Reviewed by Brunilda Nazario, MD on July 27, 2021
What Does Cutting Carbs Do?

What Does Cutting Carbs Do?

1/13

Carbs are a basic nutrient your body turns into glucose, or blood sugar, to make energy for your body to work. A very low-carb diet, like keto and the early phase of the Atkins Diet, triggers your body into nutritional ketosis. Your liver starts to make ketones -- a fuel that kicks in when your body doesn’t have enough sugar to run on -- by breaking down fat. 

How Low Is Low-Carb?

How Low Is Low-Carb?

2/13

To bring about nutritional ketosis, extreme low-carb diets cap your carb intake at less than 10% of your total macronutrient (carbs, fat, and protein) intake. That translates to 20 to 50 grams a day of carbs. Low-carb diets generally shoot for under 26% of nutrition intake, or 130 grams. Here are a few examples of common high-carb low-fat foods.

You’ll Lose Water Weight

You’ll Lose Water Weight

3/13

A sudden lack of carbs will make you lose weight. It’s mostly water weight at first, though. This is mostly because cutting carbs also wipes out the glycogen stores in your muscles. Glycogen helps your body retain water. You may also lose some salt along with the carbs you cut out. When you start to eat carbs again, the water weight comes right back. It takes 2 to 3 weeks for ketosis to rev up and start to burn fat.

You Might Get the “Keto Flu”

You Might Get the “Keto Flu”

4/13

Ketosis can lead to weakness, fatigue, dizziness, and headaches, which can feel a lot like flu symptoms. More serious side effects can happen too, like stomach pain, nausea, and vomiting. Some doctors think this is caused by not getting enough nutrients from fruits, beans, veggies, and whole grains -- foods not allowed, or strictly limited, on a keto diet.

You May Have Brain Fog

You May Have Brain Fog

5/13

You may feel foggy as your body tries to keep up normal blood sugar levels. It might be hard to sleep at first. You might feel very tired, which will make your brain feel even fuzzier for a while. 

Your Belly Might Bloat

Your Belly Might Bloat

6/13

Low-carb diets are also low on fiber. Constipation can strike, although it usually clears up in a few weeks. You can get some fiber from watery fruit like watermelon. Also, gas can get trapped in your digestive tract. Your stomach might feel too full and hurt. If you stay well-hydrated and get enough electrolytes (minerals that help balance your body water and nourish your cells), your symptoms might not be as serious or last as long. 

Your Breath Might Smell Weird

Your Breath Might Smell Weird

7/13

When your body runs on fatty acids instead of carbs, it releases  ketones through your breath as acetone. Your breath might smell fruity or sweet. Some say it tastes like decaying apple. Also, if your mouth is dry, you can have bad breath. That’s because there’s not enough saliva to get rid of bacteria and extra food particles in your mouth. So stay hydrated.

Your Blood Sugar Levels Might Dip

Your Blood Sugar Levels Might Dip

8/13

A super low-carb diet can lower your blood sugar levels. This can help if you have diabetes. But in true ketosis, hypoglycemia is a risk. This happens when your blood sugar dips too low. The go-to treatment is to have 15 grams of carbs to raise it. If it’s still too low after 15 minutes, you’ll need another 15 grams. If you have diabetes, check your sugars often and know that you may need to adjust your meds while on this diet. 

It Can Tax Your Heart

It Can Tax Your Heart

9/13

Very low-carb diets also are high-fat. Your saturated fat intake should be no more than 5% to 6% of your total. Focus on healthy fats like those in avocados, olives, and nuts. On the keto diet (as well as less strict low-carb diets like Atkins and Paleo), your triglycerides and HDL (“good”) cholesterol levels will likely get better. But you may see a rise in LDL, or “bad” cholesterol, which can also lead to heart disease. 

You Might Feel Moody

You Might Feel Moody

10/13

Your state of mind can get wonky when you sharply cut healthy carbs that send sugar to the brain. You might feel grouchy. One study found people who followed low-carb diets over time had less serotonin in their brains than those who were on low-fat diets. Healthy serotonin levels help guard against anxiety and depression. 

Your Liver May Struggle

Your Liver May Struggle

11/13

On a low-carb diet, your liver has a lot more fat to process. This can make an existing condition worse. 

Kidney Problems Can Worsen

Kidney Problems Can Worsen

12/13

Nutritional ketosis may bump up uric acid levels, leading to  kidney stones or gout flares. One study found people with mild chronic kidney disease on the keto diet did OK with close medical supervision. But others show those who eat any diet high in red meat and low in whole grains, low-fat dairy, and fruit are 97% more likely to get kidney disease. This is partly why some dietitians advise against doing extreme low-carb diets on your own.

It Can Help Control Seizures

It Can Help Control Seizures

13/13

People have used the keto diet for some 100 years to treat epilepsy, mainly in children who don’t respond to meds. It means that someone will have to watch you and do frequent lab checks and urine tests. Studies go on as to how the keto diet might also help people with brain disorders like Parkinson’s disease. There aren’t any firm results yet, though.

Show Sources

IMAGES PROVIDED BY:
1)    Watcom / Getty Images
2)    Jasmin Merdan / Getty Images
3)    fabrycs / Getty Images
4)    Roy Hsu / Getty Images
5)    Ghislain & Marie David de Lossy / Getty Images
6)    MangoStar_Studio / Getty Images
7)    tioloco / Getty Images
8)    Oscar Wong / Getty Images
9)    magicmine / Getty Images
10)    Westend61 / Getty Images
11)    SEBASTIAN KAULITZKI / Science Source
12)    Dr_Microbe / Getty Images
13)    libre de droit / Getty Images


SOURCES: 

Cleveland Clinic: “Carbohydrates,” 
Medline Plus: “Blood Sugar,” “Fluid and Electrolyte Balance.”
Harvard Health Publishing: “Ketogenic Diet: Is the Ultimate Low-Carb Diet Good for You?” “Should You Try the Keto Diet?”
Sarah Simental, MS, RD, CNSC, CSPCC, dietitian, Los Angeles.
Diabetes Teaching Center at the University of California, San Francisco: “Ketones.”
StatPearls Publishing: “Low Carbohydrate Diet,” “Ketogenic Diet.”
Arizona State University College of Health Solutions: “The Keto Diet: Is Eating More Fat the Key to Weight Loss?”
Mayo Clinic: “Is the Keto Diet For You? A Mayo Expert Weighs In,” “Dietary Fiber: Essential for a Healthy Diet,” “Hypoglycemia.”
Northwestern Medicine: “Pros and Cons of the Ketogenic Diet,” “How to Beat the Bloat,” “Surprising Causes of Bad Breath.”
American Academy of Family Physicians: “What Is Bloating?”
Journal of Breath Research: “Breath acetone as a potential marker in clinical practice.”
American Diabetes Association: “Hypoglycemia (Low Blood Sugar).”
Journal of the Endocrine Society: “A Case of Hypoglycemia Associated With the Ketogenic Diet and Alcohol Use.”
American Heart Association: “Saturated Fat,” “The American Heart Association Diet and Lifestyle Recommendations.”
Archives of Internal Medicine: “Long-term Effects of a Very Low-Carbohydrate Diet and a Low-Fat Diet on Mood and Cognitive Function.”
Nutrients: “Very Low-Calorie Ketogenic Diet: A Safe and Effective Tool for Weight Loss in Patients with Obesity and Mild Kidney Failure.”
National Kidney Foundation: “The Right Food Can Help Fight Kidney Disease.”