For decades, the battle for dietary supremacy has been waged between two seemingly opposing camps: low-fat and low-carb. The narrative has shifted dramatically over the years. In the latter half of the 20th century, public health guidelines, influenced by the prevailing wisdom that dietary fat was the primary driver of heart disease and obesity, championed low-fat eating. Grocery store shelves were flooded with “fat-free” products, often laden with added sugars to compensate for taste. Then, with the dawn of the new millennium, the tide turned. The Atkins Diet and subsequent low-carbohydrate, high-fat (LCHF) movements like keto gained immense popularity, positing that carbohydrates—not fat—were the true culprits behind expanding waistlines and metabolic disease.
This left the average consumer in a state of nutritional whiplash. Which approach is truly superior for sustainable weight loss and long-term health? The answer, as revealed by a new generation of large-scale, rigorous, long-term studies, is far more nuanced than the binary debate suggests. It is not about declaring a single winner, but about understanding that the “best” diet is profoundly individual, hinging on metabolism, adherence, and overall dietary quality.
The Historical Context: From Fat Phobia to Carbophobia
To understand the current landscape, a brief historical overview is essential. The low-fat hypothesis gained traction in the 1950s and 1960s, largely based on ecological studies and the influential Seven Countries Study by Ancel Keys, which correlated saturated fat intake with heart disease mortality (BOLD Keys A, 1966). This led to public policy that encouraged reducing total fat intake, a paradigm that held for nearly 40 years.
However, as obesity rates soared despite lower fat consumption, skepticism grew. Researchers like BOLD Taubes G (2007) in his book Good Calories, Bad Calories argued that the case against fat was built on weak science and that refined carbohydrates and sugar were the true drivers of insulin resistance and metabolic syndrome. This catalyzed the low-carb revolution, which focused on controlling the hormone insulin. The theory posits that by minimizing carbs, insulin levels remain low, allowing the body to easily access and burn stored fat for energy.
For years, the debate was fueled by short-term studies, often with small sample sizes, that showed conflicting results. Low-carb diets frequently demonstrated superior weight loss in the first 6-12 months, but these differences tended to diminish over time. The critical question remained: what happens when these diets are followed for years, not months?
The Game Changer: The DIETFITS Study
A landmark study published in 2018 fundamentally changed the conversation. The Diet Intervention Examining the Factors Interacting with Treatment Success (DIETFITS) trial was a large, randomized clinical trial designed to compare the effects of a healthy low-fat diet versus a healthy low-carbohydrate diet over a 12-month period (BOLD Gardner CD, 2018). Unlike many previous studies, DIETFITS emphasized dietary quality rather than mere macronutrient restriction.
The researchers recruited 609 overweight adults and randomly assigned them to one of the two diet groups. Crucially, both groups attended extensive classes with registered dietitians who taught them to choose high-quality foods. The low-fat group was instructed to consume more fruits, vegetables, legumes, and whole grains while reducing their fat intake. The low-carb group was taught to select high-quality fats and proteins from sources like avocado, olive oil, nuts, and pasture-raised meats, while minimizing refined carbohydrates and sugars.
The results were startling—and illuminating. After 12 months, there was no significant difference in weight loss between the two groups. On average, participants in the low-fat group lost 5.3 kg (11.7 lbs), while those in the low-carb group lost 6.0 kg (13.2 lbs), a difference that was not statistically significant. Furthermore, there were no notable differences in secondary outcomes like insulin levels or blood pressure.
The take-home message from DIETFITS was profound: When diet quality is high, the specific macronutrient ratio (low-fat vs. low-carb) may be less important for weight loss than previously thought. The study’s lead author, Christopher Gardner, famously summarized the findings by stating that the most effective diet is the one an individual can stick to. The study also highlighted significant variability within each group; some individuals lost over 60 pounds, while others gained weight, underscoring the concept of individual response.
Beyond Weight Loss: Cardiovascular and Metabolic Health
While weight loss is a common goal, the impact of these diets on cardiovascular health is paramount. Here, the evidence has also evolved from simplistic notions. Early fears about low-carb diets centered on the potential for high saturated fat intake to raise LDL (“bad”) cholesterol, a known risk factor for heart disease. Conversely, critics of low-fat diets pointed out that if fat is replaced with refined carbohydrates and sugar, it can worsen blood triglycerides and lower HDL (“good”) cholesterol, a pattern known as atherogenic dyslipidemia.
Long-term observational studies provide critical insights. The PURE (Prospective Urban Rural Epidemiology) study, which followed more than 135,000 people from 18 countries for over seven years, found that high carbohydrate intake (above 60% of total calories) was associated with a higher risk of total mortality, whereas high fat intake (up to 35% of calories) was associated with a lower risk (BOLD Dehghan M, 2017). Importantly, the study suggested that current global dietary guidelines, which recommend very low fat intake, may need reconsideration.
However, the picture is complex. Not all low-carb diets are created equal. A 2023 study published in the American Journal of Clinical Nutrition analyzed data from the Nurses’ Health Study and the Health Professionals Follow-up Study, tracking over 123,000 adults for three decades. It differentiated between animal-based and plant-based low-carbohydrate diets. The researchers found that a low-carb diet high in animal-based proteins and fats was associated with higher mortality, whereas a low-carb diet emphasizing plant-based sources of fat and protein was associated with lower mortality (BOLD Shan Z, 2023). This underscores a critical theme: the source of macronutrients is as important, if not more so, than the quantity.
Similarly, a low-fat diet based on whole grains, fruits, and vegetables is likely to have a very different health impact than one based on sugar-sweetened beverages and white bread. A 2015 meta-analysis concluded that the evidence does not support low-fat diets for long-term health, but rather supports diets that focus on the type of fat and carbohydrate consumed (BOLD Harcombe Z, 2015).
The Role of Genetics and Personalization: Is There a “Right” Diet for Your Genes?
The variability in individual responses to different diets has led to intense interest in the field of nutrigenetics—the study of how genes interact with diet. The hypothesis is that genetic predispositions might make some people more responsive to a low-fat approach and others to a low-carb approach.
For instance, research has explored genes related to insulin sensitivity and fat metabolism. A secondary analysis of the DIETFITS study itself investigated whether specific genetic patterns (related to carbohydrate and fat metabolism) or baseline insulin secretion would predict success on a low-fat or low-carb diet (BOLD Gardner CD, 2018). The results were, again, humbling. The researchers found that the genotype pattern and insulin secretion level did not help identify which diet was better for which individual.
This suggests that while genetics play a role in metabolism, their power to predict dietary success in a free-living setting may be limited compared to behavioral factors like adherence, taste preference, and cultural compatibility. The search for a simple genetic test to prescribe the “perfect” diet continues, but current evidence indicates that self-experimentation and attention to how one feels on a particular dietary pattern may be more practical tools for personalization.
Sustainability and Adherence: The Ultimate Determinant of Success
The most elegant, scientifically sound diet is worthless if it cannot be maintained. This is the Achilles’ heel of many highly restrictive diets. Long-term studies consistently show that adherence—the ability to stick to a diet—is the single strongest predictor of weight loss and health improvement, regardless of the macronutrient composition.
Very low-carb diets like keto can be difficult to sustain for years due to their restrictive nature, which can limit social dining and require significant lifestyle changes. Similarly, very low-fat diets can feel unsatisfying for some individuals, leading to hunger and eventual relapse. The DIETFITS trial succeeded in part because it promoted a “healthy” version of each diet that was less extreme and more flexible than the classic Atkins or Ornish plans.
A 2020 review of popular diets confirmed that while low-carb and low-fat diets can both be effective for weight loss, moderate approaches like the Mediterranean diet—which is not explicitly low in fat or carb but emphasizes healthy fats, whole grains, and vegetables—often show excellent adherence and impressive benefits for cardiovascular health (BOLD Ge L, 2020). The PREDIMED trial, for example, demonstrated that a Mediterranean diet supplemented with nuts or extra-virgin olive oil reduced the incidence of major cardiovascular events compared to a low-fat control diet (BOLD Estruch R, 2018).
Synthesizing the Evidence: A Practical Guide for the Perplexed
So, what does the collective evidence from these long-term studies reveal? The low-carb vs. low-fat debate, as a binary choice, is largely a false dichotomy. The following conclusions provide a more practical framework for making dietary choices:
- Quality Trumps Quantity: Focus on the quality of the foods you eat rather than obsessing over macronutrient percentages. A diet rich in whole, minimally processed foods—vegetables, fruits, lean proteins, healthy fats, and whole grains—is the foundation of good health, whether it skews slightly lower in fat or carbs.
- There is No Single “Best” Diet for Everyone: Individual factors like food preferences, culture, metabolic health, and lifestyle are far more important than following a universally prescribed plan. The best diet is the one you can maintain consistently and that makes you feel energized and healthy.
- For Weight Loss, Energy Balance is Fundamental: Both diets work by creating a calorie deficit, albeit through different mechanisms—low-fat diets tend to be lower in calorie density, while low-carb diets may suppress appetite. The diet that helps you consume fewer calories without constant hunger is the right one for you.
- Consider Your Health Profile: For someone with insulin resistance or type 2 diabetes, a moderate low-carb approach may be particularly beneficial for blood sugar control. For someone with high LDL cholesterol, being mindful of saturated fat intake, even on a low-carb diet, is prudent. Consulting a healthcare provider or registered dietitian is always recommended.
- Look to Time-Tested Patterns: Diets that have stood the test of time, like the Mediterranean diet or a whole-foods, plant-based diet, are consistently associated with longevity and reduced disease risk. These patterns naturally avoid the extremes of both very low-fat and very low-carb eating, emphasizing balance and food quality.
Conclusion
The latest long-term science has effectively called a truce in the diet wars. The simplistic narrative that one macronutrient is the villain and another the hero has been dismantled by rigorous evidence. Studies like DIETFITS have shown that when built on a foundation of whole foods, both low-fat and low-carb pathways can lead to similar health outcomes.
The true revelation is that we have been asking the wrong question. Instead of “Which is better: low-carb or low-fat?” we should be asking, “What dietary pattern of high-quality foods can I sustain for life that supports my personal health goals and brings me enjoyment?” The answer to that question is the key to long-term well-being, moving us beyond fleeting trends and toward a more personalized, sustainable, and evidence-based approach to eating. The era of one-size-fits-all dietary dogma is over, replaced by an empowering understanding that the best diet is the one you can live with, and live well on.
SOURCES
BOLD Dehghan M, (2017). Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. The Lancet, *390*(10107), 2050–2062.
BOLD Estruch R, (2018). Primary Prevention of Cardiovascular Disease with a Mediterranean Diet Supplemented with Extra-Virgin Olive Oil or Nuts. New England Journal of Medicine, *378*(25), e34.
BOLD Gardner CD, (2018). Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA, *319*(7), 667–679.
BOLD Ge L, (2020). Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials. BMJ, *369*, m696.
BOLD Harcombe Z, (2015). Evidence from randomised controlled trials did not support the introduction of dietary fat guidelines in 1977 and 1983: a systematic review and meta-analysis. Open Heart, *2*(1), e000196.
BOLD Keys A, (1966). The Seven Countries Study: A multivariate analysis of death and coronary heart disease. Harvard University Press.
BOLD Shan Z, (2023). Healthy low-carbohydrate and low-fat diets and risk of total and cause-specific mortality. American Journal of Clinical Nutrition, *118*(1), 52–61.
BOLD Taubes G, (2007). Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease. Alfred A. Knopf.
HISTORY
Current Version
Sep 26, 2025
Written By:
SUMMIYAH MAHMOOD