Are High Carb Diets Bad for Your Heart?

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Yet another study by Stanford University researchers has just been published which tries to show that a diet higher in carbohydrate and lower in unsaturated fat increases the risk of atherosclerosis.1 This study fed 8 healthy subjects a high-carbohydrate diet for two weeks then switched and fed them a high fat diet for 2 weeks. Blood lipids were tested after both diets and researchers found on the higher carbohydrate diet that:
• HDL or "good cholesterol" was lower
• Fasting triglycerides (fat in the blood) was higher
• RLP or remnant lipoprotein particles were higher. These artery cloggers are usually associated with higher triglycerides
The authors concluded, based on this and earlier studies with a similar experiment design,  that a high unsaturated fat diet is better than a high carbohydrate diet for improving blood lipids and preventing atherosclerosis. However, such a conclusion is in direct conflict with a growing body of research which proves that very high carbohydrate diets actually unclog heart arteries. To better understand the relationship between blood lipids, heart disease and diet, lets take a closer look at how a faulty experiment design can produce very misleading data.
When Lower HDL is Not Harmful
HDL often drops initially when dietary fat is replaced with carbohydrate. The primary function of HDL, simply put, is to carry excess cholesterol in the arteries back to the liver for disposal. While you would think that a lower HDL would mean less cholesterol being transported back to the liver, this is not always the case. For example it has been shown that HDL transports cholesterol back to the liver more efficiently on a very low-fat diet than it does on a high-fat diet.2 So a low HDL on a high-fat Westernized diet is a real concern. However a drop in HDL when a high-carbohydrate diet is first adopted should be of little concern.
The Body Takes Time to Adapt
It appears to take some time for the body to adapt to a higher carbohydrate intake. A group of 50 older women were placed on a low-fat, high-carbohydrate diet consisting largely of natural high-carbohydrate foods such as beans, vegetables, fruit and whole grains.3 During the first 4 months of this study, dietary carbohydrate gradually replaced dietary fat in their diets calorie for calorie. The subject's fasting triglyceride levels rose from 151 to 204.
The subjects continued to consume the same high carbohydrate, low-fat diet for another 8 months. But now they ate until they were full instead of being required to eat more to prevent weight loss. Because the subjects now felt full on fewer calories, they lost weight consuming the very low-fat diet. During this period their average triglyceride levels dropped to what they were initially.
Short Term Studies Miss Long-Term Improvements
A group of men with high cholesterol were placed on a very low-fat, near-vegetarian diet with unrestricted calorie intake (they ate until they were full). Their LDL fell 41%, fasting triglycerides dropped 15% while HDL remained about the same. After three months, they had lost 16.5 pounds while eating as much as they wanted. This study shows that a low-fat, high-fiber diet dramatically improves blood lipids.4
Fasting Triglycerides Can Be Misleading
Triglyceride levels can be measured two ways. Fasting triglyceride levels are obtained when subjects have not eaten for 10-12 hours. Postprandial triglyceride levels occur after eating. Most people spend most of their lives in a "postprandial state," so a change in blood lipids after meals may play a major role in the growth of atherosclerotic plaque. Increased fasting and postprandial triglyceride levels are associated with more potentially atherogenic RLP. Stanford researchers reported a lower HDL and higher fasting and postprandial triglycerides on their lower fat diet.
However a study by Tufts researchers which examined the impact of feeding either a very low-fat diet or a moderate fat diet on fasting and postprandial triglyceride levels is instructive. This study showed that a higher carbohydrate diet leads to much higher fasting triglycerides and higher postprandial triglycerides than a moderate fat diet when both diets were fed at the same calorie level (isocaloric). But when Tufts researchers allowed the subjects to eat until they felt full (ad libitum) postprandial triglycerides are much lower than on the moderate fat diet. In the figure below, you can see the fasting triglyceride level was still a little higher on the very low-fat diet than the moderate fat diet, but postprandial triglyceride levels during the day were now much higher on the moderate fat diet.
If most people consumed a very low-fat diet with little animal protein, consisting mostly of natural foods, they would improve blood lipids, lose weight and reduce the risk of heart disease.
Dr. James J. Kenney, PhD, RD, FACN, is the Director of Research for the Pritikin Longevity Center. He is Board Certified as a Specialist in Human Nutrition by the American Board of Nutrition.
For a comprehensive review of the research on the pros and cons of a very low-fat, near vegetarian diet versus a high-MUFA Mediterranean-style diet see 21st Century Heart. This new kit has been approved for 20 hours of continuing education by ADA. It also contains compelling, easy-to-use client materials.
This article is also available in a full text version with references and is approved for 1 CPE?hour - see the insert. FMI call 800-462-2352 or see www.foodandhealth.com.
References:
1. Abbasi F Am J Cardiol 2000;85:45-8
2. Brinton EA J Clin Nutr 1999;85:144-51
3. Kasim-Karakas SE Metabolism 1997;46:431-6
4. Thuesen L Am J Clin Nutr 1986;44:212-9.

TG

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