Tuesday, September 18, 2007

Low-carb Diet Not As Good As Higher-carb In Measure Of Cognitive Function

In my continued exploration of low-carb diets, I ran across the following study from this month's American Journal of Clinical Nutrition. It compared a low-carbohydrate, high-fat (LCHF) diet with a high-carbohydrate, low-fat (HCLF) diet - and their effects on mood and cognitive function:

Low- And High-Carbohydrate Weight-Loss Diets Have Similar Effects On Mood But Not Cognitive Performance

Study Basics
  • 93 overweight participants (mean BMI: 33 kg/m2) were randomly assigned to either a LCHF or HCLF diet for 8 weeks
  • LCHF provided 4% of energy as carb, 61% as fat
  • HCLF provided 46% of energy as carb, 30% as fat
  • Mean age of participants: 50.2 years
  • Diets were isocaloric: no difference in energy intake between the two groups
  • Women consumed about 6000 kj/day (1428 kcal)
  • Men consumed about 7000 kj/day (1667 kcal)
  • Both groups significantly reduced body weight:
    LCHF group lost an average of 7.8 kg (17.2 lbs)1
    HCLF group lost an average of 6.4 kg (14.1 lbs)

  • There was no significant difference between groups in psychological wellbeing. Both groups showed improvements in mood markers (tension, depression, anger, vigor, fatigue, and confusion).

  • There was a significant difference in cognitive function. Cognitive function, as measured by speed of processing, improved less in the low-carb group.

This study did not support the claim that low-carb diets uniquely result in elevation of mood. Mood improved the same regardless of carbohydrate intake.

I find it troubling that the low-carb diet resulted in less improvement in cognitive function. What's going on inside the bodies and brains of people eating fewer carbohydrates that caused this?

A Note On How Much Carbohydrate Constituted A Low-carb Diet

One medium apple (2 and 3/4 inch diameter), as shown in the photo, provides about 19 grams of carbohydrate. That's about 35% more carbohydrate than the women in the low-carb group in this study ate in an entire day. Most fruits have too much carbohydrate to be compatible with a low-carbohydrate diet.

If you ate just 2/3s of this apple, you could not eat any whole grains, beans, nuts and seeds, milk and other dairy foods, and most vegetables (including lettuce, spinach, and other greens) for the rest of that day, since all those foods contain carbohydrate and you would have already met your allotment.
1 The additional weight loss in the LCHF group may be explained by fluid loss associated with glycogen loss, since there were higher levels of ketones in the LCHF group.

Photo: Homegrown

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