Low-carb diets linked to reduced depression symptoms — but there’s a catch

A new study published in the Journal of Psychiatric Research suggests that not all low-carbohydrate diets affect mental health the same way. Adults who followed a low-carbohydrate diet that emphasized plant-based proteins, unsaturated fats, and fewer refined carbohydrates were significantly less likely to experience symptoms of depression. On the other hand, low-carb diets high in animal protein and saturated fats showed no such benefits. These findings point to the importance of food quality, not just quantity, in shaping mood and mental health.

Carbohydrates are a major source of energy in the human diet, but their role in mental health has been debated. Low-carbohydrate diets—often adopted for weight loss or metabolic health—reduce carbohydrate intake and compensate with higher protein and fat intake. While some studies have found that these diets improve symptoms of depression, others have reported no clear link or even suggested possible risks. One factor that may explain these conflicting results is that researchers have not always distinguished between high-quality and low-quality sources of nutrients.

To address this gap, researchers in China conducted a large cross-sectional study using data from the National Health and Nutrition Examination Survey (NHANES), a representative health survey of U.S. adults. They wanted to determine whether the type of low-carbohydrate diet someone follows—healthy or unhealthy—affects the likelihood of experiencing depression symptoms.

The researchers analyzed data from 28,791 U.S. adults aged 20 and older who participated in NHANES between 2005 and 2018. Participants were excluded if they had incomplete dietary data, implausible calorie intake, missing depression assessments, or missing background information such as education or health history. The final sample included a wide range of demographic groups, with information collected through interviews, physical exams, and two-day dietary recalls.

To evaluate how closely participants followed low-carbohydrate diets, the researchers created three types of dietary scores. The overall low-carbohydrate diet score (OLCD) was based on the total intake of carbohydrates, fats, and proteins. A higher score meant lower carbohydrate intake and higher intake of fats and proteins.

They also created two specific variations: the healthy low-carbohydrate diet score (HLCD) and the unhealthy low-carbohydrate diet score (ULCD). The HLCD emphasized plant-based proteins, unsaturated fats, and minimal low-quality carbohydrates like refined grains and added sugars. In contrast, the ULCD emphasized animal proteins, saturated fats, and reduced intake of healthy carbohydrates like fruits, legumes, and whole grains.

Depression symptoms were assessed using a widely accepted questionnaire known as the Patient Health Questionnaire-9 (PHQ-9). This tool asks participants about feelings of sadness, hopelessness, and other mood-related experiences over the past two weeks. A total score of 10 or more is considered to indicate significant symptoms of depression.

To understand the relationship between diet and depression, the researchers used statistical models that adjusted for a variety of factors, including age, sex, race, education, body weight, smoking, alcohol use, physical activity, and existing health conditions like diabetes or heart disease. They also tested whether replacing certain foods with healthier alternatives would impact depression risk.

The results showed that individuals with higher HLCD scores—those who ate fewer refined carbs and more plant-based proteins and healthy fats—had significantly lower odds of experiencing depressive symptoms. Specifically, people in the highest category of the HLCD were about 30% less likely to report depression symptoms compared to those in the lowest category. In contrast, people with higher ULCD scores showed no meaningful difference in depression risk compared to those with low scores.

The overall low-carbohydrate diet score (OLCD) also showed a weak association with lower depression symptoms, but the effect was less consistent and disappeared at the highest levels. The dose-response analysis suggested that moderate adherence to a low-carbohydrate diet was more beneficial than extreme restriction. When low-quality carbohydrates were replaced with plant-based or even animal-based proteins, participants were less likely to report depression symptoms, highlighting the potential role of nutrient substitutions.

The study also explored whether specific groups might benefit more from healthier low-carb diets. Results remained consistent across subgroups based on age, gender, race, marital status, and lifestyle factors like exercise or smoking. Sensitivity analyses that accounted for antidepressant use also supported the main findings.

One possible explanation for the link between healthy low-carb diets and reduced depression symptoms involves inflammation and oxidative stress. Diets high in added sugars and saturated fats can increase inflammation in the body, which has been linked to depression. In contrast, diets rich in nutrients like B vitamins, folate, and tryptophan—found in many plant-based foods—may support brain function by aiding neurotransmitter production, including serotonin. These dietary patterns may also improve gut health, metabolic function, and sleep, all of which influence mood.

Another key takeaway from the study is that not all low-carbohydrate diets are equal. A diet low in carbohydrates but high in processed meats, saturated fats, and lacking in plant-based foods may not support mental health. Simply reducing carbohydrate intake, without considering the source and quality of the remaining nutrients, is unlikely to provide psychological benefits.

Despite its strengths—including a large, nationally representative sample and a nuanced analysis of dietary patterns—the study has limitations. Because it was cross-sectional, it cannot prove cause and effect. It’s possible that people with depression eat differently because of their symptoms, rather than diet causing those symptoms. Dietary intake was self-reported and only measured over two days, which may not capture long-term habits. There may also be errors in how foods were categorized, despite efforts to minimize subjectivity.

Future research should follow participants over time to confirm whether these dietary patterns can prevent depression or reduce symptoms in people who are already struggling. Clinical trials that test specific dietary interventions, using more accurate tracking methods, would help clarify the underlying mechanisms and strengthen the evidence base.

The study, “Low-carbohydrate diet macronutrient quality and depression symptoms among US adults,” was authored by Chengxiang Hu, Beibei Han, Yue He, Rong Huang, Xiaoting Fan, Jia Lan, Yanan Ma, and Lina Jin.