Not Too Much, But Not Too Little Cholesterol?

Summarized by Robert W. Griffith, MD
December 10, 2004

Introduction

Cholesterol levels can't be too low, can they? Well, that depends who you ask. The new cholesterol guidelines suggest that getting the LDL ('bad') cholesterol level to 70 mg/L (1.8 mmol/L) or below would be ideal. However, there may be a risk in going too low, at least in younger people, as a study reported in the American Journal of Clinical Nutrition has pointed out. Some neurological reaction speeds are reduced. We summarize the findings here.

What was done

Some of the participants in the third National Health and Nutritional Examination Survey (1988-1994) or NHANES III, were given neurobehavioral and performance tests. Participants in an equal-sized group were given allergy tests, and acted as controls. People who were pregnant, had a history of cancer or stroke, or were taking part in a cholesterol-lowering program (i.e. on cholesterol-lowering medication) were excluded.

Fasting serum cholesterol levels (total, LDL, and HDL), blood sugar, and blood pressure were measured. The neurobehavioral tests included:

Visuomotor speed: The subjects had to press a button as soon as they saw a solid square on a computer screen. The time between when the square appeared and when the button was pressed was measured; this represented the rate at which nerve signals traveled from the eye, via different brain areas, to the finger.

Coding speed: Subjects had to pair 1 of 9 different symbols on a screen with one of the digits 1 to 9, according to a key given in a separate part of the screen. The time taken was a measure of the speed of a certain sort of mental functioning.

Learning & immediate memory: A series of digits were displayed one at a time on a screen. There were 8 digits. The subject was asked afterwards to enter the digits in the same order on the computer. A low number of errors (incorrect numbers or incorrect order) represented better learning and memory.

What was found

Participants were selected whose age ranged from 20 to 59. The average age of the roughly 2000 men and 2000 women was 37. About 15% of the men and 22% of the women were physically inactive; 13% of the men and 6% of the women had raised blood pressure. Total, HDL and LDL cholesterol levels averaged 194, 47, and 147 mg/L in men, and 191, 55, and 137 mg/L in women.

The results of the neurobehavioral tests varied with age and level of education (slower times/more errors were seen in older people and those with fewer years of education). Regular leisure-time physical activity improved the results, while lower calorie intake resulted in impaired memory and coding speeds.

Adjustments were made for age, background data, calorie intake, micronutrient intake (e.g. vitamin E, beta-carotene) and known risk factors for cardiovascular disease. Then analyses were done to see if cholesterol levels influenced the test results.

In the men, low serum total cholesterol and low LDL cholesterol levels were associated with slow visuomotor speed. There were no such associations with coding speed or learning/memory tests. And in women, there were no such associations for any of the three tests.

What this means

The results of this study are consistent with those from other studies in animals and humans. It should be noted that the participants in this study weren't taking cholesterol-lowering drugs, so it's not another 'statin-effect'. Although cholesterol is a component of lipoprotein particles that carry phospholipids and micronutrients, this cannot be a factor, as adjustments were made for micronutrient intake.

The actual differences in visuomotor speeds between low- and high-cholesterol men were quite small. However, the authors speculate that they could be relevant in view of the efforts being made to get people to reduce their cholesterol levels. In the last 10 years there have been reports of increased accidental deaths in persons with low serum cholesterols.

The main value of this study doesn't lie in a call for everyone to check their cholesterol levels and stop their healthy lifestyle practices if the levels are low. Rather, it provides a strong reminder that nothing in nature is black or white; for every move in one direction, a benefit may be accompanied by a 'side-effect', or an ill-effect by a hidden benefit. No-one said understanding patho-physiology is easy!

Source

• Serum cholesterol concentrations are associated with visuomotor speed in men: findings from the third National Health and Nutrition Examination Survey, 1988-1994 J. Zhang, MF. Muldoon, RE. McKeown, Am J Clin Nutr, 2004, vol. 80, pp. 291--298

Related Links
Even Newer Cholesterol Guidelines
Cholesterol Linked to Memory Skills in Seniors
Treating Your Raised Cholesterol Level
Can Your Blood Cholesterol Be Too Low?

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