Weight loss diets – a new study asks which are the best?

Many popular diets emphasize either carbohydrate, protein or fat as the best way to lose weight.

paleo-diet2Advocates of high protein diets claim that our Paleolithic ancestors obtained the majority of their calories from meat and thus our bodies have evolved to require a high protein intake.  There is much scientific controversy over the relative importance of animal and plant foods in the early hominid diet.  Direct evidence in the form of food remains is meagre or, at best, equivocal.  Most research relies on inference through dietary studies of other primates and archaeological evidence.  Most scientists now agree that plant foods contributed much more to the early hominid diet than did the flesh of animals. 

okinawa_diet_planHumans have adapted to their environments wherever they have settled and the balance between meat-eating and plant-eating varies substantially between populations.  Some of the leanest and healthiest societies in the world, such as in the Mediterranean and Japan, consume a diet where the majority of energy comes from carbohydrates, mainly in the form of complex carbohydrates from whole grains and vegetables.  This has led some researchers to propose that a high carbohydrate diet is better for maintenance of a healthy weight than a high protein diet.

Controversy about the role of fat in the diet has raged since the 1950s, when Ancel Keys published his landmark “Seven Countries” study and highlighted that coronary heart disease is strongly related to diet.  Low-fat diets have therefore been promoted by governments and health professionals for several decades.

With the prevalence of obesity increasing at an alarming rate, everyone wants to know which of these dietary approaches – high protein, high carbohydrate, low fat – is the most successful for weight loss.

obese-women

The scientific research conducted to date does not help much.  Some trials have shown that low-carbohydrate, high-protein diets resulted in more weight loss over the course of 3 to 6 months than conventional high-carbohydrate, low-fat diets, but other trials have not shown this effect.

A smaller group of studies that extended the follow-up to 1 year did not show that low-carbohydrate, high-protein diets were superior to high-carbohydrate, low-fat diets.  In contrast, other researchers found that a very-high-carbohydrate, very-low-fat vegetarian diet was superior to a conventional high-carbohydrate, low-fat diet.  Among the few studies that extended beyond 1 year, one showed that a very-low-fat vegetarian diet was superior to a conventional low-fat diet, one showed that a low-fat diet was superior to a moderate-fat diet, two showed that a moderate-fat, Mediterranean-style diet was superior to a low-fat diet, one showed that a low-carbohydrate diet was superior to a low-fat diet, and another showed no difference between high-protein and low-protein diets.

Small samples, underrepresentation of men, limited generalizability, a lack of blinded ascertainment of the outcome, a lack of data on adherence to assigned diets, and a large loss to follow-up limit the interpretation of many weight-loss trials.  The novelty of the diet, media attention, and the enthusiasm of the researchers may affect the adherence of participants to any type of diet.

There have been few studies lasting more than a year that evaluate the effect on weight loss of diets with different compositions of those nutrients. In a randomized clinical trial led by researchers at the Harvard School of Public Health (HSPH) and Pennington Biomedical Research Center of the Louisiana State University System, a comparison of overweight participants assigned to four different diets over a two-year period showed that reducing calories achieved weight loss regardless of which of the three nutrients was emphasized. The study, which was funded by the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health, appears in the February 26, 2009 issue of The New England Journal of Medicine.

“This is important information for physicians, dieticians and adults, who should focus weight loss approaches on reducing calorie intake,” said Frank Sacks, professor of cardiovascular disease prevention at HSPH and lead author of the study.

The trial included 811 men and women who were randomly divided into four diet groups with different target nutrient compositions:

  • Low-fat, average protein: 20% of calories from fat, 15% of calories from protein, 65% of calories from carbohydrate
  • Low-fat, high-protein: 20% fat, 25% protein, 55% carbohydrate
  • High-fat, average protein: 40% fat, 15% protein, 45% carbohydrate
  • High-fat, high-protein: 40% fat, 25% protein, 35% carbohydrate

The participants were diverse in age, sex (62% women, 38% men), geography and income. The diets followed heart-healthy principles, replacing saturated with unsaturated fat and were high in whole cereal grains, fruits and vegetables. Each participant received a diet prescription that encouraged a 750-calorie reduction per day, however none were less than 1,200 total calories per day. Participants were asked to do 90 minutes of moderate exercise each week. They recorded their daily food and drink intake in a food diary and in a web-based program that provided information on how closely they were meeting their dieting goals. Individual counselling was provided every eight weeks over two years and group sessions were held three out of four weeks during the first six months and two out of four weeks from six months to two years.

The results showed that, regardless of diet, weight loss and reduction in waist circumference were similar. Participants lost an average of 13 pounds at six months and maintained a 9-pound loss at two years. Weight loss primarily took place in the first 6 months; after 12 months, all groups began to slowly regain weight, a finding consistent with other diet studies. However, the extent of weight regain was much less, about 20%, of the average regain in previous studies. Waistlines were reduced by an average of two inches at the end of the two-year period.

Most risk factors for cardiovascular disease improved for dieters at six months and two years. HDL (“good”) cholesterol increased and LDL (“bad”) cholesterol, triglycerides, blood pressure and insulin decreased. The metabolic syndrome, a group of coronary heart disease risk factors including high blood pressure, insulin resistance and abdominal obesity, also decreased.

The main finding from the trial was that diets with varying emphases on carbohydrate, fat and protein levels all achieved clinically meaningful weight loss and maintenance of weight loss over a two-year period.

“These results show that, as long as people follow a heart-healthy, reduced-calorie diet, there is more than one nutritional approach to achieving and maintaining a healthy weight,” said Elizabeth G. Nabel, M.D., Director, NHLBI.

Another important finding was that participants who regularly attended counselling sessions lost more weight than those who didn’t. Dieters who attended two thirds of sessions over two years lost about 22 pounds of weight as compared to the average weight loss of 9 pounds.

“These findings suggest that continued contact with participants to help them achieve their goals may be more important than the macronutrient composition of their diets,” said Sacks.

fruit_and_veg11

Have you spent years embarking on every weight-loss diet going?  Have you tried cutting out entire food groups?  Have you spent a fortune on miracle foods or diet powders?  Have you eaten nothing except cabbage soup for weeks?  Have you driven your friends mad with your fervour over food combining?  Have you become obsessive about counting calories or points?  Have you spent hours jumping on and off your bathroom scales?  Do you feel hungry much of the time, exhausted and beset by cravings?

You can learn how to lose weight effortlessly without feeling hungry, whilst gaining health and vitality, at a Cooking for Health course on Managing Your Weight Naturally.  We explore why so many diets fail and explode many of the weight loss myths.  We look at cravings – how they arise and how to overcome them – and we discuss which foods the body needs to create energy and burn fat in the most efficient way.  We create a delicious meal with an array of different dishes designed to illustrate how it is possible to eat plenty without gaining weight.  The vital role of exercise in maintaining a healthy weight is also emphasised.

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One thought on “Weight loss diets – a new study asks which are the best?

  1. In the midst of menopause and diagnosed with hypothyroidism I have what many would call blocks to weightloss. Switching to a plant based diet and eliminating all processed foods has definately helped me to lose weight and increase my energy level. I fell better than I have in years and have lost that stubborn 15 lbs. You are not giving up anything you are expanding your options.

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