What is a side effect of a high protein, low carbohydrate diet?

January 1, 2026 •

4 min read

According to the Mayo Clinic, a high-protein diet that restricts carbohydrates can cause constipation, bad breath, and headaches. Understanding what is a side effect of a high protein, low carbohydrate diet is key to navigating this eating plan successfully and mitigating unwanted symptoms.

Quick Summary

Common side effects of a high protein, low carb diet include digestive issues like constipation, headaches, and unpleasant 'keto breath,' often stemming from changes in fiber, hydration, and metabolism.

Key Points

Constipation: A common side effect is constipation, primarily caused by low dietary fiber intake, often from restricting carbohydrates.
Keto Breath: The metabolic shift to ketosis can cause a temporary bad breath with a metallic or fruity odor due to the excretion of ketones.
Dehydration Risk: Increased protein metabolism requires more water, so inadequate fluid intake can lead to dehydration and worsen constipation.
Kidney Strain: A very high protein intake can place added stress on the kidneys, which is a particular concern for individuals with pre-existing kidney conditions.
Nutritional Deficiencies: Restricting carbohydrate sources can lead to a lack of essential vitamins, minerals, and fiber found in fruits, vegetables, and whole grains.
Fatigue: Initial carbohydrate restriction can cause temporary fatigue, headaches, and brain fog, sometimes referred to as the 'keto flu'.

Digestive Issues: The Link Between Fiber and Constipation

One of the most frequently reported and inconvenient side effects of a high protein, low carbohydrate diet is constipation. The primary reason for this is a significant reduction in dietary fiber intake. Many people following this diet replace fiber-rich foods like whole grains, fruits, and starchy vegetables with animal-based proteins and fats, which contain no fiber. Fiber is essential for adding bulk to stool and facilitating its movement through the intestines. Without adequate fiber, stool can become hard and difficult to pass, leading to irregularity and discomfort.

The Importance of Hydration for Digestion

Coupled with low fiber, dehydration is another factor that can exacerbate constipation. The body requires more water to metabolize and excrete the excess nitrogen from high protein intake. If fluid consumption does not increase to match this demand, the colon absorbs more water from the waste, resulting in dry, compact stools. This emphasizes that fiber needs sufficient water to function effectively, as insoluble fiber in particular needs fluid to move bulk through the digestive tract.

The “Keto Flu” and Changes in Breath

For those on a very low carbohydrate diet, the body transitions into a metabolic state called ketosis, where it burns fat for fuel instead of glucose. This can trigger a set of temporary symptoms, often dubbed the "keto flu," which can include headaches, fatigue, brain fog, and irritability, particularly during mentally or physically demanding tasks.

A more specific and potentially embarrassing side effect of ketosis is "keto breath." When fat is broken down, it produces ketones, including acetone, which is released from the body through breath and urination. This can cause a metallic, fruity, or nail-polish-remover-like odor. This effect is temporary as the body adapts, and strategies like increasing water intake or slightly adjusting macronutrient ratios can help manage it.

Potential Long-Term Health Risks

While high-protein, low-carb diets are often effective for short-term weight loss and blood sugar management, potential long-term risks require consideration. The prolonged consumption of high levels of protein can put an increased load on the kidneys, which are responsible for filtering protein waste products. People with pre-existing kidney disease or compromised kidney function are particularly at risk and should avoid this diet unless medically supervised.

Another concern, particularly with diets high in animal protein and saturated fats, is the risk to cardiovascular health. Some studies suggest a link between high animal protein intake and an increased risk of heart disease, though research is mixed and depends heavily on the specific food sources. Choosing lean proteins and prioritizing plant-based sources can help mitigate this risk.

The Risk of Nutritional Deficiencies

By restricting carbohydrates, especially nutritious sources like fruits, legumes, and whole grains, followers of this diet risk missing out on essential nutrients. Foods rich in carbs are often excellent sources of fiber, vitamins (like B vitamins), and minerals. A lack of these nutrients can negatively impact overall health and well-being.

Comparison: High-Protein, Low-Carb vs. Balanced Diet


Feature	High-Protein, Low-Carb Diet	Balanced Diet (USDA guidelines)
Primary Energy Source	Fat (ketones) and Protein	Carbohydrates (glucose)
Fiber Intake	Often low, leading to constipation	Adequate intake supports digestion
Kidney Strain	Potentially increased, especially with high animal protein	Minimal risk for individuals with healthy kidneys
Micronutrient Variety	May be lacking in certain vitamins and minerals	Rich and varied, covering all nutrient bases
Satiety	High satiety due to protein	Balanced satiety from all macronutrients

Managing Side Effects and Creating a Sustainable Plan

It is possible to follow a high-protein, low-carbohydrate eating plan without accepting these negative side effects as inevitable. A thoughtful and well-planned approach is key. To avoid constipation, ensure you are including non-starchy, fiber-rich vegetables like broccoli, spinach, and cauliflower in your meals. Choosing plant-based protein sources such as lentils, chickpeas, and tofu can also boost fiber intake. Proper hydration is non-negotiable; aim for at least 2-3 liters of water daily to support digestion and kidney function.

Conclusion

While a high protein, low carbohydrate diet can be an effective tool for weight management and blood sugar control, it carries potential side effects that should not be ignored. Common issues like constipation, dehydration, and 'keto breath' can arise from a lack of fiber, insufficient hydration, and metabolic changes. More serious long-term concerns regarding kidney health and nutrient deficiencies also exist. The key to mitigating these problems lies in careful planning, ensuring a balance of high-fiber vegetables, lean protein sources, and consistent hydration. For anyone considering a significant dietary change, it is always recommended to consult with a healthcare professional or registered dietitian to ensure the plan is safe and appropriate for their individual health needs.

For more in-depth information on dietary fiber, consult the European Food Safety Authority guidelines on dietary reference values.

Frequently Asked Questions

This diet often leads to constipation because it significantly reduces the intake of dietary fiber, which is found in carbohydrates like whole grains, fruits, and legumes. Fiber adds bulk to stool and helps regulate bowel movements.

'Keto breath' is a temporary, unpleasant breath odor caused by the metabolic state of ketosis. As your body breaks down fat, it produces ketones like acetone, which are released through your breath. To help, increase your water intake, practice good oral hygiene, or chew sugar-free gum and mints.

For individuals with healthy kidneys, a high-protein diet is generally considered safe. However, for those with pre-existing kidney disease, a high protein intake can place extra strain on the kidneys and should be avoided or carefully monitored by a doctor.

To prevent constipation, increase your intake of high-fiber, low-carb vegetables (like broccoli and spinach), include plant-based proteins (lentils, chickpeas), and ensure you are drinking at least 2-3 liters of water daily.

Initial fatigue is common as your body adapts to using fat for fuel instead of carbohydrates. Staying hydrated, ensuring adequate electrolyte intake, and being patient can help mitigate these symptoms, which typically subside as your body becomes fat-adapted.

Potential long-term risks include increased kidney stress and, depending on food sources, a higher risk of heart disease due to high saturated fat intake. It's crucial to consult a healthcare provider and choose lean, high-quality protein sources.

To ensure adequate nutrient intake, focus on a wide variety of nutrient-dense, non-starchy vegetables, incorporate seeds and nuts, and consider supplementing with any nutrients that may be lacking due to restricted food groups.