What Happens to Your Body When You Reduce Carbs?

January 3, 2026 •

3 min read

According to a 2021 study in the Journal of Nutrition, a low-carb diet can prompt rapid metabolic changes, signaling a shift in how your body sources energy. So, what happens to your body when you reduce carbs, and how does this change affect your overall health?

Quick Summary

When you reduce carbs, your body transitions from using glucose to burning stored fat for fuel, a process known as ketosis. This shift can lead to initial water weight loss, changes in energy levels, appetite control, and potentially unpleasant side effects often called the 'keto flu'.

Key Points

Initial Water Weight Loss: The body first sheds stored glycogen and its accompanying water, causing rapid but temporary weight reduction in the first week.
'Keto Flu' Symptoms: As your body adapts to using fat for fuel, you may experience temporary fatigue, headaches, and irritability.
Metabolic Shift to Ketosis: Reducing carbs prompts the liver to produce ketones from fat, shifting your body's primary energy source from glucose to fat.
Improved Appetite Control: Higher protein and fat intake associated with low-carb diets can increase feelings of fullness and reduce overall calorie consumption.
Potential Nutrient Deficiencies: Eliminating carb-rich food groups can lead to a lack of fiber, vitamins, and minerals, necessitating careful food choices.
Long-Term Health Risks: Extended low-carb diets may increase LDL cholesterol and pose risks like kidney stones, emphasizing the need for medical supervision.

Initial Phase: The "Carb Flu" and Water Weight

When you first significantly reduce your carbohydrate intake, your body will experience a noticeable transition period. As the primary and most accessible energy source, carbs provide glucose that is stored as glycogen in your muscles and liver. Glycogen also binds to water, so when your body uses up these glycogen stores, it releases a significant amount of water. This is why many people on low-carb diets experience rapid initial weight loss, which is mostly water weight rather than fat.

During this initial adjustment, many people report flu-like symptoms, commonly known as the “keto flu”. Symptoms can include headaches, fatigue, brain fog, nausea, and irritability. These effects are a result of your body's metabolic shift and can last for a few days to a couple of weeks. Staying hydrated and replenishing electrolytes, such as sodium, potassium, and magnesium, is key to managing these symptoms.

Metabolic Shift: From Glucose to Fat

After the initial phase, your body begins to adapt to its new fuel source. With limited glucose available, the liver starts converting fat into ketone bodies, which are used as energy by the brain and muscles. This state is called ketosis and is the primary metabolic mechanism behind the weight loss often seen on very low-carb diets.

Fat burning: Your body becomes more efficient at burning stored fat for energy, which can lead to a decrease in body fat over time.
Insulin sensitivity: Lowering your carb intake can significantly reduce blood sugar and insulin levels, which improves insulin sensitivity and reduces the risk of type 2 diabetes.
Appetite control: Increased intake of protein and fat, which often replaces carbs, can lead to a greater sense of fullness and reduced appetite, helping to control overall calorie intake.

Digestive and Nutritional Considerations

Reducing carbs, especially if it means cutting out whole grains, legumes, and fruits, can lead to a significant drop in dietary fiber. This can result in constipation or other digestive issues. To counter this, it's crucial to consume plenty of non-starchy, high-fiber vegetables, nuts, and seeds. Focusing on nutrient-dense, whole foods helps prevent deficiencies in vitamins and minerals that can be missed by eliminating certain food groups.

Comparison of Low-Carb vs. Keto Diet


Feature	Low-Carb Diet	Keto Diet
Daily Carb Intake	50–150 grams	Under 50 grams (very restrictive)
Protein Intake	Moderate to High	Moderate (to avoid inhibiting ketosis)
Fat Intake	Moderate to High	Very High (primary fuel source)
Metabolic State	May or may not enter ketosis	Induces a state of ketosis
Sustainabilty	Generally easier to follow long-term due to more food options	More restrictive and challenging for long-term adherence

Potential Long-Term Effects

While many people experience short-term benefits, the long-term effects of a severely low-carb diet are still under investigation. Some studies raise concerns about potential long-term risks, including increased LDL ('bad') cholesterol, kidney stones, and bone density issues. For many, a balanced approach that includes healthy, complex carbs is more sustainable and beneficial for overall health. A registered dietitian can provide personalized guidance to ensure nutritional needs are met while achieving health goals.

Conclusion: Navigating the Change

Reducing carbs can profoundly change your body, impacting your metabolism, energy levels, and even mental clarity. While the initial phase may present challenges like the "keto flu," the potential benefits of improved blood sugar control and fat burning are significant. However, it's essential to approach this change mindfully, focusing on whole, nutrient-dense foods to prevent nutritional deficiencies. For many, a moderately low-carb approach is more sustainable and less restrictive than a very low-carb ketogenic diet, offering a balanced path to better health. Consulting a healthcare professional before making drastic dietary changes is always recommended to ensure it aligns with your individual health needs and goals.

Additional Resource

For a deeper dive into the science behind low-carbohydrate diets and their metabolic effects, you can read more via the National Institutes of Health: Metabolic Effects of the Very-Low-Carbohydrate Diets.

Frequently Asked Questions

The 'keto flu' is a collection of flu-like symptoms, including fatigue, headaches, and nausea, that can occur during the first few days of significantly reducing carb intake. It is caused by the body's adjustment to using fat instead of glucose for energy and typically resolves within a week or two.

Yes, many people experience initial weight loss when reducing carbs, largely due to shedding water weight. Over time, as your body becomes more efficient at burning fat for energy, fat loss can also occur, though results vary by individual.

No. While both limit carbs, a keto diet is a more extreme form, restricting daily carbs to typically under 50 grams to induce ketosis. A standard low-carb diet is less restrictive, often allowing 50–150 grams of carbs daily.

To avoid constipation, increase your intake of fiber from low-carb sources. Focus on non-starchy vegetables like leafy greens, broccoli, and cauliflower, and incorporate nuts and seeds into your diet.

Potential long-term concerns include increased LDL cholesterol, bone density issues, and kidney problems. These risks are more prevalent in very restrictive diets and emphasize the need for professional medical guidance.

During the initial phase, you may experience a dip in energy as your body transitions from glucose to fat for fuel. Once keto-adapted, many people report more stable, sustained energy levels throughout the day without the peaks and crashes associated with high-carb intake.

Focus on sources of healthy unsaturated fats like avocados, nuts, seeds, and olive oil, while limiting saturated fats found in fatty meats and high-fat dairy. For protein, opt for lean meats, fish, eggs, and plant-based options.