By Susan M. Kleiner, PhD, RD, FACN, CNS, FISSN
The holiday season has come and gone, and your New Year’s resolutions are in effect. Whether you’re looking to melt off a few pounds in the next few weeks or you’d like to lose the extra unhealthy pounds you’re carrying around throughout this next year, the question of which diet really works is probably on your mind. The same question has been on the minds of nutrition scientists for decades, but these days we are much closer to an answer than ever before. The good news is that several different diets work pretty well. The key is choosing the right one for you.
The Science Behind the Diets
It’s not rocket science to know that if you want to lose weight fast, you cut out the carbs from your diet. But the right diet for losing weight AND keeping it off has been far less clear. Many diets have had their champions, from the low-fat, low-calorie strategy touted for the last two decades of the last century, to the 1970’s retro Atkin’s-style low-carbohydrate diet that was reborn at the beginning of this century. The Mediterranean-style diet has more recently grabbed the limelight, as has the latest version of the low-carb diet: an updated green cousin that is less restrictive and encourages the inclusion of plant foods.
Three recently-published studies have helped elucidate the problem of which diet leads to greater weight loss, long-term health and weight maintenance. A multicenter trial examined the bodyweight and metabolic outcomes after two years on a low-carbohydrate versus a low-fat diet. Unlike previous studies, this study included comprehensive behavioral treatment with long-term intervention. Subjects were randomly assigned to either the low-carbohydrate diet or the low-fat, calorie-restricted diet. All participants received comprehensive, in-person group behavioral treatment weekly for 20 weeks, then every other week for the next 20 weeks, and then every other month for the remainder of the two-year study. Behavioral treatment included behavior modification tips and techniques, and a prescribed exercise program that began at Week 4 with walking 20 minutes, 4 times per week and progressed each week by 19 to 50 minutes, 4 times per week. All subjects were instructed to take a daily multivitamin supplement provided by the study.
The low-carbohydrate diet limited carbohydrate consumption but allowed unrestricted intake of fat and protein. Participants were instructed to restrict carbohydrate to 20 grams per day of low-glycemic index vegetables during the first 12 weeks of the study. After this period, carbohydrate intake increased by 5 grams per day per week until the subjects reached their desired and stable weights. This was achieved by adding back a limited amount of fruits, whole grains and dairy products. The overall gram amount or percentage of total dietary calories contributed by carbohydrates after the initial 12 weeks was not reported.
The low-fat diet was also limited in energy consumption to 1,200 to 1,500 calories per day for women and 1,500 to 1,800 calories per day for men. Approximately 55 percent of calories came from carbohydrate, 30 percent from fat and 15 percent from protein. The dietary focus was on limiting calories and decreasing fat intake.
Both groups lost the same amount of weight: 24.2 pounds (11 percent) at one year, and a final total of 15.4 pounds (7 percent) at two years. Except for consistent improvements in HDL-cholesterol levels in the low-carb dieters, no other health parameters were significantly different at the end of two years. The researchers conclude that the most significant finding of this study is the impact of ongoing behavioral intervention. With that support, both diets were proven to be successful weight-loss strategies.
The second study was also a two-year trial that investigated the influence of a low-carbohydrate, Mediterranean, or low-fat diet on weight loss and health. Eighty-six percent of the subjects in this study were men, and they all received behavioral support throughout the study period.
The low-fat diet was calorie-restricted, with a goal of 1,500 calories per day for women and 1,800 calories per day for men. The guidelines centered around a 30 percent fat diet with low-fat grains, vegetables, fruits and legumes and a limited consumption of additional fats, sweets and high-fat snacks. The Mediterranean-style diet is moderate in fat, but was also calorie-restricted in the same fashion as the low-fat diet, with a goal of no more than 35 percent of calories from fat. The main sources of added fat were from olive oil and nuts. The low-carbohydrate diet was not energy-restricted, and aimed for 20 grams of carbohydrates per day for two months with a gradual increase to 120 grams per day to maintain weight loss. The subjects were counseled to choose vegetarian sources of fat and protein, but these intakes were not restricted.
More subjects stuck with the low-fat diet regimen to the end of the two years, compared with the other two diets: 90.4 percent adhered to the low-fat diet, 85.3 percent to the Mediterranean diet, and 78.0 percent to the low-carb diet, for the duration of the study. However, the Mediterranean and low-carbohydrate diets were the most effective for weight loss.
At the two-year mark, subjects on the low-fat diet lost 7.3 pounds, subjects on the Mediterranean diet lost 10.1 pounds, and subjects on the low-carbohydrate diet lost 12.1 pounds. Greater health benefits were seen with the Mediterranean and low-carbohydrate diets. The Mediterranean diet had a great benefit on factors associated with blood sugar control; the low-carbohydrate diet had a greater benefit on factors associated with lipid profiles.
The third study explored the impact of a low-carbohydrate diet on death rates. Data collected over a period of 20 (men) to 26 years (women) from large prospective cohort studies allowed for investigators to measure associations between animal-based low-carbohydrate diets and plant-based low-carbohydrate diets and incidence of death. Although the researchers called the diets low-carbohydrate, they in fact averaged about 36 percent of calories from carbohydrates (152 g/day for women and 166 g/day for men). They found that animal-based low-carbohydrate diets were associated with a greater risk of death from all causes as well as death from cardiovascular disease and cancer. In contrast, a plant-base low-carbohydrate diet was associated with a lower risk of death from all causes, as well as a lower risk of cardiovascular disease-associated death.
A Win-Win Situation
The question regarding which diet to choose is no longer based on which diet works, but which diet works for you. For long-term weight loss, what diet can you live with best? Because that’s what you’ll stick with, and that’s the bottom line.
All the diets are safe and healthy. It appears that depending on your current health, one or other of the diets may be a better choice. So think about the foods that you can’t live without and the style of the diet. Are you someone who can’t live without bread and pasta, but you don’t mind counting calories? Then the low-fat diet might be the better choice for you. If you already eat a Mediterranean-style diet, including fish, legumes, vegetables, fruits, whole grains, olives and olive oil in your diet regularly, then just cutting down on your portion sizes may be all you need to do. If you want to drop pounds quickly and don’t ever want to count a calorie, then going low-carb may be the diet for you.
You can also mix and match. The very low-carb start to the diets in the studies leveled out to a more moderate carbohydrate intake after a few months. It might make sense for you to start with a low-carbohydrate diet but then switch to a Mediterranean-style diet once you’ve gotten your motivation in gear. The plant-based low-carbohydrate diet is also a sustainable and healthy strategy for the long term.
Your food should be enjoyable and interesting, and you should keep track of what you’re eating in a daily log. These strategies are some of the most important keys to losing weight and keeping it off.
The fat loss wars are over, and your journey to a healthy bodyweight is just beginning. Take time to carefully choose the diet that is right for you. No two people are alike, and the diet that’s right for you may not be the same diet that works for your sibling, your neighbor, your friend or your spouse. Find what feels balanced so that you can stick with it. You will be successful this time.
Foster G, et al. Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet. Ann Intern Med, 2010;153:147-157.
Shai I, et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. N Eng J Med, 2008;359:229-241.
Fung TT, et al. Low-carbohydrate diets and all-cause and cause-specific mortality. Two cohort studies. Ann Intern Med, 2010;153:289-298.