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Data presented at the World Heart Federation’s World Congress of Cardiology & Cardiovascular Health 2016 (WCC 2016) in Mexico City may radically change our perspective on how carbohydrates and different types of fats affect blood cholesterol and other lipid biomarkers. The presentation was based on data from the Prospective Urban Rural Epidemiological (PURE) study. The data have not been published yet, and the results are only available in an abstract (1).
The study, presented by researchers from McMaster University, Hamilton, Canada, addresses how carbohydrate and fat intake affected blood lipid profiles in 145.000 individuals living in nineteen low- to high-income countries. The researchers conclude that the message to reduce the intake of saturated fats for the purpose of lowering cholesterol and thus decrease risk of cardiovascular disease may be misleading
For decades, blood cholesterol was assumed to be a robust surrogate marker to predict the risk of heart disease. A reduction in saturated fats is recommended to reduce cholesterol levels, and carbohydrates are placed at the bottom of the food pyramid, mainly because they tend to lower cholesterol levels. Public health authorities have recommended that 60 percent of daily calories should come from carbohydrates.
However, the low-fat, high-carbohydrate approach has recently been challenged, and the authors of the recent study point out that there are no data from low and middle-income countries where more than 80 percent of cardiovascular disease occurs.
The PURE Data on Nutrition and Lipids
The goals of the study were to describe the association between nutrient intake and blood lipids and to examine the effect of iso-caloric replacement of nutrients on blood lipids.
The habitual food intake of 145,275 participants in 19 high, middle and low-income countries who were enrolled in the PURE study was prospectively measured using validated food frequency questionnaires.
The lipid biomarkers addressed in the study:
- Total cholesterol (TC)
- LDL cholesterol (LDL-C)
- HDL cholesterol (HDL-C)
- Triglycerides (TG)
- Apolipoprotein A (ApoA)
- Apolipoprotein B (ApoB)
The macronutrients addressed in the study:
- Saturated fatty acids (SFA)
- Monounsaturated fatty acids (MUFA)
- Polyunsaturated fatty acids (PUFA)
Higher carbohydrate intake was associated with lower TC and LDL-C but also with lower HDL-C and ApoA levels, leading to higher TC/HDL-C and ApoB/ApoA ratios and higher TGs. The apoB/apoA ratio has repeatedly been shown to be a better marker of risk than lipids, lipoproteins and lipid ratios (2)
A higher intake of SFAs was associated with higher LDL-C and lower TG levels. Higher MUFA intake was associated with lower TC, LDL-C, and higher ApoA. Higher PUFA intake was associated with lower TC and LDL-C and paradoxically higher ApoB level.
Iso-caloric replacements of carbohydrates with SFAs increased TC by 3%, LDL-C by 5% and HDL-C by 1% and decreased TG by 5%. Replacement of carbohydrates with MUFA led to a 2% decrease in LDL-C, 3% decrease in TC/HDL-C ratio, and 1% decrease in ApoB/ApoA ratio. Replacing carbohydrates with PUFAs was associated with little change in lipid markers.
The authors concluded that higher carbohydrate intake has the most adverse impact on lipid profiles and replacing it with saturated fat improved HDL-C and TG and replacing it with MUFAs improved TC/HDL-C and ApoB/ApoA.
“These data from a large global study indicate that guidelines on dietary fats and carbohydrates require re-evaluation.”
The Bottom Line
Public health authorities, including the American Heart Association (AHA) and the World Health Organization (WHO) recommend that 60% of calories should come from carbohydrates and only 5% to 6% of calories from saturated fat.
In the above study, the only benefit of a high carbohydrate diet was a lowering of TC and LDL-C. However, the effect on other lipid biomarkers such as HDL-C, TG, and ApoB/ApoA ratio may be harmful.
A diet rich in SFAs raised TC and LDL-C but lowered TG while a diet rich in MUFAs improved all lipid biomarkers. A diet high in PUFAs had a mixed effect on lipid biomarkers.
The study suggests that placing carbohydrates at the bottom of the food pyramid based on their effect on blood cholesterol was a mistake. In fact, the data show that replacing dietary carbohydrates with different types of fat may improve lipid profile.
In an interview on Medscape, Dr. Mahshid Dehghan, the principal author of the abstract said (3):
To summarize our findings, the most adverse effect on blood lipids is from carbohydrates; the most benefit is from consumption of monounsaturated fatty acids; and the effect of saturated and polyunsaturated fatty acids are mixed. I believe this is a big message that we can give because we are confusing people with a low-fat diet and all the complications of total fat consumption, and WHO and AHA all suggest 55% to 60% of energy from carbohydrates.
Today, most experts agree that diets high in SFAs or refined carbohydrates are not be recommended for the prevention of heart disease. However, it appears that carbohydrates are likely to cause a greater metabolic damage than SFAs in the rapidly growing population of people with metabolic abnormalities associated with obesity and insulin resistance.
I assume we all agree that partially hydrogenated fats (trans-fats) should be avoided. However, the singular focus on reducing the intake of SFAs, and dietary fats in general, may have been counterproductive and promoted the rapidly growing popularity of refined carbohydrates. The nutritional data from the PURE study clearly suggest that it is time to shift our focus away from reducing fat in our diet towards reduced consumption of carbohydrates.