What happens when you eat a very low-fat diet? The surprising health risks

October 20, 2025 •

4 min read

Over the years, fat has been unfairly demonized, but it's an essential macronutrient for numerous bodily functions, including energy production and hormone synthesis. Answering the question, "What happens when you eat a very low-fat diet?" reveals that while some fat reduction can be beneficial, an overly restrictive intake can lead to a host of negative health consequences.

Quick Summary

Explore the adverse health effects of a diet with severely restricted fat intake, including nutrient malabsorption, hormonal disruption, compromised mental clarity, and negative impacts on heart health.

Key Points

Nutrient Deficiency: A very low-fat diet can prevent the absorption of essential fat-soluble vitamins A, D, E, and K, leading to a range of health issues.
Hormonal Disruption: Inadequate fat intake can cause imbalances in sex hormones, affecting menstrual cycles in women and testosterone levels in men.
Compromised Brain Function: With nearly 60% of the brain being fat, a very low-fat diet can lead to 'brain fog', mental fatigue, and mood swings.
Negative Lipid Profile: Counterintuitively, restricting all fat can lower protective HDL cholesterol and raise triglycerides, negatively impacting heart health.
Satiety Problems: Fat promotes a feeling of fullness, so a very low-fat diet can leave you constantly hungry, potentially leading to increased consumption of unhealthy processed carbs and sugars.
Long-Term Sustainability Issues: Severely restrictive low-fat diets are often difficult to maintain due to blandness and persistent cravings, making them unsustainable for long-term health.

For decades, low-fat diets were promoted as the key to weight loss and heart health. However, modern nutritional science paints a more complex picture, revealing that healthy fats are indispensable for optimal health. A very low-fat diet, typically defined as one where less than 15-20% of daily calories come from fat, can trigger a cascade of issues that negatively impact the body and mind.

Impaired Nutrient Absorption and Deficiencies

One of the most immediate and significant consequences of a very low-fat diet is the body's inability to properly absorb crucial nutrients.

Fat-Soluble Vitamin Deficiencies: Vitamins A, D, E, and K are fat-soluble, meaning they require dietary fat for proper absorption and transport throughout the body. A deficiency in these vitamins can lead to a range of issues:
- Vitamin A: Can cause vision problems, including night blindness, and weaken the immune system.
- Vitamin D: Essential for bone health, deficiency can lead to soft bones (osteomalacia) and increase fracture risk.
- Vitamin E: A powerful antioxidant, its deficiency can cause nerve damage and muscle weakness.
- Vitamin K: Crucial for blood clotting, a lack can lead to excessive bleeding and easy bruising.
Essential Fatty Acid Deficiency: The body cannot produce essential fatty acids like omega-3 (alpha-linolenic acid) and omega-6 (linoleic acid) and must obtain them from food. These fats are critical for cellular function, brain health, and managing inflammation. Symptoms of deficiency include dry, scaly skin, hair loss, and reduced immune function.

Hormonal Imbalances and Reproductive Issues

Dietary fat plays a foundational role in the production of many hormones, including sex hormones like estrogen and testosterone.

Impact on Sex Hormones: Very low-fat diets can lead to a drop in sex hormone levels, which can cause significant problems. In women, this can lead to irregular menstrual cycles or even amenorrhea, disrupting fertility. In men, it can decrease testosterone, which affects mood, muscle mass, and energy levels.
Effects on Metabolism: Fat intake influences thyroid function and the body's overall metabolic rate. Severely restricting fat can slow metabolism and compromise the production of essential hormones, hindering weight management and overall vitality.

Poor Heart Health, Despite the Intent

Paradoxically, attempting to reduce heart disease risk by cutting out all fat can sometimes be counterproductive, particularly when focusing only on total fat rather than type.

Dysregulated Cholesterol: While some saturated and trans fats are detrimental, very low-fat diets can negatively affect cholesterol profiles. Restricting all fat can decrease beneficial HDL cholesterol and increase triglycerides, both risk factors for cardiovascular disease. Moderate-fat diets, especially those rich in monounsaturated and polyunsaturated fats, have been shown to improve these markers more effectively.
Inflammation: Healthy omega-3 fatty acids have potent anti-inflammatory effects. A diet devoid of these fats can lead to increased systemic inflammation, a key contributor to many chronic diseases.

Cognitive Decline and Mood Changes

The brain is composed of nearly 60% fat, making it highly dependent on a steady supply of healthy fatty acids to function properly.

Brain Fog and Reduced Focus: Deficiencies in omega-3s, which are vital for brain cell membranes, can manifest as mental fatigue, poor concentration, and memory issues.
Mood and Mental Health: Healthy fat intake is linked to mental well-being. Low consumption can increase the risk of mood imbalances, irritability, and depression due to its impact on neurotransmitters and brain function.

Comparison: Moderate Fat vs. Very Low Fat Diet


Feature	Moderate Fat Diet (25-35% of calories)	Very Low-Fat Diet (Under 20% of calories)
Effect on HDL ('good') Cholesterol	Maintains or increases levels.	Can significantly decrease levels.
Effect on Triglycerides	Tends to keep levels low.	Can increase levels, especially if high in refined carbs.
Nutrient Absorption	Supports optimal absorption of fat-soluble vitamins (A, D, E, K).	Hinders the absorption of fat-soluble vitamins, leading to deficiencies.
Hormonal Health	Supports balanced hormone production.	Can disrupt hormone production, affecting fertility and metabolism.
Satiety and Cravings	Increases feeling of fullness, reducing cravings.	Can lead to constant hunger and increased cravings for sugar/carbs.
Brain Function	Provides essential fatty acids for cognitive health.	Can lead to 'brain fog' and mood imbalances.

The Unintended Consequences of Extreme Restriction

Beyond the specific physiological effects, a very low-fat diet comes with significant lifestyle drawbacks. Fat adds flavor and texture to food, and eliminating it can make meals feel unsatisfying and bland, making long-term adherence difficult. Many people compensate for the lack of fat by consuming excess processed foods high in refined sugars and carbohydrates, which can lead to weight gain and metabolic issues. This was a common trend in the 90s when fat-free foods were popular, only to later be linked to an obesity crisis. Focusing on consuming the right types of fat is far more important than attempting to eliminate all fat.

Conclusion: Seeking Balance Over Restriction

Ultimately, a very low-fat diet is not a sustainable or healthy strategy for long-term well-being. While a moderate reduction in unhealthy fats can be beneficial, severe restriction can do more harm than good, impacting everything from hormonal balance and cognitive function to nutrient absorption and heart health. A balanced nutritional approach that emphasizes healthy fats from sources like avocados, nuts, seeds, and fatty fish is far more effective for maintaining optimal health and vitality. Learn more about the benefits of healthy fats on your brain.

Frequently Asked Questions

A very low-fat diet can reduce your levels of HDL ('good') cholesterol and increase your triglyceride levels, which can increase your risk of heart disease.

Yes, severely restricting fat intake can disrupt hormone production, particularly sex hormones like estrogen and testosterone, which can lead to reproductive issues and mood changes.

Since the brain is largely composed of fat, a very low-fat diet can cause cognitive issues like brain fog, reduced memory, and mood imbalances due to a lack of essential fatty acids.

Fat-soluble vitamins are A, D, E, and K. They are crucial for vision, bone health, antioxidant protection, and blood clotting, and they require dietary fat for absorption.

While it can lead to short-term weight loss, often initially due to water weight, very low-fat diets are not proven to be more effective than other diets for long-term weight management. The rebound effect can be common due to persistent hunger.

Good sources of healthy, unsaturated fats include avocados, nuts, seeds, fatty fish (like salmon and mackerel), and olive oil.

Yes, research indicates that a moderate-fat diet, focusing on healthy fats, often results in a more favorable lipid profile, better satiety, and improved overall health outcomes compared to a severely restrictive low-fat diet.