In recent years, social media has been flooded with trendy diets like low-carb and ketogenic diets, with claims of controlling blood sugar levels and even preventing diabetes. But how effective are these diets in reality? A groundbreaking study conducted by Harvard University over three decades, tracking nearly 200,000 participants, reveals surprising insights. The study shows that the quality of the carbohydrates you replace in your diet is just as important as how many carbs you cut out. Discover how different low-carb eating patterns impact the risk of developing type 2 diabetes and which ones could truly help protect your health.
Have you ever been tempted to give up rice and noodles in the name of weight loss or better blood sugar control? The rise of “low-carb” diets, especially on social media, has sparked a conversation about the link between carb restriction and health benefits. The promise seems simple—cut carbs, control blood sugar, and prevent conditions like diabetes.
But, as with all things in nutrition, the answer is not as clear-cut as it may seem. To answer this question scientifically, researchers from Harvard University embarked on a monumental study that spanned over three decades, involving nearly 200,000 participants. What did they find? Well, it turns out, the benefits of low-carb eating depend a lot on what you’re eating in place of those carbs. Let’s dive into the details of this comprehensive research to see what truly protects us from diabetes.
In recent years, have you been scrolling through your social media feed and been bombarded by posts about “low-carb weight loss” or “ketogenic diets”? Are you also wondering if saying goodbye to rice and noodles could help manage your blood sugar or even prevent diabetes?
To answer this question, Harvard University's research team conducted a groundbreaking study spanning 30 years, tracking nearly 200,000 participants. This extensive research not only debunks the myth of “low-carb equals healthy,” but it also provides us with a key takeaway: the effectiveness of low-carb diets in diabetes prevention depends not just on how much carb you cut, but also on what you replace it with!
Participants and Research Design
This study combined data from three major U.S. health cohorts: the Nurses’ Health Study (NHS), which began in 1984 with 71,244 women; the Nurses’ Health Study II (NHSII), started in 1991 with 88,801 women; and the Health Professionals Follow-up Study (HPFS), initiated in 1986 with 38,961 men. In total, nearly 200,000 participants were tracked for an average of 28 years—a study truly designed to last.
https://doi.org/10.2337/dc25-140110.2337/dc25-1401
Every 2-4 years, participants filled out detailed food frequency questionnaires (FFQs) to record their dietary habits. Researchers then categorized their eating patterns into five different low-carbohydrate diet scores:
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Overall Low-Carb Diet: Focuses solely on reducing carbohydrate intake.
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Animal-Based Low-Carb Diet: Emphasizes animal protein and fats.
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Plant-Based Low-Carb Diet: Focuses on plant-based proteins and fats.
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Unhealthy Low-Carb Diet: Builds on animal-based low-carb, further restricting whole grains and other healthy carbohydrates.
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Healthy Low-Carb Diet: Centers on plant-based foods and reduces refined carbs and added sugars.
To ensure that the results were purely from dietary patterns and not influenced by other lifestyle factors, the researchers adjusted for a wide range of confounding variables, including age, race, smoking, alcohol consumption, physical activity, health conditions (such as BMI, high blood pressure, and cholesterol levels), and total calorie intake.
Figure: Experimental Flowchart
What’s Protecting You, and What’s Digging a Hole?
In total, the study accumulated nearly 5 million person-years of follow-up data, recording 20,452 new cases of type 2 diabetes. After adjusting for factors such as age, lifestyle, and baseline BMI, the data revealed some striking findings.
The first major takeaway was that not all low-carb diets are beneficial. When compared to participants who followed a diet with the lowest level of overall carbohydrate intake, those adhering to a basic low-carb diet (which reduces carbs but doesn’t specify the quality of fats or proteins consumed) had a 31% higher risk of developing type 2 diabetes (HR 1.31, 95% CI 1.25-1.37).
Table: Age-Standardized Characteristics of Participants in HPFS (1986–2018), NHS (1984–2018), and NHSII (1991–2019) Cohort Studies
But the story took a major turn when the quality of macronutrients in the diet was considered. It became clear that low-carb diets based on animal sources—such as those emphasizing red meat and processed meats—were linked to worsened outcomes. Participants following an animal-based low-carb diet had a 39% higher risk of developing diabetes (HR 1.39).
The worst offenders were those following an unhealthy low-carb diet, which reduced high-quality carbs like whole grains in addition to cutting sugars. Participants following this diet had a 44% higher risk of diabetes (HR 1.44). This suggests that reducing healthy carbs in favor of refined carbs and unhealthy fats can have a detrimental effect on metabolic health.
On the other hand, the research found some protective patterns. The plant-based low-carb diet showed a modest benefit, reducing diabetes risk by 6% (HR 0.94). But the real standout was the healthy low-carb diet, which replaced refined sugars and carbs with high-quality plant foods such as beans, nuts, and whole grains. This diet reduced the risk of diabetes by 16% (HR 0.84), showing the most protective effect.
The Warning for Non-Obese Individuals
Interestingly, the study also found a notable phenomenon regarding non-obese participants. The positive association between overall low-carb, animal-based, and unhealthy low-carb diets with diabetes risk was especially strong in participants with a baseline BMI <30 (non-obese). Further analysis showed that weight fluctuations were a key mediator in how these unhealthy low-carb diets increased diabetes risk, particularly in non-obese individuals. In fact, weight changes accounted for a substantial proportion (44%-68%) of the increased risk. This indicates that unhealthy low-carb diets could trigger harmful weight fluctuations, thereby raising diabetes risk—especially for those who are not obese to begin with.
In contrast, the plant-based low-carb diet did not show this BMI-related effect, meaning its benefits appeared more universal, regardless of baseline weight.
Conclusion
This 30-year study with nearly 200,000 participants has shed light on the complex relationship between low-carb diets and type 2 diabetes risk. The key takeaway is that not all low-carb diets are created equal. The focus should not only be on reducing carbohydrates, but also on replacing them with nutrient-dense, high-quality foods. A healthy low-carb diet, based on plant proteins and fats, can offer substantial protection against diabetes, while diets centered on animal proteins and fats may have the opposite effect.