“An over-emphasis on low-fat diets has likely contributed to the obesity epidemic in the US by encouraging an over-consumption of foods high in carbohydrates.”
Overweight and obese people looking to drop some pounds and considering one of the popular low-carbohydrate diets, along with moderate exercise, need not worry that the higher proportion of fat in such a program compared to a low-fat, high-carb diet may harm their arteries, suggests a pair of new studies by heart and vascular researchers at Johns Hopkins School of Medicine.
“Overweight and obese people appear to really have options when choosing a weight-loss program, including a low-carb diet, even if it means eating more fat,” says lead investigator Kerry Stewart, EdD, an exercise physiologist.
His team’s latest analysis is believed to be the first direct comparison of either kind of diet’s effects on vascular health, using the real-life context of 46 people trying to lose weight through diet and moderate exercise.
The research was prompted by concerns of people who wanted to try one of the low-carb, high-fat diets, such as Atkins, South Beach or Zone, but were wary of these diets’ higher fat content.
In the first study, to be presented June 3 at the annual meeting of the American College of Sports Medicine in Denver, the Hopkins team studied 23 men and women, weighing on average 218 pounds and participating in a six-month weight-loss program that consisted of:
• Moderate aerobic exercise and lifting weights,
• Plus a diet made up of no more than 30% of calories from carbs such as pastas, breads and sugary fruits,
• With as much as 40% of their diet coming from fats from meat, dairy products and nuts.
After shedding an average of 10 pounds on this diet at the 45-day mark, the low-carb group showed no change in two key measures of vascular health:
• Finger tip tests of how fast the inner vessel lining in the arteries in the lower arm relaxes after blood flow has been constrained and restored in the upper arm (the so-called reactive hyperemia index of endothelial function),
• And the augmentation index, a pulse-wave analysis of arterial stiffness.
By comparison, another group of 23 people randomly assigned to a low-fat diet took on average 70 days – a month longer – to lose 10 pounds.
The low-fat group’s diet consisted of no more than 30% from fat and 55% from carbs.
“Our study should help allay the concerns that many people who need to lose weight have about choosing a low-carb diet instead of a low-fat one” in terms of immediate risk to vascular health, says Dr. Stewart. “More people should be considering a low-carb diet as a good option.”
While the effects of eating low-carb, higher-fat diets versus low-fat, high-carb options over a longer period of time remain unknown, he does contend that an over-emphasis on low-fat diets has likely contributed to the obesity epidemic in the United States by encouraging an over-consumption of foods high in carbohydrates.
• High-carb foods are, in general, less filling, he says,
• And people tend to get carried away with how much low-fat food they can eat.
More than half of all American adults are estimated to be overweight, with a body mass index, or BMI, of 26 or higher; a third are considered to be obese, with a BMI of 30 or higher.
Dr. Stewart says the key to maintaining healthy blood vessels and vascular function seems - in particular, when moderate exercise is included - is less about the type of diet and more about maintaining a healthy body weight without an excessive amount of body fat.
A Preliminary Study, the McDonald’s 'Meal Challenge'
Among the researchers’ other key study findings, to be presented separately at the conference, was that consuming an extremely high-fat McDonald’s breakfast meal, consisting of two English muffin sandwiches, one with egg and another with sausage, along with hash browns and a decaffeinated beverage, had no immediate or short-term impact on vascular health.
Study participants’ blood vessels were actually less stiff when tested four hours after the meal, while endothelial or blood vessel lining function remained normal.
Researchers had added the McDonald’s meal challenge immediately before the start of the six-month investigation to separate any immediate vascular effects from those to be observed in the longer study. They also wanted to see what happened when people ate a higher amount of fat in a single meal than recommended in national guidelines.
Previous research had suggested that such a meal was harmful, but its negative findings could not be confirmed in the Johns Hopkins’ analysis.
The same meal challenge will be repeated at the end of the six-month study, when it is expected that participants will still have lost considerable weight, despite having eaten more than the recommended amount of fat.
“Even consuming a high-fat meal now and then does not seem to cause any immediate harm to the blood vessels,” Dr. Stewart concludes - though he strongly cautions against eating too many such meals because of their high salt and caloric content.
He says this single meal - at over 900 calories and 50 grams of fat - is at least half the maximum daily fat intake recommended by the American Heart Association and nearly half the recommended average daily intake of about 2,000 calories for most adults.
All study participants were between ages 30 and 65, and healthy, aside from being overweight or obese.
Source: Johns Hopkins School of Medicine news release, June 1, 2011