Blog | Wednesday, October 15, 2014

A comparison of low carb and low fat diets


The joys of September! Parents gleefully shove their reluctant children onto school buses, the palm trees in Los Angeles don’t change color, and everyone realizes that they gained 20 pounds during their summer vacation. It’s time to get serious again about losing weight.

But how should you eat to best help you shed the extra pounds? Many people are passionate about their favorite diet, but there is very little data comparing different diets to each other. Some swear by low carbohydrate diets (like Atkins), while others insist that low fat diets (like Weight Watchers and others) yield more weight loss and achieve healthier cholesterol numbers.

A study published in Annals of Internal Medicine attempted to shed some light on this question. The study enrolled 148 men and women who were obese (BMI 30 to 45) but didn’t have diabetes or cardiovascular disease. The participants were randomized into 2 groups. One group was counseled to eat a low carbohydrate diet, with less than 40 grams of carbohydrates per day. The second group was counseled to eat a low fat diet, with less than 30% of total calories from fat, and less than 7% from saturated fat. Neither group was counseled about limiting total calories or about exercise. Both groups received ongoing periodic dietary counseling throughout the study.

The subjects were followed for a year and had periodic assessments of their weight, diet, cholesterol, blood pressure, and other blood tests measuring cardiovascular risks.

At the end of the study the group eating a low fat diet lost an average of 4 lbs. while the group eating a low carbohydrate diet lost an average of 12 lbs. Even more impressive was that the low-fat group lost lean body mass (muscle weight) and gained fat weight, while the low-carbohydrate group lost fat weight and gained muscle. This is especially surprising since average caloric intake and physical activity was similar between groups. One frequent criticism of low carbohydrate diets, that it results in an increase of LDL (bad cholesterol), was dispelled. Total cholesterol and LDL levels remained similar between groups, but the low-carbohydrate group had bigger increases of HDL (good cholesterol).

This all suggests that a low carbohydrate diet leads to more weight loss than a low fat diet while improving body fat composition and some cholesterol measures. For those who are losing weight on a low carbohydrate diet but were worried that the excess fat intake was increasing their cardiovascular risk, this is good news.

Though the results were trumpeted as a major vindication for low carbohydrate diets, I interpret the results differently. Sure, the low carbohydrate group fared better than the low fat group, but what I find striking is how disappointingly modest the results in both groups were. The participants had a BMI of 30 to 45 which means that at minimum they were 35 pounds overweight, some much more. An average weight loss of 12 lbs. is a laudable step in the right direction but is a small fraction of the weight that should be lost. Considering the fact that this weight loss took 12 months and that all longer term studies suggest that some of this lost weight will be regained, the results seem quite discouraging.

So I conclude from this study that any diet that helps you eat less and that you can maintain indefinitely will help you lose weight but that for meaningful weight loss you have to make a more radical change in your diet than the groups in this week’s study. If you feel full and not deprived on a low carbohydrate diet, then do it and stick to it. But you should probably have even less carbohydrates than 40 gm per day until you reach your target weight. This study at least reassures you that your cholesterol and body fat composition won’t get worse. If you do best with a low fat diet, consider a diet that is radically low in fat, like a plant-based vegan diet without processed foods. My patients who have stuck with either strategy have done well. This study is also a reminder that without exercise, changing what you eat will only achieve modest results. Frequent exercise can accelerate weight loss while maintaining muscle mass.

And for people who are over 100 lbs. overweight, especially those with diabetes, studies increasingly suggest that weight loss surgery has healthier outcomes than diet and exercise alone.

So let’s all make a plan and get started. Thanksgiving is just around the corner.

Learn more:
A Call for a Low-Carb Diet That Embraces Fat (New York Times)
Cutting Back On Carbs, Not Fat, May Lead To More Weight Loss (NPR)
Effects of Low-Carbohydrate and Low-Fat Diets: A Randomized Trial(Annals of Internal Medicine)

Some of my past posts on diet and weight loss:
Why Losing Weight Is So Hard
Startling Scientific Finding: Dieting Leads to Weight Loss
Scientifically Proven Weight Loss Method: Eat Less

Albert Fuchs, MD, FACP, graduated from the University of California, Los Angeles School of Medicine, where he also did his internal medicine training. Certified by the American Board of Internal Medicine, Dr. Fuchs spent three years as a full-time faculty member at UCLA School of Medicine before opening his private practice in Beverly Hills in 2000. Holding privileges at Cedars-Sinai Medical Center, he is also an assistant clinical professor at UCLA's Department of Medicine. This post originally appeared at his blog.