No matter what popular diet you go on, you’re likely to lose at least some weight and see your blood pressure drop by the end of six months.
But you’re also likely to see those lost pounds return — and your blood pressure slide back up — within another six months.
That’s the reality-check offered by a new international review of dozens of previous studies of popular diets. The review was published last week in The BMJ (formerly known as the British Medical Journal).
“Differences between diets are … generally trivial to small, implying that people can choose the diet they prefer from among many of the available diets without concern about the magnitude of benefits,” the study’s authors conclude.
That finding underscores a message that health experts (those not trying to sell a particular book or diet program) have been trying to get us to heed for years: The best “diet” is the one you can maintain for the rest of your life.
Each year, almost half of American adults try to lose weight, including two-thirds of the 40 percent of adults who have obesity. In their attempt to shed those pounds, they spend an estimated $72 billion annually on weight-loss diets, products and services.
Comparing the diets
For the review, a team of researchers from the United States, Canada, China, Switzerland and Iran analyzed data from 121 prior studies involving more than 21,000 people. The average age of the studies’ participants was 49, and their average body mass index (BMI) was 33. (A BMI of 30 or higher is considered obese.)
The studies were all randomized controlled clinical trials — ones in which participants were assigned to follow a particular popular diet or to continue their usual eating habits. Sometimes (although rarely) two popular diets were pitted against each other.
A total of 14 different popular diets were investigated. The review’s authors divided the diets into three categories based on the general make-up of their “macronutrients” — in other words, on whether they were low in carbohydrates, low in fat, or something in between:
- Low carb: Atkins, South Beach, Zone
- Low fat: Ornish, Rosemary Conley
- Moderate macronutrients (similar to low fat, but with slightly more fat and slightly less carbohydrates): Biggest Loser, DASH, Jenny Craig, Mediterranean, Portfolio, Slimming World, Volumetrics, Weight Watchers
The data showed that, compared with the participants’ usual diet, all three types of popular diets led to a similar modest reduction in weight — an average of about 8 to 12 pounds — after six months. Blood pressure also dropped modestly, an average of about 5 mm Hg for systolic blood pressure and about 3 mm Hg for diastolic blood pressure.
The diets that appeared to have the largest effect on both weight loss and blood pressure compared with the participants’ usual diet were Atkins, DASH and Zone. But at the 12-month follow-up period, the improvements had all but disappeared for people who had been on those three diets as well as for those on all the others.
None of the diets appeared to result in significantly improved levels of HDL cholesterol or C-reactive protein at 12 months, and only one — the Mediterranean diet — did so at six months. (Higher levels of HDL cholesterol and lower level of C reactive protein are associated with a lower risk of heart disease.)
Limitations and implications
The primary limitation of this review, say its authors, is that the 121 studies used for the analysis were rife with “inconsistency and imprecision” and a “risk of bias.” Few directly compared different diets, a factor that contributes to bias. Another common problem with the studies was a lack of reporting on how well participants adhered to the different diets.
Still, the review’s findings are in line with previous analyses of the claims made by various popular diets. Those analyses led the American Heart Association, the American College of Cardiology and the Obesity Society to jointly state in 2013 that the evidence was too inconclusive to recommend any particular diet.
The review’s findings should cause all of us to resist the marketing hype and overstated claims that surround many popular diets.
“The extensive range of popular diets analysed provides a plethora of choice but no clear winner,” write Helen Truby and Terry Haines, professors of nutrition and primary health care at Monash University in Australia, in an editorial that accompanies the review. “So conversations should shift away from specific choice of diet and focus instead on how best to maintain any weight loss achieved.”
“Although it is scientifically interesting to explore the distributions of macronutrient in diets, we eat foods, not nutrients,” they add.
Instead of focusing on the latest dietary trend — including getting dragged into the ongoing low-fat versus low-carb feud — we should be following basic healthy eating habits. As Truby and Haines stress, that means consuming “more vegetables, legumes and whole grains and less sugar, salt, and alcohol.”
You don’t need to buy a diet book to do that.