Low-Carb vs. Low-Fat Diets: Which Is Better for Lifespan?
Comparing low-carb and low-fat diets: health impacts, longevity, and how diet quality matters more than you think.
Low-Carb vs. Low-Fat Diets: What Does the Latest Science Say About Longevity?
Which is better for living a longer, healthier life: a low-carb diet or a low-fat diet? While diet trends continually shift, robust new research adds important detail—and a surprising dose of nuance—to the ongoing debate.
A large-scale study of hundreds of thousands of adults sheds light on how these common eating patterns affect your risk of dying from any cause, heart disease, or cancer. Importantly, the study emphasizes that the quality of foods within these diets matters as much, or more, than the macro breakdown itself.
The Research Behind the Headlines
A massive, prospective study published in 2023 in the Journal of Internal Medicine tracked over 370,000 people aged 50–71 years for more than two decades. This research is among the most comprehensive to date to investigate the impact of low-fat and low-carb diets on overall and specific causes of mortality.
The study evaluated how closely participants followed low-carb or low-fat diets and further distinguished between ‘healthy’ and ‘unhealthy’ versions of each dietary style. “Healthy” low-carb and low-fat diets were defined by higher intakes of plant protein, whole grains, and unsaturated fats, and lower amounts of saturated fat and refined carbohydrates.
Key Findings: Not All Low-Carb and Low-Fat Diets Are Created Equal
- Overall low-carb diets were associated with higher risks of death from all causes, heart disease, and cancer, particularly when the diets consisted mainly of animal fats, saturated fats, and low-quality carbohydrate sources.
- A “healthy” low-carb diet—emphasizing plant protein, unsaturated fats, and high-quality carbs from veggies and whole grains—slightly reduced overall mortality risks.
- Overall low-fat diets reduced the risk of death from all causes, as well as from heart disease and cancer, especially when low in saturated fat and high in plant-based foods and quality carbs.
- The “healthiest” low-fat diets (low saturated fat, high plant protein, and high-quality carbs) lowered the risk of dying from any cause by up to 18%, from heart disease by 16%, and from cancer by 18% compared to the least healthy low-fat diets.
Comparison Table: Diet Quality and Mortality Outcomes
| Diet Pattern | Diet Quality | Mortality Risk | Key Characteristics |
|---|---|---|---|
| Low-Carb | Unhealthy | ↑ Higher (esp. heart disease, cancer) | High in animal fat, saturated fat, low plant foods |
| Low-Carb | Healthy | ↓ Slightly lower | Plant proteins, unsaturated fats, veggies, whole grains |
| Low-Fat | Unhealthy | Neutral or small reduction | Low in saturated fat, but high in refined carbs |
| Low-Fat | Healthy | ↓ Up to 18% lower | Low saturated fat, high plant proteins, high-quality carbs |
Why Does Diet Quality Matter More Than Carb or Fat Intake?
This research underscores a consistent theme in modern nutrition: what you eat matters as much as what you limit. Cutting carbs or fat without regard to sources and food quality produces highly variable health effects.
- A low-carb diet filled with red meats, butter, and minimal plants can increase mortality, even as it reduces total carbs.
- Low-carb diets based on healthy fats (olive oil, nuts, seeds), plant protein (tofu, legumes), and abundant vegetables may actually promote longevity.
- Low-fat diets overloaded with refined grains and sugars may not help and could worsen health; those optimizing whole grains, beans, and fruits—with minimized saturated fat—are linked to living longer.
Does Replacing Saturated Fat With Other Nutrients Help?
One particularly notable finding: substituting just 3% of calories from saturated fat with plant protein, unsaturated fats, or high-quality carbohydrates significantly lowered risks of death from all causes, heart disease, and cancer. This supports longstanding guidance to prioritize unsaturated fat over saturated, and plant-based protein over animal, whenever possible.
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What Are “High-Quality Carbohydrates”?
Not all carbohydrates are the same. High-quality carbohydrates include:
- Whole grains (e.g., oats, brown rice, quinoa, whole wheat)
- Starchy vegetables (e.g., sweet potatoes, corn, peas)
- Beans and lentils
- Fruits
In contrast, low-quality carbohydrates refer to refined grains (like white bread, white rice, pastries), added sugars, and highly processed snack foods. These are associated with increased mortality in observational research.
How Do Low-Carb and Low-Fat Diets Affect Weight and Heart Health?
Previous short-term studies have established that both low-carb and low-fat eating plans can promote weight loss and reduce markers of poor heart health. For example:
- Low-carb diets often improve insulin sensitivity and raise HDL (“good”) cholesterol, while lowering triglycerides.
- Low-fat diets, when based on high-quality plant foods, also support weight loss and heart health by lowering bad cholesterol (LDL) and improving metabolic markers.
- However, the content and quality of what replaces the carbs or fat—whether it’s plant-based and unprocessed or more animal-based and processed—is key to determining health outcomes.
Practical Tips for a Healthier Low-Carb or Low-Fat Diet
- Choose plant sources of protein and fat: Opt for beans, lentils, tofu, nuts, and seeds rather than relying heavily on meats and cheeses.
- Prioritize whole fruits and vegetables: Load both diet types with a variety of colorful, fiber-rich produce.
- Limit saturated fats: Use less butter, cream, processed meats, and fried foods. Swap these for olive oil, avocado, and fatty fish.
- Cut out refined carbs: Steer clear of white bread, sugary cereals, pastries, and most packaged snacks.
- Eat more whole grains (low-fat) or low-starch veggies (low-carb): Match side dishes and snacks to your diet focus, but choose unprocessed options either way.
- Watch added sugars: Regardless of diet type, minimize sugar-sweetened beverages and desserts.
Expert Insights: What Do Dietitians Recommend?
Nutrition experts agree that individual needs, preferences, and medical history play an important role in choosing the best eating pattern. For the general public aiming for longevity, common advice includes:
- Instead of focusing narrowly on macronutrients (carbs vs. fat), prioritize unprocessed, nutrient-dense whole foods.
- Balance rather than extreme restriction: Overly rigid low-carb or low-fat diets may be hard to sustain and can harm nutrient balance unless carefully planned.
- Monitor portion sizes and avoid excess calories, regardless of macronutrient ratios.
- Focus on plant-based diversity: The healthiest patterns (like the Mediterranean or DASH diets) combine healthy carbs, fats, and proteins primarily from plants and seafood.
Addressing Common Myths about Carbs and Fats
- Myth: All carbs are bad.
Reality: Carbs from whole plant sources are consistently linked to better health. The risk comes mostly from ultra-processed and sugary carbs, not whole grains, fruits, or legumes. - Myth: Eating fat always leads to weight gain.
Reality: Unsaturated fats from plants and fish are essential for brain and heart health and, when included in a balanced diet, do not cause unwanted weight gain. - Myth: A low-carb, high-protein diet always extends lifespan.
Reality: Diets high in processed meats and saturated fats are linked to higher mortality, even if low in carbs. Plant-based proteins and healthy fats are protective.
Frequently Asked Questions (FAQs)
Q: Which diet is better for weight loss: low-carb or low-fat?
A: Both can help with weight loss if calorie intake is managed, but weight maintenance and health outcomes depend more on food quality and an individual’s medical profile.
Q: Can I eat a healthy low-carb diet if I don’t eat meat?
A: Yes. Emphasize nuts, seeds, tofu, eggs (if not vegan), non-starchy vegetables, and low-sugar dairy or plant alternatives as your main protein and fat sources.
Q: Should I avoid all fats for longevity?
A: No. Unsaturated fats, found in fish, nuts, seeds, and plant oils, are recommended for heart and brain health. The key is to minimize saturated and trans fats.
Q: Are low-fat diets safe for people with diabetes?
A: If they emphasize whole grains, legumes, fruits, and veggies and control overall carbohydrate intake, low-fat diets are safe and beneficial. However, some people with diabetes find their blood sugar is better managed on moderate-carb, higher-protein diets. Consult your healthcare provider.
Q: What about “keto” and other very low-carb diets?
A: Very low-carb, ketogenic diets have demonstrated benefits for specific groups (e.g., people with refractory epilepsy, some with Type 2 diabetes) but are not required for most people to achieve long-term health or longer life. They can pose risks if high in animal products and deficient in fiber or plants.
Conclusion: Key Takeaways for Healthier Living
- Healthy eating patterns—not strict carb or fat limits—predict longer life.
- Quality trumps quantity: Emphasize unsaturated fats, plant proteins, and whole, high-fiber carbohydrates.
- Personalization matters: Find a sustainable diet that works for your health, cultural, and taste preferences.
- Periodically re-evaluate your approach based on new research and your individual health status.
For sustainable health and longer life, dietary patterns should be balanced, rich in whole and minimally processed foods, and tailored to your unique health needs. It’s not just about what you exclude, but what you consistently include—so make every meal count for the long run.
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