For most of the past 50 years, medical and public health experts have instead embraced low-fat diets, concerned about the health effects of too much saturated fat on cardiovascular risk factors like LDL cholesterol. As a result, low-fat and fat-free foods have proliferated — many of them high in refined carbohydrates.
A clinical trial from Boston Children’s Hospital, one of the largest and most rigorous study of its kind, now challenges that thinking.
It demonstrated that low-carb diets — even though higher in saturated fat — produced better cardiovascular and metabolic profiles than low-fat, higher-carb diets.
Carbohydrates, insulin resistance and disease
While high LDL cholesterol is the traditional risk factor for heart disease, a group of other risk factors is increasingly being tied to both heart disease and diabetes: high triglycerides, low HDL (“good”) cholesterol, high blood pressure, high blood sugar, chronic inflammation, a tendency toward blood clotting and fatty liver.
These factors are hallmarks of metabolic syndrome, also known as insulin resistance syndrome, because the body’s cells lose their sensitivity to signals from insulin to remove sugars from the blood. Mounting evidence implicates increased consumption of carbohydrates, especially highly processed carbs like refined grains and added sugars.
As people switch to low-fat diets, carbohydrates make up more and more of what they eat. This is one reason why metabolic syndrome is rising — while obesity remains an epidemic.
Comparing low-carb and low-fat diets
The researchers wanted to test the idea that a low-carb diet would improve people’s cardiometabolic risk profiles as compared with a low-fat diet.
In partnership with Framingham State University, they enrolled 164 adults with overweight or obesity who had lost 10 t 14 per cent of their body weight on a reduced-calorie diet.
The participants then followed one of three weight-loss maintenance diets for five months, assigned at random:
- Low-carb diet (20 percent carbs, 60 percent fat, 20 percent protein)
- Moderate-carb diet (40 percent carbs, 40 percent fat, 20 percent protein)
- High-carb diet (60 percent carbs, 20 percent fat, 20 percent protein)
All participants received fully prepared, customized meals that they could eat in cafeterias or take to go. This protocol ensured that they stuck to the diets, unlike many other studies that just give people nutritional guidelines.
The meals were designed to keep participants at the same weight throughout the five months, so that everything observed during this study would be independent of weight loss.
In all three diets, 35 percent of the fat consumed was saturated fat. This meant that the low-carb diet had three times the saturated fat of the high-carb diet (21 vs. 7 percent), well above the range of current recommendations.
But of note, the low-carb diet was not restrictive very-low-carb diet. There was room for whole fruits, beans and small amounts of grains.
The benefits of restricting carbs
As compared with higher-carb, lower-fat diets, the low-carb diet improved the profiles of a range of blood lipids related to cardiovascular disease and insulin resistance. It also increased adiponectin, a hormone made by fat cells that promotes sensitivity to insulin and protects against atherosclerosis (the formation of fatty plaques in the arteries).
The low-carb diet also reduced blood levels of lipoprotein(a), a risk factor for atherosclerosis, heart disease and stroke that previously was not thought to be influenced by diet.
Specifically, lipoprotein(a) fell by an average of nearly 15 percent with the low-carb diet, versus a 2 percent reduction with the moderate-carb diet and a slight increase of 0.2 percent with the high-carb diet.
Refined carbohydrates — not excess calories — have been implicated in fueling the obesity epidemic. Foods like white bread, white rice, most breakfast cereals and highly-processed snack foods cause spikes in blood sugar and insulin that slow metabolism, increase hunger, and set the stage for weight gain, the authors write.
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