By: Kade Gilbert
I will cut straight to the chase here and give you the quick answer: no, carbs will not make you gain body fat.
Fortunately, we have plenty of real-world anecdotes as well as evidence-based data to back this claim.
Carbohydrates, when eaten in excess, are rarely converted to and stored as fat. This process is called de novo lipogenesis (DNL). DNL rarely occurs under normal circumstances and is energetically inefficient (2). Instead, the body more readily stores the dietary fat you eat as body fat, regardless of how many carbs you eat. In fact, in several overfeeding studies overfeeding people carbohydrates or fat, ~90-95% of overfed fat was stored, vs 70-75% of overfed carbohydrates (1).
You may have heard that if you increase your fat intake, you increase your ability to oxidize stored body fat. This is true to an extent. However, excess dietary fat that isn’t burned off through digestion and metabolic processes will end up being stored as body fat. So, any increase in oxidation you experience tends to have a null effect.
Think of it like this: if you are burning 10 units of fat per hour at baseline and consume 100 units of fat and burn 50 units, you have increased your fat oxidation by 40 units but are now storing 50 units.
Now, back to carbohydrates.
The following is an example of the effects of DNL when overeating carbs (3).
Researchers overfed individuals by 175% of their estimated maintenance calories for a period of four days. These individuals were fed about 750 grams of carbs every day for four days. After this period of overfeeding, DNL increased by 296%. For an individual weighing 80kg (176lb), this amounts to about 48 grams (0.12lb) of fat gained.
In this same study, the control group only gained 12 grams (0.03lb) of fat through DNL. I again want to reiterate that the maintenance calories were estimated, which in itself could amount to the fat that was gained. Anyone that has spent a week tracking knows that it is possible to make a tracking error that can amount to more than 100 calories. Then we also have to take into account label inaccuracies. The FDA allows up to a 20% margin of error on nutrition labels (4). For example, an item listed as having 200kcal could actually have up to 240kcal. Now, compound that across multiple items and you’ll see my point. Given the possibility of tracking inaccuracies through labels or human error, it would be foolish to worry about fat being gained from eating carbs.
If we were to follow the calculations given in this study for the control group. Smaller individuals whose maintenance calories are around 1500, would have eaten about 185g of carbs per day. Larger individuals with a maintenance of around 2500 would require an intake of about 310g carbs per day.
One of the more common statements we get from those new to tracking once they receive their macros from the calculator is: “this many carbs will make me fat.” If they’ve entered their information correctly, our response will likely be, “No, this amount of carbs will not make you fat.” This conclusion is based on decades of hard data studying this exact question, so you are highly unlikely to have a relatively small increase in carbohydrates cause you to gain fat.