What is a “carb leak”?

If you start a low-carb diet, but it does not seem to be working nearly as well as you had hoped, one reason could be a “carb leak” in your diet. The effect of a carb leak is that you are taking in many more grams of carbs per day than you believe, and therefore you are not getting all of the benefits of a low-carb diet.

Carb leaks are much easier if you do not keep a food diary. A food diary, if you keep an accurate one, will tell you exactly how many grams of carbs you are eating each day. Therefore you will be able to see a carb leak immediately. But even if you do keep a food diary, you can record things incorrectly and create a carb leak without realizing it.

The goal of a low-carb diet is to keep total grams of carbs per day below some upper limit. A person on a strict keto diet, for example, will try to stay below 20 grams of carbs per day. I find that if I stay below 50 grams of carbs per day, I am OK. Your body will have some threshold of its own, depending on your size, level of obesity, natural metabolic rate, your amount of daily exercise, etc. You will hear people in low-carb circles talk about being “in ketosis” – by staying below your threshold, you stay “in ketosis”.

So where do carb leaks come from? If you are not keeping a food diary, and you are mentally trying to “not eat high-carb foods”, you might mentally believe that “fruit is OK”, or “milk is OK”. It turns out that these beliefs are incorrect. A big glass of milk contains a lot of carbs (in the form of lactose), and a banana does too (like 30 grams of carbs in one banana).

If you are keeping a food diary, you would understand that milk and bananas are problematic because they would stand out like sore thumbs in your daily stats.

The way to create a carb leak while keeping a food diary is to keep an inaccurate diary. Let’s say that you are eating vanilla yogurt every day, but you are recording it as plain yogurt because in your mind “vanilla” is “plain”. It turns out that vanilla yogurt in the store comes loaded with sugar, and is therefore high-carb. Yougurt that is truly plain had no sugar added, so it is low-carb. It pays to read labels and make sure the food you are eating really is low in carbs. Another way to create a carb leak is to guesstimate weights or volumes rather than actually measuring them. even plain yogurt can bring in many grams of carbs if you eat lots of it without recording things accurately. To take an extreme case, if you are eating a 32-ounce container of plain yogurt but recording it as a cup of plain yogurt, you have a carb leak.

If you want to experiment with it, use a food diary (obviously) and be very meticulously accurate in recording what you eat. Try to go for a week where you strictly stay below 20 grams of carbs per day. See how your weight loss goes. Then try 30, then try 40, and so on. I know that once I get much up above 50 grams a day, my body starts storing water. It is obvious in my daily weigh ins.

So, if you are trying to have a low-carb diet, and it does not seem to be working, make sure there are eliminating all of the carb leaks. Also, keep an accurate food diary – record what you are actually eating in the amounts you are actually eating. If you have an accurate food diary, you cannot have a carb leak.