The Truth About When “Carbs Turn to Fat”

July 11, 2011

in All About Carbs, Nutrition

Excess carbs get turned into fat.

It’s true.

But it’s nothing to worry about. Here is why.

You may have heard that excess carbs are converted to fat or will be stored as fat (implying conversion from carb to fat first). Usually this idea is discussed when talking about weight gain. Your uncle Peter will tell you that whatever carbs your body can’t use right away, or store in muscle, will get turned to fat. He heard it at the gym.

I saw a recent statement from an “authoritative” health and fitness site that read:

“These are all refined grains that lack nutrients but are loaded with rapidly digested carbohydrates that can lead to weight gain or prevent weight loss”.

Without proper context, it would be easy to come away thinking that carbs are readily turned to fat. Many people will read a statement like this, recall Uncle Peter telling them that carbs get turned to fat, and end up making a connection that barely exists. Very few people have the knowledge to carefully analyze the information and understand that it is misleading.

The Reality:

The conversion of carbs to fat, called de novo lipogenesis (DNL), is found throughout the animal kingdom but happens to be relatively rare in humans. It can happen under two conditions. First, under extreme conditions of severe carbohydrate overfeeding. A tribe in Cameroon has a “fattening” tradition where adolescent boys are fed 3-4 times the calories that they would normally eat, with most of the extra calories coming from carbs. A study of this tradition found that the boys gained an average of 12kg of body fat over 10 weeks while eating only 4 grams of fat. In this situation, the body converted carbs directly to fat in response to massive quantities of carbs being ingested. Keep in mind, the chance of this happening under normal circumstances is slim to none.

It’s very rare that anyone will eat 3-4 times their normal amount of calories. All these nightmare dietary scenarios that terrify us into thinking that too many carbs will make us fat are just that, nightmare scenarios. They just don’t happen under normal dietary conditions.

A Couple Studies

A 2001 study appearing in the American Journal for Clinical Nutrition compared de novo lipogenesis in lean subjects and obese subjects. The details of what occurred aren’t that important because the conclusion of the research was that although rates of lipogenesis increase in both lean and obese people due to over-feeding (by 50%) the rate of de novo lipogenesis is relatively small to the point of being insignificant.

In a 1999 review of de novo lipogenesis in humans, the researcher observed that only when carbohydrate intake exceeded daily caloric expenditure would DNL occur to any significant degree. If your daily caloric expenditure is 2,000 calories, you’d have to eat at least 2,000 carb calories alone before any DNL took place–and even then the DNL response would be very tiny. No normal person would eat that many carb calories unless you figured out a way to mainline them into your veins. You don’t eat carbs alone; you eat meals which consist of carbs, protein, and fat. Eating 2,000 carb calories as well as normal amounts of protein and fat would push your daily caloric intake way past the 2,000 that you burn. You’ll get fat even without de novo lipogenesis.

The Carb/Fat relationship

Your body can store around 500 grams of carbs in your muscles and liver as glycogen. A small amount is always present in the bloodstream as blood sugar (glucose). If those stores are full, where do the carbs go? The fear is that the body will have to convert the extra carbs into fat for storage. But what studies actually show is that the body responds by simply burning off more of the carbs for energy and lipogenesis rates are low or insignificant. When the carbs are burned off, the body goes back to burning fat for its main fuel source.

When people get fat, it happens in one of two ways. If their extra calories are coming in the form of carbs, the body spends so much time burning the carbs that body fat doesn’t get burned off. Fat stores increase. You get fat indirectly.

If their extra calories are coming from fat, dietary fat is getting stored in fat cells faster than it can be burned off, so, again, fat stores increase. You get fat directly from storing too much dietary fat.

Either way, you get fat, and the key is the extra calories.

If you are in energy balance, there is absolutely nothing to worry about. What carbs come in get burned off. It’s only when you start to gain weight that the possibility of carb to fat conversion could occur. Even then, the only way that this process occurs to any significant degree is in response to massive over-feeding of carbs.

The other condition is which the conversion of carbs to fat can occur is in times of severe dietary fat restriction. If dietary fat drops below 10% of your calories, the body will convert carbs to fat to maintain fat stores. Beyond being an interesting tidbit of information, it’s not something to spend much time thinking about.

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