What are the downsides of a low-carb diet?

December 12, 2025 •

4 min read

According to a 2018 study, low-carbohydrate diets may be linked with an increased risk of early death, underscoring that considering what are the downsides of a low-carb diet is crucial. This popular eating approach, while beneficial for some short-term goals, can trigger a variety of negative health effects that many people overlook.

Quick Summary

Understand the health drawbacks linked to reducing carbohydrate intake, from initial side effects like 'keto flu' to serious long-term risks, including nutrient deficiencies, cardiovascular issues, and kidney problems.

Key Points

Keto Flu Symptoms: Initial fatigue, headaches, and nausea are common as the body adapts to burning fat instead of carbs.
Nutrient Deficiencies: Restricting food groups like grains and fruits can cause a lack of essential vitamins, minerals, and fiber, leading to health issues.
Cardiovascular Risks: High saturated fat intake, common in some versions of low-carb diets, can increase 'bad' LDL cholesterol and potentially raise heart disease risk.
Kidney and Bone Health: Very low-carb diets, particularly those high in animal protein, can increase the risk of kidney stones and contribute to bone mineral loss over time.
Mental and Social Challenges: The restrictive nature can cause mood swings, irritability, and social isolation, potentially leading to disordered eating.
Reduced Athletic Performance: A lack of carbohydrates can hinder high-intensity exercise capacity and reduce endurance.

Short-Term Side Effects: The 'Keto Flu'

When first transitioning to a low-carb or ketogenic diet, many people experience a cluster of flu-like symptoms, often called the 'keto flu'. This occurs as the body adapts to burning fat for fuel instead of its preferred carbohydrate source.

Symptoms typically include:

Fatigue and lethargy
Headaches and dizziness
Nausea and gastrointestinal distress
Irritability and mood swings
Muscle cramps due to electrolyte loss

These side effects are largely caused by dehydration and an electrolyte imbalance, as reduced carbohydrate intake causes the body to flush out water and key minerals like sodium, potassium, and magnesium. While these symptoms usually subside within a few weeks, proper hydration and electrolyte management are essential for mitigating their severity.

Potential for Nutritional Deficiencies

By restricting entire food groups like fruits, whole grains, and starchy vegetables, low-carb diets can lead to significant nutrient gaps over time. These foods are vital sources of essential vitamins and minerals, and eliminating them without careful planning poses a risk of deficiencies.

Inadequate Fiber Intake

Low-carb eating patterns often result in low dietary fiber intake, a nutrient crucial for digestive health. A lack of fiber can cause persistent constipation and may negatively affect the gut microbiome, which is linked to overall health and immunity. Research suggests that low-fiber diets can increase the risk of bowel cancer.

Deficiencies in Vitamins and Minerals

Depending on the specific foods restricted, low-carb dieters may miss out on key micronutrients. Studies have shown a link between low-carb diets and decreased intake of:

Vitamin C
Vitamin B1 (Thiamine)
Folate
Magnesium
Calcium
Iron
Iodine

Mental and Cognitive Health Impacts

Carbohydrates are the brain's preferred fuel source. A severe reduction can lead to notable cognitive and mood changes as the brain adapts to using ketones for energy.

Brain Fog: Many people report feeling mentally foggy or having trouble concentrating during the initial transition period.
Mood Instability: Some individuals experience increased irritability and mood swings. A study noted that people on long-term low-carb diets had less serotonin, a neurotransmitter important for mood regulation, than those on low-fat diets.
Social Isolation: Strict dietary restrictions can be challenging to maintain in social settings, potentially leading to feelings of isolation or disordered eating patterns. Food is a central part of many social gatherings, and avoiding it can strain relationships.

Long-Term Health Risks

For some individuals, especially when poorly executed, low-carb diets pose several long-term health risks.

Cardiovascular Concerns

While some studies show improved triglycerides and HDL cholesterol on low-carb diets, a focus on animal-based sources of fat and protein can lead to elevated LDL ('bad') cholesterol. A 2018 meta-analysis linked animal-based low-carb diets with a higher risk of cardiovascular death. This risk depends heavily on the quality and type of fat and protein consumed.

Increased Risk of Kidney Stones

Diets very low in carbohydrates and high in animal protein can increase the amount of acid and calcium excreted in the urine. This altered urine chemistry can increase the risk of developing kidney stones. Individuals with pre-existing kidney disease should be especially cautious and seek medical supervision.

Potential for Bone Weakening

Research indicates that ketosis, the metabolic state triggered by very low carb intake, can increase the risk of bone loss. Acidosis, a condition where bodily fluids have too much acid, can cause minerals to be leached from the bones. Some studies in athletes on the keto diet showed increased markers for bone breakdown.

Low-Carb vs. Balanced Diet: A Comparison


Feature	Restrictive Low-Carb Diet	Balanced, Whole-Food Diet
Carb Source	Severely limited; typically from non-starchy vegetables.	Variety of whole grains, fruits, vegetables, legumes.
Energy Source	Shifts to fat (ketones); potential for initial fatigue.	Primary energy source is glucose from complex carbs.
Fiber Intake	Often low, leading to constipation and gut health issues.	High fiber intake from diverse plant sources.
Micronutrients	Potential for deficiencies in key vitamins and minerals.	Rich in vitamins and minerals from varied foods.
Sustainability	Can be difficult to maintain long-term due to restrictions.	More flexible and easier to adhere to over time.
Mood	Potential for initial irritability and mood swings.	Supports steady energy and mood regulation.

Conclusion: Evaluating the Overall Impact

While low-carb diets can be an effective short-term strategy for weight loss and blood sugar management for some, they are not without significant drawbacks. The initial side effects of the 'keto flu,' the potential for nutrient deficiencies, and the long-term risks to heart, kidney, and bone health must all be seriously considered. The sustainability of such a restrictive diet is a major challenge, often leading to diet fatigue and potential weight regain. Furthermore, for certain populations, such as pregnant women or individuals with pre-existing health conditions, this diet can be dangerous. Ultimately, for most people, a balanced diet that prioritizes nutrient-dense, whole foods offers a more sustainable and less risky path to long-term health and wellness. Always consult a healthcare professional before making significant dietary changes. A balanced approach that focuses on consuming high-quality, fiber-rich carbohydrates from whole foods may be a better option for lasting health.

Harvard Health on the “Keto Flu”

Frequently Asked Questions

The 'keto flu' is a collection of flu-like symptoms, including fatigue, headache, and dizziness, that occurs during the initial days of a low-carb diet. It is primarily caused by dehydration and an imbalance of electrolytes as the body adjusts to burning fat instead of glucose for energy.

Yes, constipation is a common side effect of low-carb diets, especially very low-carb ones like keto. This is because the diet often restricts high-fiber foods such as whole grains, legumes, and fruits that are crucial for healthy bowel function.

Yes, some people experience mental and cognitive side effects, including brain fog, irritability, and mood swings. This is because the brain relies heavily on glucose, and the shift in energy source can temporarily disrupt its function.

Low-carb diets, particularly those high in saturated animal fats, can lead to elevated LDL ('bad') cholesterol, which is a risk factor for heart disease. Some studies have linked animal-based low-carb diets to higher rates of cardiovascular death.

Very low-carb, high-protein diets can increase the risk of kidney stones in susceptible individuals by changing urine chemistry. It is not recommended for people with pre-existing kidney disease without medical supervision.

Certain groups should avoid or use extreme caution with a low-carb diet, including pregnant or breastfeeding women, children, people with kidney or liver conditions, those with osteoporosis, and individuals with a history of eating disorders.

Yes, restricting or eliminating food groups rich in carbohydrates often results in deficiencies of essential nutrients, including fiber, calcium, magnesium, potassium, vitamin C, folate, and various B vitamins.

Beyond short-term symptoms, long-term risks may include an increased risk of heart disease, kidney stones, bone density loss, and a negative impact on gut health.