September 14, 2007 — A low-carbohydrate, high-fat (LCHF) diet and a high-carbohydrate, low-fat (HCLF) diet both improve weight loss and mood, but speed of cognitive processing improves less with the LCHF than the HCLF diet, according to the results of a study published in the September issue of the American Journal of Clinical Nutrition.

"Low-carbohydrate diets are often used to promote weight loss, but their effects on psychological function are largely unknown," write Angela K. Halyburton, from the Commonwealth Scientific and Industrial Research Organisation–Human Nutrition in Adelaide, Australia, and colleagues. "Considering the lack of well-controlled intervention studies, the purpose of the present study was to compare the effects on mood and cognitive function of a moderately energy-restricted LCHF diet with those of an isocaloric, conventional HCLF diet in overweight and obese persons."

In this study, 93 overweight or obese participants were randomized to receive an energy-restricted (about 6000 - 7000 kJ, 30% deficit), planned isocaloric LCHF diet or an HCLF diet for 8 weeks. Mean age was 50.2 ± 0.8 years, and mean body mass index was 33.6 ± 0.4 kg/m². At baseline and every 2 weeks, participants were weighed and underwent measurement of psychological well-being on the Profile of Mood States, Beck Depression Inventory, and Spielberger State Anxiety Inventory instruments. Cognitive functioning, including working memory and speed of cognitive processing, were evaluated at baseline and at week 8.

Compared with the HCLF diet, the LCHF diet was associated with significantly greater weight loss (7.8 ± 0.4 vs 6.4 ± 0.4 kg; P = .04). Improvements in psychological well-being were similar in both groups, with the greatest effect observed during the first 2 weeks. Although working memory was similar in both groups (P = .68), there was a significant time/diet interaction for speed of processing (P = .04), with less improvement on this measure in the LCHF than in the HCLF diet group.

"Both dietary patterns significantly reduced body weight and were associated with improvements in mood," the authors write. "There was some evidence for a smaller improvement in cognitive functioning with the LCHF diet with respect to speed of processing, but further studies are required to determine the replicability of this finding."

Study limitations include difficulty separating practice effects from treatment effects; inability to determine the effects of concurrent exercise training; generalizability limited to healthy, overweight, and obese young to middle-aged adults with normal mood state and no cognitive impairment who begin structured diets; and limited measures of cognitive performance.

"Short-term consumption of a moderately energy-reduced LCHF diet has an effect on the psychological well-being of overweight and obese persons similar to that of consumption of an isocaloric conventional HCLF diet," the authors conclude. "However, there was evidence of a somewhat smaller improvement in speed of processing with an LCHF diet. Further studies are required to determine the replicability of this finding and to determine whether similar outcomes are evident over the long term."

The National Heart Foundation of Australia and the National Health and Medical Research Council of Australia supported this study. The authors have disclosed no relevant financial relationships.

Am J Clin Nutr. 2007;86:580-587.

Clinical Context

Study Highlights

• Included were 121 men and women with body mass index from 26 to 43 kg/m² and age from 24 to 64 years with abdominal obesity and 1 other metabolic risk factor as defined by the International Diabetes Foundation.
• Excluded were those with medical or psychiatric morbidities.
• Matched participants were randomized to an energy-restricted diet that was either LCHF or HCLF with a 30% daily energy deficit.
• At baseline, anthropometric measurements, mood and cognitive function tests, and laboratory tests were performed.
• Participants were asked to maintain their usual lifestyle.
• The LCHF diet provided 35% of energy as protein, 61% as fat (20% saturated fat), and 4% as carbohydrate.
• The HCLF diet provided 24% of energy as protein, 30% as fat (< 8% saturated), and 46% as carbohydrate.
• Daily energy provided was about 6000 kJ for women and 7000 kJ for men.
• Key uncooked foods were provided to the participants fortnightly.
• A qualified dietician provided recipes and scales for weighing food, and a semiquantitative
  3-day food record was completed every 2 weeks.
• Participants attended the clinic every 2 weeks for weighing and mood assessment.
• Mood assessment was completed in a quiet, temperature-controlled room using the validated paper-based Profile of Mood States, Beck Depression Inventory, and Spielberger State Anxiety Inventory.
• Cognitive function assessment was completed at baseline and week 8 by computer-based digit span backwards and inspection time tests, which assess working memory and speed of processing, respectively.
• Both cognitive tests were conducted with PRESENTATION software (Neurobehavioral Systems, Inc).
• Of 121 participants, 93 completed the study.
• Mean age was 50 years, there were more women (56 women) vs men, mean body weight was 95 kg, mean body mass index was 33 kg/m², mean systolic blood pressure was 134 mm Hg, and mean diastolic blood pressure was 75 mm Hg.
• Mean cholesterol was 5.4 mmol/L, and fasting glucose was 5.7 mmol/L.
• There was no significant difference in total energy intake between the 2 groups.
• Carbohydrate intake was significantly higher and fat intake lower in the HCLF vs LCHF group ( P < .001).
• There was no difference in plasma ketone levels at baseline, but by 2 weeks, ketone levels were higher in the LCHF group; this persisted for the remaining 6 weeks, which indicated adherence to the LCHF diet.
• There was significant weight loss in both groups, with the LCHF group showing significantly greater loss ( P = .005) vs the HCLF group (weight loss, 8.0 and 6.6 kg, respectively).
• There was no effect of sex or age on weight loss.
• By intent-to-treat analysis, the respective weight losses were 7.6 and 6.3 kg for LCHF and HCLF groups with a significant difference ( P = .01).
• At baseline, there were no differences in Beck Depression Inventory, Spielberger State Anxiety Inventory, or Profile of Mood States scores; scores were normal.
• Both groups showed significant mood improvement, occurring by week 2 ( P < .002 for time).
• Diet composition did not affect improvement seen for mood.
• All 6 subscales of Profile of Mood States (tension, depression, anger, vigor, fatigue, and confusion) showed significant improvement for both groups.
• At baseline, there were no differences in cognitive function.
• At week 8, both groups improved significantly for digit span backwards and inspection time tests.
• The HCLF diet provided significantly greater improvement in speed of processing vs the LCHF diet.
• There was no effect of weight lost, plasma ketones, or nutrient intake on improvements in memory or processing.

Pearls for Practice

• Use of an LCHF or an HCLF calorie-restrictive diet is associated with significant weight loss during 8 weeks, with greater weight loss for the LCHF diet.
• Mood and cognitive function are both improved with an LCHF and an HCLF diet, but the HCLF diet is associated with greater improvement in speed of processing.

CME Test