Following a low calorie diet is linked to a heightened risk of depressive symptoms, finds research published in the open access journal, BMJ Nutrition Prevention & Health.
Men and those who are overweight may be especially vulnerable to the effects of restrictive eating, the findings suggest.
A 'healthy' diet rich in minimally processed foods, fresh fruits and vegetables, whole grains, nuts, seeds, lean proteins and fish, is generally associated with a lower risk of depression, while an 'unhealthy' diet, dominated by ultra-processed foods, refined carbs, saturated fats, processed meats and sweets, is generally associated with a heightened risk, explain the researchers.
But people follow many different types of diet for health or medical reasons, including those that restrict calories or particular nutrients, and it's not clear if these other dietary patterns might be associated with a risk of depressive symptoms, they add.
To explore this further, they drew on 28,525 adult respondents (14,329 women and 14,196 men) to the nationally representative US National Health and Nutrition Examination Survey (NHANES) for the years 2007–18, who had completed the Patient Health Questionnaire-9 (PHQ-9) for depressive symptom severity.
In all, 2508 people (just under 8%) reported depressive symptoms, and 7995 participants (29%) had a healthy weight; 9470 (33%) were overweight; and 11060 (38%) were obese.
Participants were asked if they were following any particular diet either to lose weight or for other health reasons, and if so, which of the 9 diet options set out in all 6 cycles of NHANES they were on.
Dietary patterns were categorised into 4 groups: (1) calorie-restrictive; (2) nutrient-restrictive (low in fat/cholesterol, sugar, salt, fibre, or carbs); (3) established dietary patterns (adapted for diabetes, for example); and (4) not on a diet.
Most participants (25,009, 87%) said they weren't on any specific diet, while 2026 (8%) followed a calorie-restrictive diet, 859 (3%) a nutrient-restrictive diet, and 631 (2%) an established dietary pattern.
When stratified by sex, a greater proportion of men (12,772; 90%) than women (12,237; 85%) said they weren't on a diet. Calorie restriction was most commonly reported by obese participants (1247;12%) and those who were overweight (594; 8%), while nutrient-restrictive and established dietary patterns were less commonly reported, with the highest proportion of established dietary pattern users among obese participants (359; 3%).
PHQ-9 scores were 0.29 points higher in those on calorie-restrictive diets than in those not on any specific diet.
The scores were higher among those who were overweight and following a calorie-restrictive diet: their PHQ-9 scores were 0.46 points higher, while a nutrient-restrictive diet was associated with a 0.61 point increase in PHQ-9 scores.
Calorie-restrictive diets were also associated with higher cognitive-affective symptom scores (measure of relationship between thoughts and feelings) while nutrient-restrictive diets were associated with higher somatic symptom scores (excessive distress and anxiety about physical symptoms).
These scores also varied by sex: a nutrient-restrictive diet was associated with higher cognitive-affective symptom scores in men than in women not on a diet, while all 3 types of diet were associated with higher somatic symptom scores in men.
And people living with obesity following an established dietary pattern had higher cognitive-affective and somatic symptom scores than those of a healthy weight not on a diet.
This is an observational study, and as such no firm conclusions can be drawn about causality. Respondents may not have accurately classified their diets either, say the researchers.
The findings also contradict those of previously published studies suggesting that low calorie diets improve depressive symptoms. But the researchers explain: "This discrepancy may arise because prior studies were primarily randomised controlled trials (RCTs) where participants adhered to carefully designed diets ensuring balanced nutrient intake.
"In contrast, real-life calorie-restricted diets and obesity often result in nutritional deficiencies (particularly in protein, essential vitamins/minerals) and induce physiological stress, which can exacerbate depressive symptomatology including cognitive-affective symptoms." Another possible explanation might be a failure to lose weight or weight cycling—losing weight and then putting it back on, they suggest.
By way of an explanation for the observed gender discrepancies, the researchers point out that glucose and the fatty acid omega-3 are critical for brain health. "Diets low in carbohydrates (glucose) or fats (omega-3s) may theoretically worsen brain function and exacerbate cognitive-affective symptoms, especially in men with greater nutritional needs," they suggest.
Professor Sumantra Ray, Chief Scientist and Executive Director of the NNEdPro Global Institute for Food, Nutrition and Health, which co-owns BMJ Nutrition Prevention & Health with BMJ Group, comments: "This study adds to the emerging evidence linking dietary patterns and mental health, raising important questions about whether restrictive diets which are low in nutrients considered beneficial for cognitive health, such as omega-3 fatty acids and vitamin B12, may precipitate depressive symptoms.
"But the effect sizes are small, with further statistical limitations limiting the generalisability of the findings. Further well designed studies that accurately capture dietary intake and minimise the impact of chance and confounding are needed to continue this important line of inquiry."
