How Low-Carb Diets Can Affect Your Blood Sugar Levels

Low-carb diets — including meals with plenty of protein and veggies — may cause low blood sugar.
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Low-carb diets can help people with diabetes avoid high blood sugar and improve their overall health, according to the 2019 Standards of Medical Care in Diabetes from the American Diabetes Association (ADA).


But low-carb diets aren't for everyone. They can can be very hard to follow, and many people who start them find they can't stick with these restrictive eating plans for more than a few days.

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Here's what you need to know about how low-carb diets affect blood sugar before adopting a new eating plan.

Read more: 4 Diet Tips for Dealing With Hypoglycemia

Carbs and Your Blood Sugar

First, a refresher: When you eat, your body coverts the carbohydrates in food into glucose, aka blood sugar, which your cells use for energy. When you change to a low-carb diet, however, the body needs to get most of its fuel from another source. For the first few days, your body will use stored glucose. But when that supply runs out, you start to burn fat for energy.


In the process of breaking breaks down fat, your body creates ketones. Ketones become a central energy source in very low-carb diets. In fact, the ketogenic diet, or keto diet (which is extremely low-carb), aims to keep the body in a continued state of ketosis, where the body uses only fat for fuel, according to Harvard Health Publishing.

The Centers for Disease Control and Prevention (CDC) recommends that 45 percent of your daily caloric intake come from carbs if you have diabetes. Based on a 2,000-calorie diet, that means 900 daily calories should come from carbs (about 225 grams per day).

People with diabetes struggle with high blood sugar because their bodies either can't use or don't have enough insulin, a hormone critical in lowering blood glucose levels. Low-carb diets help people with diabetes easily reduce the amount of glucose available to the body. Weight loss can be an added benefit of low-carb diets for people with type 2 diabetes, who often need to manage their weight as part of their diabetes care.


Per the ADA, a low-carb diet is one in which 25 to 45 percent of your daily calories come from carbs. By contrast, in very low-carb diets, fewer than 25 percent of your calories come from carbs. Some very low-carb plans even recommended getting just 25 to 50 grams per day in order for your body to break down fat for energy.

Read more: What Is a Healthy Blood Sugar Reading in the Morning?



Craft Your Low-Carb Diet Carefully

However, Jo-Anne Rizzotto, RD, director of educational services at Harvard's Joslin Diabetes Center, urges caution for people with diabetes starting low-carb diets. "If you want to go on a low-carb diet, it's really important to talk with your diabetes care team, especially if you're taking blood sugar-lowering medications like insulin," she tells The reason? If the body isn't getting very many carbs, certain diabetes medications could lower blood sugar levels too far, causing hypoglycemia.


If you take medication for diabetes, your doctor may direct you to adjust your dose when following a low-carb diet, according to the University of Iowa. It'll be important for you to monitor your blood sugar levels closely. If you discover that your blood sugar has dropped too low, you can then correct it with a small, high-carb snack or glass of juice.


It's important to distinguish ketosis from the similar-sounding diabetic ketoacidosis (DKA). DKA is a rare but life-threatening condition usually associated with extreme high blood sugar in people with type 1 diabetes. Though ketosis and DKA both involve the breakdown of fats into ketones, only DKA is toxic to the body. The reason for this difference is not yet understood, according to a December 2018 study in Neural Regeneration Research.

Low-Carb Diet Side Effects

Whenever the body transitions to using fat for energy, it can cause what is known as keto flu. The most common symptoms, according to a March 2018 study published in PeerJ, include:


  • Headache
  • Bad breath
  • Fuzzy thinking
  • Mood swings
  • Weakness
  • Muscle cramps
  • Skin rash
  • Constipation or diarrhea

Per the PeerJ study, these symptoms are actually the result of low sodium and potassium levels: Lowering your carb intake lowers your insulin levels, which can in turn cause excess urination that flushes too much of these vital nutrients out of your system.


Read more: How Long Does It Take for Blood Sugar Levels to Peak After Eating?

Headaches and/or changes in mood or cognitive function are also some of the symptoms of low blood sugar, and they're the body's way of signaling that it wants more glucose, according to the Mayo Clinic. In general, these symptoms go away in the first few days of starting a low-carb diet.

If you suddenly reduce your carb intake drastically, you are more likely to experience adverse side effects, according to the Mayo Clinic. Lowering your carbs gradually will reduce the severity of adverse symptoms, including those associated with low blood sugar.


A low-carb regimen "is not recommended at this time for women who are pregnant or lactating, people with or at risk for disordered eating or people who have renal disease, and should be used with caution in patients taking the diabetes medicine called an SGLT2 inhibitor," according to the ADA's Standards of Medical Care in Diabetes.