Summary

Eight-week consumption of a low-carb high-fat diet had similar effects on the psychological wellbeing of overweight and obese adults as consuming a high-carb low-fat diet with the same calorie content. However, improvement in the speed of processing visual information was slightly less with the low-carb high-fat diet.

Introduction

There has been some dispute between high-carbohydrate, low-fat (HCLF) diets and low-carbohydrate, high-fat (LCHF) diets (e.g. the Atkins diet) amongst those trying to lose weight. Protagonists of the HCLF approach have worried that LCHF may alter serotonin activity in the brain, leading to mood and mental functioning changes. Indeed, LCHF diets have been reported to be associated with higher levels of anxiety and depression. To investigate this further, Australian researchers conducted a study in overweight or obese people. The findings are reported in the American Journal of Clinical Nutrition, and are summarized here.

What was done

One hundred and twenty-one overweight or obese men and women were recruited by advertisement. Those with a history of liver, cardiovascular, respiratory, or gastrointestinal disease, or diabetes, cancer, or a psychological disorder were excluded.

The participants were randomly assigned to eat a calorie-restricted LCHF diet or an HCLF diet with the same number of calories, for 8 weeks. At baseline and after the 8-week diet period the participants attended for a series of psychological mood assessment questionnaires, and cognitive function tests measuring working memory and speed of brain processing.

The subjects also had a weight check and mood questionnaires every two weeks. They were asked to maintain their usual lifestyle (e.g. physical activity) during the study.

The diets were composed as follows:

	LCHF	HCLF
Protein	35%	24%
Fat	61% (20% saturated)	30% (less than 8% saturated)
Carbohydrate	4%	46%

The two diets were isocaloric, set at approximately 1,450 calories for women and 1,675 calories for men.

The mood assessments were based on the Profile of Mood States (POMS), Beck's Depression Inventory, and the Spielberger State Anxiety Inventory. Brain functioning (cognitive function) was assessed by using the computer-based 'digital span backwards' and 'inspection time' tests, which measure working memory and speed of visual information processing.

What was found

A total of 26 subjects withdrew from the study before it was completed, and two additional subjects were not analyzed as they were clearly outliers; the final analysis was therefore made on 93 participants. Their average age was 50, and average BMI was 33.5. Men made up 66% of the study population. There were no significant differences between the two diet-assigned groups with respect to age, weight, BMI, or cardiovascular disease risk factors.

There was a significant difference in the amount of weight lost by the two groups over the 8 weeks. The LCHF members lost, on average, 8% of their starting weight, compared with 6.6% by the HCLF group.

The tests of mood revealed improvements in psychological well-being at week 2 in both dietary groups, with no differences between the groups; these improvements in depression, anxiety, and mood continued for the balance of the study.

Memory test results showed that scores increased in both dietary groups, but again with no significant differences between the groups. The speed of visual information processing improved too, but in this case there was a significant difference between the groups. The HCLF diet was associated with a greater improvement than the LCHF group, and the difference remained significant after allowing for the weight loss.

Individual participant's changes in cognitive functioning speed were positively correlated with the individual's percentage of energy obtained from fat, those subjects with higher fat intake having less improvement in speed.

What the findings mean

Both low- and high-carb weight-loss diets significantly reduced body weight and improved mood (less depression, anxiety, and general mood). In addition, there was evidence that the speed of information processing improved less with the LCHF than with the HCLF diet.

The apparent lack of a difference between diets in the improvement in mood may be because the effects of weight loss with both diets overshadowed any effects of composition. The changes in visual information processing speed - significantly less improvement with the LCHF diet - are interesting, but too much importance should not be attached to them. This was only a short-term study, and results might be very different with longer periods of dieting. As the authors of the study say, more research is needed, including exploration of other areas of cognition.