Why doesn't low carb work for everyone?

November 27, 2025 •

5 min read

Recent studies reveal significant individual variation in metabolic response to low-carbohydrate diets, suggesting that genetics, hormones, and other factors determine success. Understanding why doesn't low carb work for everyone involves moving beyond a one-size-fits-all approach and examining the complex interplay of biological and psychological variables that influence outcomes.

Quick Summary

This article explores the multiple reasons low-carb diets fail for some individuals, including genetic predispositions, hormonal imbalances, and psychological challenges. It outlines the importance of metabolic individuality and quality whole foods for successful dieting.

Key Points

Genetic Predisposition: Your DNA influences how your body processes carbs and fats, meaning low carb diets work better for some genetic profiles than others.
Hormonal Imbalances: Low carb diets can disrupt hormone levels, including thyroid hormones (T3) and cortisol, which can slow metabolism and increase stress.
Metabolic Individuality: A person's insulin sensitivity or other pre-existing metabolic conditions can affect their response, with some even seeing worse insulin metabolism on low carb.
Psychological Hurdles: For many, the social restrictions and mental fatigue associated with strict low-carb eating lead to burnout and poor adherence.
Performance and Energy: Athletes engaging in high-intensity exercise often require more carbs for optimal performance and recovery than a strict low-carb diet allows.
Micronutrient Deficiencies: Excluding major food groups can lead to a lack of key vitamins and minerals like iron, magnesium, and folate, impacting overall health.
Whole Foods over 'Dirty Keto': Processed low-carb products can sabotage a diet's benefits. Focusing on whole foods is crucial for nutritional adequacy.

The Biological Complexities of Low-Carb Diets

For many, low-carbohydrate diets, such as the ketogenic or paleo approaches, promise rapid weight loss and improved metabolic health. However, a significant number of individuals find themselves hitting a plateau or feeling worse than when they started. The notion that 'one diet fits all' is a myth, as our bodies respond differently based on a complex web of factors.

Genetic Influences on Diet Response

Your DNA plays a pivotal role in how your body metabolizes food and responds to dietary changes. Research in nutrigenomics shows that certain genetic markers can predict how effectively a person processes different macronutrients.

FTO Gene: Variations in the fat mass and obesity-associated (FTO) gene can influence an individual's fat metabolism and appetite regulation. Some individuals with certain FTO variants may experience greater weight loss on a low-carb diet, while others are less affected.
APOE Gene: The apolipoprotein E (APOE) gene, which affects lipid metabolism, also plays a role. Those with the APOE4 allele, for example, may have an increased risk of elevated cholesterol on high-fat, low-carb diets, making it a poor choice for their cardiovascular health.

Genetic testing can provide personalized insights, revealing why one person might thrive on a low-carb, high-fat diet while another struggles with negative side effects and poor results.

Hormonal and Metabolic Factors

Drastic carbohydrate restriction can trigger a cascade of hormonal and metabolic changes that can undermine weight loss efforts for certain people. The body's intricate systems are designed for balance, and upsetting this balance can have unintended consequences.

Thyroid Function: For some, prolonged, very low-carbohydrate intake can lower levels of T3, the active thyroid hormone, as T4 conversion to T3 can slow down. This metabolic slowdown can lead to increased fatigue, weight gain, and general sluggishness, counteracting the diet's intended effects. Active women, in particular, may be more sensitive to these thyroid fluctuations.
Cortisol and Stress: A very low-carb diet can mimic a state of fasting or stress, leading to elevated levels of the stress hormone cortisol. Chronically high cortisol can encourage fat storage, particularly around the midsection, and exacerbate feelings of anxiety and fatigue.
Insulin Resistance: While low-carb diets are often used to address insulin resistance, some individuals, especially those with pre-existing low insulin levels, may not respond positively. In some cases, low carb diets can even worsen insulin metabolism, leading to issues with blood sugar regulation.

The Psychological and Social Hurdles

Beyond the biological, psychological and social factors heavily influence a person's ability to sustain any restrictive diet long-term. Willpower is often less important than the diet's compatibility with an individual's lifestyle and mental state.

Sustainability and Adherence: For many, the strict nature of very low-carb diets is simply unsustainable. The diet requires meticulous planning, especially in social settings or when traveling. This can lead to a 'weekday warrior' mentality, where strict adherence during the week is undone by weekend binges, stalling progress.
Mood and Energy Levels: As the brain's primary fuel source, glucose availability affects cognitive function and mood. While adaptation to ketosis can provide stable energy for some, others experience fatigue, brain fog, and irritability, commonly known as the 'keto flu'. Moderate carbohydrate intake is sometimes linked to better mood and lower depression risk, especially in women.
Social Isolation and Food Relationships: Following a restrictive eating plan can lead to social isolation, anxiety around food, and a negative relationship with eating. When a diet becomes an obsession rather than a healthy choice, it can lead to psychological distress.

Low Carb vs. Higher Carb: A Comparison

When considering a low-carb approach, it's useful to compare its potential benefits and drawbacks with a more moderate or higher-carbohydrate diet, especially concerning specific health outcomes and lifestyle factors.


Feature	Low-Carbohydrate Approach	Moderate-to-High Carbohydrate Approach
Best for...	Insulin-resistant individuals, those with metabolic syndrome, some types of athletes.	Endurance athletes, insulin-sensitive individuals, those with thyroid issues.
Performance Impact	Can compromise high-intensity exercise, though long-term adaptation is possible for endurance.	Optimal for fueling high-intensity training and rapid muscle glycogen replenishment.
Fat Oxidation	Can significantly increase the body's ability to burn fat for fuel, even at higher exercise intensities.	More reliant on carbohydrates for energy, with lower fat oxidation rates during exercise.
Dietary Quality	Requires a strict focus on whole, nutrient-dense foods to avoid deficiencies.	Supports a wider range of food groups, making it easier to meet fiber and micronutrient needs.
Adherence & Mood	Higher adherence rates for some, especially insulin-resistant individuals, but can cause mood issues for others.	Often easier to sustain long-term and may positively influence mood for certain individuals.

Making It Work: Personalizing Your Approach

Instead of abandoning a low-carb diet entirely, many find success by personalizing the approach to their individual needs and goals. This often involves adopting a more moderate strategy, focusing on diet quality, and listening to your body.

Prioritize Food Quality: Focus on whole, nutrient-dense, and minimally processed foods, including healthy fats, lean proteins, and low-carb vegetables. Avoid processed low-carb replacements that may lack nutritional value.
Periodize Your Carbs: For athletes or active individuals, matching carbohydrate intake to training intensity can optimize performance and recovery. Consume more carbs on heavy training days and less on rest days.
Include Complex Carbohydrates: For those with hormonal sensitivities, especially women, including moderate amounts of whole-food carbohydrates like root vegetables and berries can support hormone balance and thyroid function.
Focus on Lifestyle Factors: Don't rely solely on diet. Stress management, adequate sleep, and regular exercise are crucial for hormonal balance and metabolic health. Poor sleep and high stress levels can increase cortisol, undermining dietary efforts.
Work with a Professional: Consulting a registered dietitian or nutritionist can help tailor a plan to your specific genetic makeup, hormonal profile, and health goals. This personalized guidance can prevent nutritional deficiencies and optimize your diet for long-term success.

Conclusion: Finding the Right Path for You

Ultimately, the effectiveness of any diet, including low-carb, is a highly individualized matter. The notion that why doesn't low carb work for everyone? has a simple answer is misleading. The solution involves acknowledging the roles of genetics, hormones, metabolism, and psychology in dictating how our bodies respond to different eating patterns. Instead of viewing diet success as a measure of willpower, it should be seen as a journey of personalization. Focusing on nutrient-dense, whole foods, listening to your body's unique signals, and adjusting your approach as needed is the key to finding a sustainable, healthy path forward.

For more information on the intricate links between diet, genes, and health, you can explore the principles of nutrigenomics.

Frequently Asked Questions

Yes, to some extent. Certain genetic markers, such as variations in the FTO and APOE genes, can affect how your body metabolizes fats and carbs. Genetic testing can provide insight into whether you are more likely to succeed or face challenges on a low-carb diet.

Significant carbohydrate restriction can lower active thyroid hormone (T3) levels, slowing your metabolism. It can also increase cortisol (the stress hormone), which promotes fat storage, and disrupt reproductive hormones, especially in women.

The 'keto flu' is a collection of symptoms like fatigue, headaches, and brain fog that can occur during the initial adaptation to a ketogenic diet. It's often caused by electrolyte imbalances and the body transitioning from using glucose to fat for energy.

The impact on athletes varies. While some studies show long-term keto-adapted endurance athletes maintain performance, high-intensity exercise is often compromised due to the body's reliance on carbohydrates for quick energy. Periodization of carbohydrates is often a better strategy.

The initial rapid weight loss on a low-carb diet is often due to shedding water weight, as the body uses its glycogen stores. When carbohydrate intake increases, these glycogen and water stores are replenished, leading to rapid weight regain. The diet's long-term unsustainability is also a factor.

Because low-carb diets often restrict fruits, vegetables, and whole grains, they can lead to deficiencies in important nutrients like fiber, potassium, magnesium, and B and C vitamins. Supplementation or careful food choices are often necessary.

Alternatives include a moderate low-carb approach (75-150g carbs/day), a Mediterranean diet, or periodizing carbohydrate intake around exercise. Prioritizing whole, nutrient-dense foods and balancing protein and healthy fats is key regardless of carb intake level.