A diet with almost a third of its calories from saturated fat raised LDL cholesterol in a two-year trial, yet the blood fats doctors also worry about went down

Saturated fat is the nutrient most heart guidelines tell us to limit, mainly because it pushes up LDL cholesterol. Fans of low-carb eating argue that LDL is only part of the picture. A two-year trial from Bergen, Norway, published on 24 October in the journal eBioMedicine, put both ideas to a hard test by giving one group a diet far richer in saturated fat than most low-carb studies have tried.

The CARBFUNC trial randomly assigned 193 men and women aged 20 to 55 with obesity to one of three diets with the same calories and protein. Two were higher-carbohydrate diets in line with usual advice (45% of energy from carbohydrate, 10 to 12% from saturated fat): one built on flour-based foods such as wholegrain bread and pasta, the other on intact grains, beans and unpolished rice. The third was a low-carbohydrate, high-fat diet planned at 8% carbohydrate and 75% fat, with 30% of energy from saturated fat. Food records showed the low-carb group really did eat that way, with carbohydrate at 10 to 15% of energy and saturated fat at 30 to 31%.

LDL cholesterol rose on the low-carb diet. At nine months it was up 12.8% from the start, while it fell 8.5% on the flour-based diet. But other blood fats moved the opposite way. HDL cholesterol rose 13.5% by twelve months on the low-carb diet, against 5.6% on the flour-based diet. At two years, triglycerides were down 20.4% and remnant cholesterol, carried in the triglyceride-rich particles that are also linked to heart disease, was down 23.3%, while on the flour-based diet they barely changed. The extra LDL was mostly in larger particles, and apolipoprotein B, a count of the particles that can lodge in artery walls, did not change significantly on the low-carb diet. Weight loss was similar on all three diets, 5 to 7% over the first year, and the two carbohydrate diets did not differ consistently from each other.

The big caveat is who was left. Only 30 of the 192 people who stayed in the trial after randomisation finished all two years, and blood samples at two years came from just 9 or 10 people per diet. Self-rated adherence fell to 40 to 60% in the second year. These blood-fat results were secondary outcomes, many markers were tested, and nobody was followed long enough to count heart attacks. Artery calcium scans at 6 and 12 months showed no overall worsening, but most volunteers had no calcium to begin with. The authors call the pattern a possible sign of faster fat turnover that needs more study; it is not proof that a high saturated fat diet is safe for the heart.