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