What happens when people eat much more saturated fat than dietary guidelines recommend? A new study found that the answer isn't as simple as watching one cholesterol number go up or down.
In a Norwegian trial, adults following a very low-carbohydrate diet high in saturated fat saw improvements in several blood markers associated with heart health. Their triglycerides dropped and their HDL, often called good cholesterol, increased. But their LDL, or bad cholesterol, also rose.
The findings, published in eBioMedicine, offer a closer look at how different diets affect cholesterol and other fats in the blood. They don't establish that eating more saturated fat lowers heart disease risk, particularly because so many participants left the study before it ended.
Researchers at the University of Bergen assigned 193 adults with obesity to one of three diets. Two included more carbohydrates, either from foods such as bread and pasta or from less processed choices such as intact whole grains and beans.
The third diet was very different. It provided about 75% of daily calories from fat, including 30% from saturated fat, while keeping carbohydrates below 10%. Most dietary guidelines recommend limiting saturated fat to less than 10% of daily calories.
All three diets were designed to provide similar amounts of calories and protein, with little added sugar and plenty of vegetables.
During the first several months, the high-fat group experienced some notable changes. Triglycerides, a type of fat in the blood, fell by roughly 20% to 35%. HDL cholesterol also increased. Both changes are generally considered favorable.
But LDL cholesterol rose by approximately 5% to 13%, depending on when it was measured.
Researchers looked more closely at that increase and found that it mainly involved larger cholesterol-carrying particles. Another measurement, called apoB, which helps estimate the number of particles that can contribute to plaque buildup in arteries, didn't rise significantly.
In other words, the amount of LDL cholesterol increased without a clear increase in the number of potentially harmful particles.
That may help explain why the results appeared mixed, but it doesn't mean the rise in LDL cholesterol was harmless. Researchers acknowledged that it could still matter for long-term heart health.
Participants in all three groups lost similar amounts of weight during the first year, and the diets produced comparable improvements in how their bodies responded to insulin.
The researchers also checked for calcium buildup in the arteries supplying the heart. They found no clear differences between the diets over 12 months. However, most participants had no detectable buildup at the beginning, and the study wasn't long enough to establish whether any of the diets changed the risk of heart disease.
There was another major limitation: Most participants didn't complete the trial.
Of the 193 people originally assigned to a diet, only 57 remained at one year and just 30 completed the full two years. Participants also reported finding it increasingly difficult to stick with their assigned diets.
That makes the longer-term findings much less certain.
It's also important to understand that the high-fat group wasn't simply eating more butter or other sources of saturated fat. Participants replaced much of their usual carbohydrate intake with fat, so researchers couldn't determine whether the changes came from eating more fat, fewer carbohydrates or both.
The study adds to evidence that different eating patterns can affect cholesterol measurements in different ways. But it doesn't show that a diet exceptionally high in saturated fat is safer for the heart or better than other approaches to healthy eating.
The research was supported by the Trond Mohn Research Foundation, the University of Bergen, Sweden's Kostfonden and the Norwegian Health Association. One author disclosed serving as a reviewer for a Novo Nordisk Foundation nutrition grant program.