there. (13)

Compared to "whole" carbs, refined carbs are much less filling and cause a larger insulin response. Because of this, dieters who consume refined carbs will tend to consume more calories and store more fat. In the Stanford study, dieters consumed "whole" carbs so both low-fat and low-carb groups experienced the same level of weight loss. In the other studies, dieters were allowed to consume refined carbs which is why the low-carb groups experienced more weight-loss. All carbs are not created equal and, generally speaking, the more refined they are, the worse they are for you.

TL;DR

Not all carbs are the same. Refined carbs cause larger spikes in blood sugar and are less satiating than "whole" carbs. That tends to lead to more fat storage and overconsumption.

What Should You Do?

The original question that we started with was "do carbs make you gain weight?" The answer to which is nuanced. Overconsumption of anything will cause weight gain — even protein. Refined carbs are very easy to overconsume because they're not filling and they're hyperpalatable. They're also cheap and they're everywhere. They cause weight gain for many people and are the main driver of obesity in the United States.

Having said that, one or two cookies after dinner will not ruin you. Minimizing refined carbs will be best for your overall health, but it's up to you to decide how you want to balance that with your enjoyment of life.

TL;DR

Refined carbs should be minimized for optimal health. Balance with occasional deviations to maximize health and happiness.