What Really Happens to Your Body When You Drastically Cut Carbs

Very low-carbohydrate diets — hello, keto — have become so popular, most people have either tried them or know someone else who has. While the long-term risks and benefits of these diets are the subject of ongoing debate, nixing grains, dairy and fruit has a significant effect on your body and overall health.

Your body doesn't only need energy for movement and exercise. It uses energy constantly — to digest food, breathe, think, maintain its temperature, sustain hormone levels and keep the heart pumping. So, to say the least, it's essential to have a constant supply of energy. And carbohydrates — glucose (sugar), specifically — is the body's primary source of energy.

So how does the body use carbs for energy? It transforms them into glucose, which is the easiest and most accessible fuel source, says Yasi Ansari, RD and certified specialist in sports dietetics. The body also maintains a supply of stored glucose — known as glycogen — in the muscles and liver. The average person stores around 2,400 calories from glycogen, according to an April 2018 article in ‌Nutrition Reviews‌, and that fuel supply can be drastically depleted within 1 to 2 days if someone nixes carbs.

But the human body is resourceful, and when carb intake is limited, there are other ways to get the energy it needs.

When you slash your carb intake to below 50 grams per day or about 5 to 10 percent of total calories coming from carbs — a level common in the keto diet or in the early phases of the Atkins diet — changes occur in the way the body produces energy. And in some situations, this can provide therapeutic value. In fact, since the 1920s, the ketogenic diet has been used as a treatment for epilepsy, since the metabolic changes associated with a very low-carb intake can reduce or prevent seizures, according to November 2008 research in ‌Epilepsia‌.

In the short term, eating very low-carb leads to lower levels of glucose and insulin, along with increased production of glucagon, a hormone that helps the body break down glycogen for energy.

As glycogen stores run low, this triggers a shift in the body's energy production, causing processes known as gluconeogenesis and ketogenesis, according to a March 2019 report in ‌StatPearls‌.

Cue the sciencey stuff: Gluconeogenesis is a process in which glucose is created from the backbone of fat molecules known as triglycerides, and from amino acids, which are the building blocks of protein. Ketogenesis occurs when the liver transforms stored fat into ketone bodies, which the body also uses for energy. If your diet is consistently and extremely low in carbs, your body enters a state of ketosis, where ketone bodies supply most of your needed energy.

In simple terms, these processes prompt the body to use fat as the primary fuel. (Yep, that's the whole premise behind the ketogenic diet.)

But if you add carbs back into your diet during a refeed, the muscle and liver glycogen stores get built back up pretty quickly, which means that your body will revert back to using that glycogen in your muscles and liver rather than body fat.

Most people who follow the keto diet for weight loss aim to keep their body in a mild state of ketosis. However, folks following a keto diet to control seizures will require a higher level of ketosis and will need to maintain a diet with even fewer carbs, according to a June 2020 ‌Cochrane Review‌.

But ketosis is not always the desired state, as it can be a hallmark of unhealthy conditions, such as starvation, unintentional low food intake related to depression or health conditions or the severely restricted food intake associated with some eating disorders.

And people following a very low-carb diet for weight loss aren't necessarily immune to its side effects. For instance, the "keto flu" is a typical complaint among keto dieters, according to the Academy of Nutrition and Dietetics.

"The keto flu is a collection of symptoms that some people experience when they first enter ketosis, which happens as a result of the fast excretion of sodium and fluids during carbohydrate restriction," Malina Malkani, RDN and creator of the Wholitarian Lifestyle. This happens when ketones enter your bloodstream — a result of your body burning fat for fuel. Symptoms typically include poor energy and mental function, headache, sleep issues, nausea, digestive discomfort and decreased exercise performance. "These arise during what is commonly known as the 'shock period' of a low-carb diet," adds Ansari.

But symptoms from ketosis can vary from person to person. While some people may feel fatigued, others may simply notice bad breath from the excess ketone bodies in the blood. Some people may also experience dehydration and muscle cramps when going super low-carb, since glycogen stores run low and glycogen is stored with water. And while dehydration will surely make you feel thirsty, it also comes with a slurry of side effects like dizziness, heart palpitations or feeling faint.

You may have heard all the hype around the keto diet's cognitive benefits — and there's some evidence that links low-carb, high-fat diets to brain-boosting effects. Elevated ketone levels in the blood was observed to improve working memory, visual attention and the ability to switch between tasks in healthy older adults, per a small October 2016 study published in ‌Psychopharmacology‌.

What's more, a state of ketosis may also be beneficial for people with Alzheimer's disease. In a study of 152 people with Alzheimer's, those who added a ketone-producing MCT supplement to their diet for 90 days had better cognitive performance than the control group, according to the August 2009 ‌Nutrition & Metabolism‌ study.

While the study mimicked a low-carb, high-fat diet by giving the participants an MCT supplement rather than restricting them to a low-carb diet, the study suggests that the ketones may be responsible for the beneficial cognitive effects — and therefore, these effects may be replicated with a low-carb diet.

Recent reviews regarding the keto diet's effect on dementia and cognitive function — including one published by the ‌Journal of Geriatric Psychiatry and Neurology‌ in January 2020 and another published in November 2022 by the international journal ‌Nutritional Neuroscience‌ — seem to suggest possible benefits. However, although preliminary data are promising, large confirmatory studies are needed.

The attractive feature of a low-carb diet — weight loss — can stem from several processes. Initially, you're likely to experience rapid weight loss due to the fluid loss that comes with glycogen depletion. Not to mention, restricting the variety of foods you eat can also cause you to eat fewer calories, which may lead to weight loss, Malkani adds.

Keto dieters also often report reduced cravings and appetite, which may suggest that the diet interferes with hunger hormones. And although more research is needed to clarify the complex relationship between ketosis and these hormones, a small study published in the May 2013 issue of ‌European Journal of Clinical Nutrition‌ showed that participants who lost weight on the keto diet had reduced levels of several appetite hormones, including the "hunger hormone" ghrelin.

Low-carb diets are known for their not-so-pretty effects on the gastrointestinal (GI) system. And while the mechanisms of how these diets affect our intestinal bacteria isn't fully understood, it remains an important question for researchers, since the gut microbiome plays a significant role in immunity and health.

Very low-carb diets are usually deficient in fiber (a type of carb), which you can find in fruits, certain vegetables, legumes and whole grains — foods known to be beneficial for promoting a healthy gut microbiome, according to a September 2015 article in ‌Gut‌. Low-carb diets also tend to be high in fat, and a study published in the February 2019 issue of ‌Gut‌ linked high-fat diets to unfavorable effects on the gut microbiome, such as changes that are associated with increased inflammation and increased risk of getting heart disease.

That's why keto dieters sometimes complain of constipation, according to the Academy of Nutrition and Dietetics, which can be a result of dehydration and not getting enough fiber.

If you're still set on going low-carb, there's a healthy way to do it. Make sure your body gets the nutrients it needs to function properly by following a few simple steps.

• ‌Eliminate processed carbs.‌ You don't have to go full-blown keto to lose weight. Instead, focus on cutting out processed carbs such as refined grains and added sugars. This will not only slash your empty calorie intake, but will also leave more room in your diet for healthy, fiber-rich eats.
  
  
• ‌Focus on veggies.‌ Keto dieters are at risk for missing some important nutrients simply because food choices can be so limited. Very low-carb diets can be deficient in many vitamins and minerals, as noted in a November 2008 study published in ‌Epilepsia‌, which found that the lowest carb plan provided adequate amounts of only 3 of the 28 vitamins and minerals studied. Vitamins keto dieters fell short on included vitamin C and folate, which are found in many plant foods.
  
  "Omitted foods such as fruits, beans, whole grains and starchy vegetables like peas, yam and corn provide valuable nutrition to the body in the form of vitamins, minerals, phytochemicals and antioxidants that support overall health," Malkani says.

‌Read more:‌ Everything You Need to Know to Start a Plant-Based Keto Diet

‌A medical note for people with diabetes:‌ Ketosis is not the same thing as diabetic ketoacidosis (DKA). According to the American Diabetes Association, DKA is a life-threatening complication that is mostly a risk in people with type 1 diabetes.

DKA occurs when there is very little or no insulin in the blood, which prevents the body using glucose for energy. The body shifts to producing more fat for fuel and making ketone bodies, which in the absence of insulin can build up quickly, and lead to high levels of ketones and acid in the blood. A person with DKA requires prompt medical treatment.