How Your Body Runs without Carbs: Low or No Carbs? (Part 1 of 2)

How Your Body Runs without Carbs: Low or No Carbs? (Part 1 of 2)

Carbohydrates provide energy for your body and brain. A diet that is high in quick-digesting carbs blocks weight loss. So, how do you strike a balance between providing your body and brain with energy while allowing weight loss to happen? I’ll answer that question in this first part of our two-part series on how your body runs without carbs.

How Your Body Runs without Carbs [Video]

In this post, you’ll learn…

  • What insulin and glucose do for the body.
  • What happens to your body when you restrict carbs.
  • How to make sure your body and brain have enough energy on a low-carb diet.

Do you need carbs to survive?

The Dietary Reference Intakes published by the National Academies Press states that “The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed.” (1)

In essence, what they are saying is that you do not need to eat carbs to survive as long as you are taking in a sufficient amount of protein and fat.  

carbohydrates are not essential

How does your body run without carbohydrates? 

Let’s take a look at what happens inside of you when you cut carbs.

Your digestive tract breaks down the carbs you eat into simple sugar molecules called glucose. The amount of glucose in your blood affects two important hormones that influence your metabolism; they are insulin and glucagon. 

What is insulin?

Insulin is a fat-storing hormone, so it is necessary to keep insulin levels low by cutting carbs. This low insulin state triggers the secretion of glucagon.

Glucagon can be thought of as the exact opposite of insulin. Its job is to raise your blood sugar. It does this by stimulating glycogenolysis, which is a term that literally means the glycogen break down. 

What is glycogen?

Glycogen is the storage form of glucose. It is basically just glucose molecules bonded together.

You have about 100 grams or 400 calories worth of glucose stored in your liver.

When you stop eating carbs, your liver breaks down its glycogen stores and sends the free glucose molecules out into your bloodstream in order to keep your blood glucose at an acceptable level. 

Water Weight: It’s worth mentioning that glycogen holds a lot of water in your body. This explains why low-carb dieters have an initial drop in water weight: you stop eating carbs, glycogen stores get depleted, and water is excreted.

It also explains why low-carb dieters that have a “cheat meal” that is high in carbs will see a temporary, but seemingly unfair weight gain when they step on the scale the next morning. The carbs replenished your glycogen stores and water retention causes a spike in water weight. 

The process of gluconeogenesis.

We just learned that your liver can break down and release stored glucose when you first start your low-carb diet.

If you continue to restrict carbs, your liver ramps up another process called gluconeogenesis, which literally means ‘making new glucose from non-carbohydrate substrates or particles’.

In other words, even when you don’t eat carbs, your body has some pretty amazing mechanisms to ensure that you maintain an adequate level of glucose in your blood.

Your body makes glucose when you don't eat carbs

Some of you might think that this is a silly or frustrating thing for your body to do.

After all, you are cutting carbs, so you can keep glucose and insulin low. Your body seems to be working against your efforts by making brand new glucose.

As it turns out, our bodies are much smarter than us. You always need some glucose in your body because some of your cells must have it.

What happens when you continue to restrict carbs?

We now know that your liver can free stored glucose and it can make new glucose.

If you are taking in too few dietary carbs, however, your body needs another form of energy to properly run your body and brain. This is when your body switches fat-burning into high gear. 

Fat is stored in your fat cells as triglycerides. It is basically three fatty acids attached to a glycerol backbone.

fatty acid break down

Enzymes cleave the fatty acids off of the backbone. The free fatty acids go into your bloodstream where your tissues can now grab them and take them in and use them directly for energy.

Your heart and skeletal muscles are two tissues that run very well on fatty acids. But, not all of your cells can use fatty acids directly for energy.

For instance, your red blood cells cannot use fatty acids because they do not have mitochondria, which are the tiny powerhouses of your cells that convert these more complex fatty acids into energy.

Red blood cells, however, can run on glucose, which is a very simple molecule. Fueling your red blood cells is one of the reasons that your liver continually makes glucose even when you are not eating carbs or you’re fasting. 

Your brain on a low-carb diet

The brain is another example of an organ that runs on glucose, but cannot run directly on free fatty acids because the fatty acid molecules are too big to get through your blood-brain barrier

Your brain is a big energy user and obviously very important, so your body works very hard to keep it supplied with enough energy.

This is a balancing act when you reduce carbs. If you’re following a well-formulated low-carb diet, you are taking in the right mix of carbs, fat, and protein to meet the needs of your body and brain.

Many people will find that a low-carb diet allows them to lose weight at a nice steady and consistent pace.

low-carb diet macros

Ketones are Energy

When glucose is insufficient, your liver makes ketone bodies from the breakdown products of fatty acid oxidation.

There is no clear line that your body draws when it comes to making ketones. If you’ve been on a low-carb, but not keto diet for a period, it is not unusual for your body to be making ketones as they are needed.

I am an example of this. I follow a low-carb diet that hovers between 50 to 100 grams of carbs per day. My typical diet is not keto, but it is not unusual for me to test my blood after intermittent fasting or a particularly low-carb day and see that I have ketones circulating in my blood. 

The Ketogenic Diet

Some people will find that their bodies will not burn fat efficiently unless they drastically reduce the number of carbs that they are eating, which drops them into the ketogenic range.

A ketogenic diet is a very-low-carb diet. So low, in fact, that your liver may not be able to make enough new glucose to properly feed your brain.

Luckily, your brain is a hybrid organ and runs well on two fuels: glucose and ketones, which are both small enough to cross over the blood-brain barrier. 

Going Carnivore?

If fewer carbs are beneficial, would the no-carb Carnivore diet be the best diet? I will share my insights on that topic in next week’s post.

If you’re interested in trying a low-carb or keto diet to see if it will work for you, I encourage you to take one of my 21-Day Challenges. Thank you for reading!

Dr. Becky's 21-Day Challenge

Reference: 

(1) Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press, 2005.

One thought on “How Your Body Runs without Carbs: Low or No Carbs? (Part 1 of 2)

  1. I’m a 60 year old pharmacist and enjoy your YouTube channel. My question is about carbohydrate needs during exercise of greater than 1 hour. I have done 2 half marathons and have followed the standard advice to supplement carbs/electrolytes during runs longer than 1 hour so long training runs and during the race itself. I am currently following your low carb eating guidelines and find it easy to follow with using an eating window of 1100 to 2000 daily. I’m planning to start trading again soon for a 10K in Nov but also am trying to loose 20 lb. Are the carbs need for exercise of long duration?

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