Low-fat and low-carbohydrate diets can be equally effective at helping obese adults shed weight over the longer term, but cutting carbs may have an advantage when it comes to some heart risk factors, a study published Monday suggests.
Researchers say the findings offer reassurance that low-carb diets — which tend to be relatively high in fat — are not a threat to heart health.
Instead, the study found that over two years, people on the low-carb plan had a greater increase in “good” HDL cholesterol than those on the low-fat regimen. They also had a more significant dip in diastolic blood pressure, the bottom number in a blood pressure reading.
Still, that does not mean that everyone hoping to lose weight should go low-carb, according to lead researcher Dr. Gary D. Foster of Temple University in Philadelphia.
Both diets, he told Reuters Health in an interview, helped people shed pounds and improve their risk factors for heart disease. So the bottom line is that individuals should choose the diet changes that they can live with for the long haul, according to Foster.
“With either diet, you’re looking pretty good,” he said.
The caveat, though, is that everyone in the study adopted their diets as part of an overall program focused on lifestyle change.
They were instructed to start exercising regularly — mostly brisk walking — and learned tactics for weight management, such as writing down what they ate every day and setting reasonable short-term goals (if you normally eat 10 candy bars a week, for instance, first try cutting out a couple rather than going cold-turkey.)
And that behavioral shift may be key in helping people keep the weight off in the long run. Foster cautioned against “becoming myopically focused on what you should or shouldn’t eat.”
The study findings, which appear in the Annals of Internal Medicine, are based on 307 middle-aged obese adults who were randomly assigned to either a low-fat or low-carb diet, plus the lifestyle program.
People in the low-carb group followed an Atkins-style plan, strictly limiting carbohydrates for the first 12 weeks to 20 grams, or about 80 calories’ worth of carbs, per day – with vegetables as the only source. After that phase, they gradually added small amounts of carbs from certain fruits, grains and dairy. They were allowed unlimited amounts of fat and protein.
People in the low-fat group cut their calories to between 1,200 and 1,800 per day, depending on their sex and initial body weight, and aimed to get 55 percent of their calories from carbs, 15 percent from protein and 30 percent from fat.
Over the first year, the whole study group averaged a weight loss of about 22 pounds; after the second year, they’d managed to keep off about 15 pounds, on average. There was no significant difference between the low-fat and low-carb groups, Foster’s team found.
In the first six months of the study, the low-fat group had the edge when it came to “bad” LDL cholesterol. On average, their LDL fell by about 10 mg/dL, from a starting point of 124 mg/dL; LDL levels below 100 mg/dL are considered “optimal.”
In the low-carb group, LDL initially rose during the first three months — probably, according to Foster, because of participants’ newfound freedom to eat unlimited fat. But by year two, both diet groups had LDL levels that were several points lower than their starting numbers, with no significant difference between the groups.
Similarly, triglycerides (another type of blood fat) and systolic blood pressure (the top number in the reading) declined to a comparable degree in both groups.
When it came to HDL, the average for the low-carb group rose by nearly 8 mg/dL, from a starting point of 46 mg/dL; HDL levels below 40 mg/dL are considered a risk factor for heart disease, and ideally, levels should be at least 60 mg/dL.
HDL levels in the low-fat group rose by almost 5 mg/dL, on average, from a starting point of 45 mg/dL.
The low-carb group also had a modest advantage when it came to diastolic blood pressure, showing a three-point decline at year two, versus a half-point dip in the low-fat group.
Some side effects were more common among low-carb eaters. At the six-month mark, 45 percent reported hair loss, versus 21 percent of the low-fat group. After three months, nearly two-thirds said they had problems with bad breath, compared with 37 percent of the low-fat group.
The only persistent side effect, however, was constipation. After two years, 39 percent of the low-carb group reported constipation, versus 17 percent of the low-fat eaters. It’s not yet clear why the low-carb diet created a greater increase in HDL cholesterol, according to Foster. But the findings suggest that increased fat intake, itself, may be responsible.
However, whether that HDL advantage actually translates into greater heart-health benefits is unclear.
For now, Foster said, the message is that either a low-fat or low-carb diet can work in the longer term. He also noted that people do not necessarily have to join a formal program to learn the behavioral changes that can help them succeed, as the same type of information is available online and in books.
