Is a Low-Carb, High-Fat Diet a Heart-Healthy One?

One of the questions that comes up repeatedly for me -- from  colleagues as well as friends -- is whether following a low-carbohydrate, high-fat diet could be detrimental to heart health.   A chief concern is  that eating foods high in fat, particularly saturated fat, will raise cholesterol levels, thereby increasing the risk of atherosclerotic heart disease.  It doesn't really surprise me; after all, for more than 30 year  fats  have been considered  the primary food we should all be cutting back on if we want to avoid coronary artery disease.  The USDA's  Dietary Guidelines for Americans and  My Plate promote whole grains, nonfat milk, fruits, and other foods that are high in carbohydrates and low in fat as a way of decreasing cardiac risk.   On the other hand, there is a large body of research showing that lowering carb intake and  increasing consumption of fat (both saturated and unsaturated) can result in favorable changes in serum lipids.  

Below are a some of the cardioprotective benefits of low-carbohydrate, moderate-protein, high-fat diets:

1. Significant decrease in serum triglycerides.  Carbohydrates are a potent stimulator of  hepatic triglyceride synthesis and  plasma concentration, particularly in the presence of insulin resistance.  Lowering carbohydrate intake can reduce triglyceride levels, resulting in lower cardiac risk.
2. Increase in HDL cholesterol.  Higher fat intake is positively correlated with improvements in HDL levels, and  high HDL cholesterol is considered cardioprotective.
3. Improvement in LDL particle size, glycation, and oxidation.  While triglycerides levels almost invariably decline with carbohydrate restriction,  LDL cholesterol response appears to be more individualized.  LDL has been classified as the "bad"  cholesterol for years, and elevated levels are often  seen as  increasing one's  risk of arterial plaque formation and heart disease. However, simply looking at the amount of serum LDL itself gives us very little information about cardiac risk.  It is primarily when LDL is oxidized and its particle size  small that this lipoprotein becomes most problematic.  Restricting carbohydrate intake has been shown to reduce glycation and  subsequent oxidation of LDL.  A lower-carb, higher-fat diet tends to produce an increase in LDL particle size (known as Pattern A), whereas an abundance of  dietary carbohydrate typically results in smaller, denser particles (Pattern B) that increase the likelihood of atherosclerosis. 

I also often hear, "If  people don't eat whole grains and legumes, how can they consume adequate fiber?"  Fiber, particularly the soluble type, has many health benefits.  A  low-carb diet can easily supply sufficient fiber if it contains plenty of nonstarchy  vegetables, nuts and nut butters, seeds, berries, and avocados.  Technically a fruit, an average avocado contains about 12 grams of fiber, as well as 16 grams of monounsaturated fat. 

As a registered dietitian, I can't endorse a low-carbohydrate diet consisting of 6 eggs fried in butter with 4 slices of bacon for breakfast, 3 hamburger patties for lunch, and a 20-oz steak with a tiny green salad for dinner.  While certainly nearly carb-free, it's missing a lot of beneficial phytochemicals found only in plant foods and contains only a couple of grams of fiber.  But I firmly believe that a carbohydrate-restricted plan that includes the high-fiber plant foods listed above can be a very heart-healthy way to go.  

Resources
1.   Tay, J., et al.  Metabolic effects of weight loss on a very-low-carbohydrate diet compared with an isocaloric high-carbohydrate diet in abdominally obese subjects.  J Am Coll Cardiol,  2008. 51:59-6
2. Volek, J.S., et al. Dietary carbohydrate restriction induces a unique metabolic state positively affecting atherogenic dyslipidemia, fatty acid partitioning, and metabolic syndrome. Prog Lipid Res, 2008, doi: 10.1016/j.plipres.2008.02.0033. 
3.  Hayek, T, et al. Dietary fat increases high density lipoprotein (HDL) levels both by increasing the transport rates and decreasing the fractional catabolic rates of HDL cholesterol ester and apolipoprotein (Apo) A-I. J Clin Invest, 1993; 91(4);1665-16714. 
4.  Siri-Tarino, P.W., et al. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. Am J Clin Nutr doi: 10.3945/ajcn.2009.27725