A Low Fat Diet Can Make You Angry, Irritable and Depressed

Research shows that not eating enough fat increases the risk of suicide, depression, irritability and anger. To protect your mood, you should eat a normal amount of the right types of fat every day as part of a healthy, balanced diet.

Fat has been demonised because some types of fat, such as cholesterol, are thought to raise the risk of heart attacks and other cardiovascular problems. However, a meta-analysis of cholesterol-related trials totalling 24,847 people found an unexpected side-effect. People on cholesterol-lowering diets were significantly more likely to die from suicide, accidents or violence [1]. Later evidence corroborated the link between low cholesterol levels and death from suicide specifically as well as attempted suicide or suicidal ideation [2], [3]. Researchers theorize that lowering cholesterol has an impact on serotonin and this, in turn, increases impulsivity, aggression and depression which raise suicide risk [2].

Looking at dietary fatty acids more generally, research shows that people who eat a low-fat diet are significantly more likely to become depressed, irritable, angry and hostile. Most of the studies are experiments, randomised-controlled trials or longitudinal studies therefore they can tell us about the causal effects of low-fat diets.

• A long-term study of 12,404 people assessed over 10 years found that men on a low-fat diet were 26% more likely to be depressed after 1 year than other men [4]. After 10 years, their risk of depression remained 14% higher. Women on a low-fat diet were 37% more likely to depressed after 1 year than other women. After 10 years, their risk of depression remained 30% higher. Low-fat diets were defined in the research as regularly eating fat-free meals, fat-free dairy, and low-fat dairy alternatives such as margarine.
• Another long-term study of 12,059 people over 11 years found that, the less people consumed olive oil, polyunsaturated fatty acids and monounsaturated fatty acids, the more likely they were to become depressed [5]. The more people ate trans-fatty acids the more likely they were to become depressed.
• A randomised controlled trial over five years asked 359 women to eat a low-fat diet of 15 to 20% of calories from fat, whereas 382 women were asked to eat their usual diet averaging 29 to 30% of daily calories from fat [6]. Both sets of women got dietary advice and they had normal weights of 62 to 63kg. The women on the lower-fat diet experienced significantly higher levels of stress and sleep problems. Both groups of women had equivalently low depression levels with no significant differences between them which might be because the women on the lower-fat diet were actually consuming a reasonable amount of fat every day.
• An experiment over two months took a group of 19 people, fed them a diet comprising 41% of calories from fat for a month and then measured their mood [7]. The experiment then switched about half of the people to a lower-fat diet comprising 25% of calories from fat. The rest of the people remained on the previous diet of 41% calories from fat. After one month, the experiment measured mood and found that people on the lower-fat diet had become significantly more angry/hostile and depressed/dejected. In contrast, the higher-fat diet reduced negative indicators of mood by making people less angry or hostile and tense or anxious.
• A randomised-controlled trial over a few weeks had a cross-over design in which the researchers fed 37 people a low-fat diet for four days, measured their depression, anxiety and mood, then gave them a two-week break followed by another type of diet [8]. For instance, a person within the trial was fed a high saturated fat diet for four days, filled in the questionnaire measures, and then they had a two-week break. The researchers repeated this process with a high polyunsaturated fat diet, and then a control diet, each with a break. The trial randomised the order in which each person completed each dietary phase. The study found no significant differences in depression, negative emotions or anxiety because of changes in diet perhaps because of the short study duration. The study found that people were significantly less irritable and felt significantly more positive emotions after eating a high saturated fat diet or a high polyunsaturated fat diet compared to a low-fat diet or a control diet.

Two long-term studies, therefore, show that low-fat diets increase the likelihood of someone becoming depressed, and one long-term study showed that low-fat diets increase the risk of stress and sleep problems. One short-term experiment shows that low-fat diets increase the risk of becoming more irritable, angry, hostile, tense and anxious. Another short-term experiment showed that low-fat diets increase the risk of irritability and less positive emotions.

You should, therefore, eat enough dietary fat every day as part of a healthy, balanced diet with enough protein, carbohydrates, fibre, vitamins and minerals [9]. Get dietary fat from natural sources (e.g. olive oil, oily fish, meat, nuts or seeds) because research shows that artificially created trans-fats increase your risk of depression [5], as does eating dietary fat in the context of sugary foods such as desserts or pastries [4]. Dietary fat is important for mental health and physical health. For instance, it helps your body absorb fat-soluble vitamins such as vitamin A, D and E and, therefore, official guidelines recommend that you eat about 70g of the right types of fat every day [9].

Eating a balanced, healthy diet with enough from every food group – including dietary fat – supports good mental health.