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Low Fat or Low Carbs?

Low-Fat vs. Low-Carb: Making Sense of all the Diet Hype

By Julia Blank, MD 

So you're driving down Wilshire, past the familiar tract of fast-food joints, and suddenly you do a double-take.  Was that Burger King offering a low-carb menu?  And was that a Subway sign touting Atkins-friendly wraps alongside its 6-grams-of-fat sandwiches?

Everywhere you go, low-carb is all the rage:  lettuce-wrap burgers at McDonald's, low-carb chips at the deli, carb-controlled candy bars at the supermarket check-out, the South Beach Diet and Dr. Atkins Diet Revolution on the bookstore shelf.

What ever happened to "low fat"?  Despite the bad rap that low fat diets have recently gotten in the press, low-fat/reduced-fat/fat-free products are still available-and, it seems, as popular as ever.

With so many foods and diets to choose from, how do you decide what to eat?  And what is the best approach to weight loss-and maintenance of healthy weight?

A Weighty Problem

If you're concerned about your weight, you're not alone:  studies suggest that at any given time, up to 44% of Americans are trying to lose weight.  In the process, they spend $40 to $60 Billion dollars a year on everything from diet drinks and health club memberships to diet books and commercial weight loss programs.

Despite this, the number of overweight and obese Americans continues to climb.  According to the latest National Health and Nutrition Examination Survey (NHANES III), 64.5% of American adults are overweight-up from 46% two decades ago.  Of overweight adults, 30.5% are considered clinically obese (i.e. BMI of 30 or more), up from 14.4% in 1980.

The problem with excess weight is that it leads to significant health complications such as diabetes, high blood pressure, elevated cholesterol, heart disease, arthritis, and some forms of cancer.  Obesity increases overall risk of death from all causes by 50-100%.   In other words, obesity can shave off an estimated 2-5 years of life expectancy.  The health problems associated with obesity are also responsible for a significant increase in healthcare costs.  For example, 2001 healthcare costs for diabetes associated with excess weight rose to $98 Billion.

Even more alarming, the percentage of children who are overweight and obese continues to rise as well.  Between 1980 and 2001, the percentage of overweight and obese children doubled to 20%. Studies show that children who are overweight tend to grow into adults who are overweight; they also develop obesity-related health complications at earlier ages.

How Much Should You Weigh?

Assessment of healthy weight is usually done on the basis of Body Mass Index (BMI) and waist size or waist-to-hip ratio (abdominal obesity in particular is thought to cause a significant increase in cardiovascular risk).  Large population studies have shown increased risk of associated disease and death at extremes of weight; the following chart summarizes these findings (adapted from "Partnership for a Healthy Weight"):

Risk of Associated Disease According to BMI and Waist Size

BMI Weight Category Waist Size
less than or equal to
40 in. (men) or
35 in. (women) 
Waist Size
greater than
40 in. (men) or
35 in. (women)
18.5 or less Underweight  N/A  N/A
18.5 - 24.9 Normal N/A  N/A 
25.0 - 29.9 Overweight Increased  High
30.0 - 34.9  Obese (I)  High Very High 
35.0 - 39.9  Obese (II) Very High  Very High
40 or greater  Extremely Obese (III)   Extremely High            Extremely High

Dietary Trends

Several factors have contributed to the "fattening of America".  Portion sizes have doubled and tripled over the last few decades, resulting in a tremendous increase in daily caloric intake in the American diet.  You see this trend everywhere:  from the oversized bagels and muffins on sale at your local Starbucks, to the double-sized burgers and fries at every fast-food dive, to the family-size entrees at the chain restaurant down the street.  A sample comparison of portion sizes today versus ten years ago shows the following:

French Fries 2 oz. Over 4 oz.
Deli Bagel 2 oz. 4-7 oz.
Muffin 2 oz. 6-8 oz.
Soda 6.5 - 8 oz 12-20 + oz.

Even our advertising reflects this trend:  remember the television commercial for the all-you-can-eat buffet, featuring a satisfied customer who says, "I ate a lot, but I sure didn't pay a lot"?!  Unfortunately, more food for your dollar isn't necessarily the best choice for your health.

At the same time that our food portions have been increasing, our physical activity has been declining.  We have become a sedentary culture.  We go from car to desk to car to couch, with fewer than 33% of American adults getting the minimum recommended amount of physical activity on a routine basis (i.e. 30-60 minutes of light or moderate physical activity 5 times or more per week).  Forty percent of American adults do not do any leisure-time physical activity at all!

So, we eat more than ever, and exercise less than ever.  Any wonder, then, that we continue to gain weight?

The solution seems obvious:  limit portion size, and increase exercise.  So why are so many of us still struggling?  And why are we still chasing after a quick fix?

There is no end to fad diets (remember the grapefruit diet, the cabbage diet, the "Think Yourself Thin" diet?), mostly because there is no end to the demand for something structured that "guarantees" weight loss.  The problem is that no matter how we lose the weight, once we go off the diet-as we inevitably do, since few diets are realistically sustainable in the long-run-we regain the lost weight and more.

This raises the following question:  what is a "healthy diet", and how can it help us to lose weight and/or successfully maintain a healthy weight?

The Low-Fat Solution

Traditionally, the recommended diet has been a low-fat, calorie-controlled diet based on the now-familiar food pyramid.  With this diet, carbohydrates provide 55-65% of total calories, protein provides 15%, and fat provides up to 30%.

Many formal diet programs, such as Weight Watchers, Jenny Craig, and Nutrisystem are based on this approach.  Likewise, this dietary break-down provides the basis for the "Step I" and "Step II" diets recommended by the American Heart Association for patients with heart disease and high cholesterol.

The problem with the "low-fat" approach is that it is too often applied without an appropriate-and necessary-element of common sense.  "Low-fat" does not mean "calorie-free" (if you read the fine print on many low-fat, fat-free, and sugar-free products, you'll find the following statement:  "not intended for weight control").  The flood of low-fat and fat-free products on the market have unfortunately also given free reign to guilt-free over-consumption of these foods, leading to even greater weight gain. 

Anecdotal evidence and resulting media hype regarding other dietary approaches have challenged conventional wisdom regarding the low-fat approach.  The Mediterranean diet, for example, advocates a higher proportion of fat in the diet (mainly from "heart-healthy" mono-unsaturated fats, like those found in olive oil).  While people who live in the Mediterranean and follow this diet have traditionally had a lower incidence of heart disease, it is not yet clear whether the diet alone is responsible:  factors such as higher levels of physical activity may also play a significant role.  To date, no long-term clinical studies have been conducted evaluating the health consequences of the Mediterranean diet.

The Low-Carb Solution

A more recent popular phenomenon has been the low-carbohydrate high-protein diet.  Examples of this diet include:

  • Atkins diet - advocates eating less than 20 grams of carbohydrate per day (during the weight-loss phase) and unlimited protein and fat.  Critics of this diet charge that this leads to increased consumption of saturated fats (such as those found in bacon, beef, butter, etc.), which are known to cause atherosclerosis, or deposition of fatty "plaques" in the arteries, leading to the narrowing and eventual blockage of arteries and resulting in heart disease, heart attacks, and stroke.  Furthermore, increased protein intake can stress the kidneys, potentially resulting kidney failure (particularly in patients at risk for this due to underlying high blood pressure and/or diabetes).
  • The ZoneTM - a more moderate diet consisting of 40% carbohydrate, 30% protein, 30% fat.  Carbohydrates are further ranked on the basis of glycemic index, which is a measure of how quickly the carbohydrates get absorbed into the blood in the form of glucose, and how much insulin is produced in response to this blood glucose:  the higher the glycemic index, the "worse" the food is supposed to be for you.  However, this needs to be interpreted with caution, since some high-nutrient high-fiber foods like carrots have a higher glycemic index than low-nutrient high-fat foods like chocolate.  A more recent-and perhaps more rational-measure of the relative "value" of particular carbohydrates is glycemic load.
  • South Beach Diet - this is a somewhat regimented diet that relies on glycemic index to identify "good" and "bad" carbohydrates.  It also steers dieters away from saturated animal fats, and toward unsaturated fat sources such as olives and nuts.  As with many other fad diets, this one relies on a strict regimen that essentially limits caloric intake to 1300-1500 calories per day, without requiring the dieter to actually count calories.  Critics of this diet contend that it is too rigid and difficult to follow in the long-run, resulting in high drop-out rates.

The controversy over low-carb diets was heightened in 2003 with the publication of two studies in the New England Journal of Medicine, comparing low-carbohydrate versus low-fat diets.  The studies were small (63 and 132 patients), with high drop-out rates for both low-carb and low-fat groups (about 40%).  The studies were also short-term, following patients for 6 months in one study, and 12 months in the other.  The results showed greater weight loss at 6 months in the low-carb group, though weight gain in both groups by 12 months resulted in a smaller (though still statistically significant) difference favoring the low-carb group.    The low-carb group also had greater reduction in triglyceride levels and fasting blood sugar (a marker for diabetes).  There were no significant or consistent differences in other parameters such as total cholesterol, LDL, HDL, and blood pressure.  (For a summary on cholesterol, LDL, HDL, and triglycerides, see "Cholesterol:  the Good, the Bad, and the Ugly" page.)

Critics of these studies argue that weight loss in the low-carb group resulted from greater caloric restriction.  This may be because low-carbohydrate/high-protein diets induce a metabolic state called "ketosis," which leads to appetite suppression (as well as higher stress on the kidneys and bad breath).

Still, these results call into question long-held conventional wisdom regarding the health benefits and efficacy of low-fat diets, as well the potential risks of low-carb diets.  However, because of the small size, high attrition rates, and short follow-up, the results should also be interpreted with caution.  More long-term data is needed before any conclusion can be made regarding low-fat versus low-carb diets.

One thing, though, is for sure:  both studies underscored how difficult it is to stick with any diet.

Getting It Off and Keeping It Off:  The Bottom Line

The National Weight Loss Registry is an ongoing database of over 3,200 people who have lost 30 pounds or more and successfully maintained the weight loss for at least a year.   How did they do it?  The following are some of the lessons learned from these "success stories":

  • Although there was no single method for the initial weight loss (some lost weight on their own, some through commercial weight-loss programs, others through very-low-calorie liquid diets), the majority of people maintained their weight loss successfully through a combination of calorie-restricted low-fat diet (approximately 56% carbohydrate, 19% protein, 24% fat) and exercise
  • Over 90% reported engaging in some form of regular physical activity

So, that's it:  no secret formula, no quick-fix diet, no magic pill.  Just making sure you get enough exercise and keeping an eye on portion size and calories consumed (the old formula of "calories in=calories out" still holds true for weight maintenance).

Of course, there are exceptions.  Some people who are clinically obese and have obesity-associated health conditions such as high blood pressure, high cholesterol, and diabetes may benefit from medication to help them lose weight (e.g. OrlistatTM and MeridiaTM).  Some people who have 100 or more pounds to lose may even benefit from surgical treatment (see "gastric bypass" and "lap-band").  There is also ongoing research into hormonal, metabolic, genetic, and other factors contributing to obesity, with potential implications and product development for weight management.

In the meantime, however, most of us need to rely on common sense and moderation.

Whether you choose the low-fat or the low-carb route, or something in between, remember the following:

  • Calories still count (though not all calories are the same)
  • Fresh, high-fiber, high-nutrient foods are still better for your health than highly-processed foods
  • Exercise is still the best way to help control appetite and weight

And, as always, before starting any diet or exercise program, be sure to check with your physician.  (For a listing of SMBP physicians, offices, and office hours see "Our Physicians" or "Locations".)

For more information on weight management, exercise, and the low-fat/low-carb debate, see the following: