Diet

Nutritional Pearls: Low Carb vs. Low Fat Diets

Melissa is a 40-year-old patient who wants to lose 25 lbs. She’s tried various diets but recently read that a low-carbohydrate diet produces the best results. She decided to switch her diet and is now monitoring her food to keep her fat intake under 30% of her daily caloric count.

She informs you of her diet change and her long-term weight goals at her next appointment.

How should you advise her?     

What is the correct answer?
(Answer and discussion on next page)

 

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Answer: Understand the research before buying into the latest diet craze. Neither low-fat nor low-carbohydrate diets are best for your patient.

New research published in the Annals of Internal Medicine1 seems to show that a low-carbohydrate diet increases weight loss and improves cholesterol scores more than a low-fat diet. While proponents of the Atkins diet are understandably celebrating, the media, as a whole, is taking this study as the last word on healthy eating.

The Research

For the study, researchers followed 148 adult men and women who were considered clinically obese (with a body mass index [BMI] over 30), free from cardiovascular disease, type 2 diabetes, or kidney disease, had not had weight-loss surgery, and had maintained a stable weight for the 6 months leading up to the start of the study. Height, weight, cholesterol scores, body fat, and other demographic and clinical scores were recorded at the start of the study.
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The participants were randomly assigned to 1 of 2 styles of eating. (Note: I use the term “styles of eating” because they were not instructed to reduce their caloric intake.) Half were instructed on a low-fat diet and the other half were instructed to follow a low-carbohydrate diet.

The Results: After 1 year, individuals following a low-carb diet lost more weight than those on a low-fat diet. Further, their HDL improved while their total cholesterol and triglycerides remained about the same.

Looking pretty bad for a low-fat diet, isn't it?

Here's what the media coverage doesn't tell you: This is an extremely poorly designed study.

Other Considerations

Those on a low-fat diet were instructed to keep their fat intake under 30% of their daily caloric intake, with less than 7% of that being saturated fat. That's not a low-fat diet—30% of daily calories from fat is about the same as the usual American diet. A truly low-fat diet is more often 10% of calories from fat or less. Further, they were also instructed to keep their carbohydrate intake under 55% of calories. 

Those on a low-carb diet, by contrast, were instructed to limit their digestible carbohydrate intake to less than 40 grams per day, which is a big change from the typical American diet. Those on a low-carb diet ate fewer calories than those on the low-fat diet. Of course, they lost more weight! 

Tracking the participants’ adherence to the diets was limited to dietary recalls done once every 3 months over the course of the study. Each dietary recall was for 2 days: a weekday and a weekend day. Dietary recalls (in which a study participant reports what they have eaten over the last 24 hours) are notoriously unreliable. Do you remember everything you ate yesterday? What about last weekend? 

Further, 40 grams of carbohydrate (or roughly 3 slices of whole wheat bread) is tremendously restrictive for our patients. The problem with this study is it presumes patients have to give up great food to be healthier.

A study in the New England Journal of Medicine2 showed that a Mediterranean diet was at least the equal of a low-carbohydrate diet for weight loss and more importantly, it is a normal, non-restrictive diet. Here, study participants in the Mediterranean diet group actually reduced their calories less than those in the low-carbohydrate group and they lost the same amount of weight. There were subtle but not really significant differences in their blood sugars (low-carb better) and cholesterol (Mediterranean diet better). The take away? A Mediterranean diet, which allows individuals to eat real food, can help patients lose weight eating larger quantities of food than a low-carbohydrate diet. 

What’s the “Take Home”?

Ignore the hype. An unsustainable diet (low-carb) versus a disproven diet (low-fat) equals bad science on its face. When you factor in a poor quality study design, there is potential for existing health and dietary misinformation. Instead, instruct your patients on better eating principles, such as the Mediterranean diet. Here is a breakdown of the 9 principles of the Mediterranean diet.3

References:

  1. Bazzano L, Hu T, Reynolds K, et al. Effects of low-carbohydrate and low-fat diets: a randomized trial. Ann Intern Med. 2014;161(5):309-318.
  2. Shai I, Schwarzfuchs D, Henkin Y, et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. N Engl J Med. 2008;359:229-241.
  3. Harlan T. The Mediterranean diet. Dr Gourmet website. www.drgourmet.com /mediterraneandiet/#.VA3C8EhZGA8. Accessed September 8, 2014.