The skinny on low carb diets
Do they work? Are they healthy?
Sound Consumer | June 2004
by Jennifer Lovejoy, Ph.D.
(June 2004) — With the huge array of low carb products on the market and an increasing number of low carb diet books, consumers today are bombarded by messages to decrease their carbohydrate intake.
While it's easy to find anecdotal evidence from friends and relatives that a low carb diet makes you lose weight, it's important to evaluate the various types based on data, not on media or marketing hype. One way to look at the popular diet craze is to ask why any specific diet works or doesn't, and what that means for you.
Why does a diet work?
When trying to lose weight, the only thing that matters is to burn more calories than you take in. There is no magic fat-burning nutrient or substance that will help your body violate the laws of physics. A popular diet works because, when you follow its preset menus, you end up significantly restricting calories. When you restrict calories (especially if you increase physical activity at the same time), you lose weight.
The bottom line is that, whatever diet you stick to for the long haul, it results in weight loss. However, a diet you follow to lose weight initially may not be a diet that will work for maintaining that loss.
Does a low carb diet work for weight loss?
Diets currently promoted by most governmental and medical associations are based on scientific evidence that suggests those high in vegetables, fruits, whole grains and low-fat dairy products enhance feelings of fullness and reduce risk for chronic diseases such as heart disease, cancer and diabetes. Several published studies that analyzed and summarized many clinical trials (called "meta-analyses") show that when people increase fat and decrease carbohydrates in their diet, body weight goes up.
The converse also is true — when people decrease the fat in their diets and increase carbs, their body weight drops. A typical low carbohydrate diet is high in protein and fat; high in meats, cheeses, nuts and oils and relatively low in fruits, vegetables, legumes and grains.
There have been few scientific studies on the long-term efficacy of a low carb diet. Clinical trials in nutrition rarely are longer than three months and most of the clinical trials on dietary fat/carbs have run four to six weeks in length. Short-term studies (one to three months in length) suggest that a low carb diet does produce weight loss; however, the lost pounds are due to reductions in calorie intake.
Low carb terminology
If you decide to limit your carbohydrate intake, here's some information to help navigate common terms that may appear on food labels.
"Low carb" and "Reduced carb"
"Total carbs" vs. "Net carbs"
Typically, net carbs are calculated by subtracting fiber grams from total carbohydrate grams, since fiber is non-digestible. Some diets also subtract sugar alcohols such as sorbitol from total carbs in their net carb calculations.
The problem is that the "net carb" term is not standardized and can have different meanings. Also, fiber and sugar alcohols still contain calories and thus are not truly "free" when you are trying to lose weight.
Recently, data from long-term studies (one year in length) have been released and these studies find no difference in weight loss between a low carb and lowfat/high carb diet. Again, any diet that gets you to restrict calories and that you can stick to will cause you to lose weight.
Is a low carb diet healthy long term?
While some people may have short-term weight loss success with a low carb diet, the real concern is with long-term consumption of a diet high in protein and fat and low in healthy foods like fruits, vegetables and whole grains. Even before the high-protein/low carb diet craze, most Americans ate more protein than the recommended amounts.
Excess protein makes extra work for the kidneys, which have to break the protein down for excretion. When the kidneys are overworked, they may not regulate calcium levels properly, which can lead to bone loss and osteoporosis. People who have kidney problems or are at risk for kidney problems (including people with diabetes) should not follow a high protein diet plan. Studies have also associated a diet high in animal protein with a greater risk of certain cancers, such as colon cancer.
Another concern with the long-term use of a low carb diet is the lack of important nutrients from fruits, vegetables and whole grains. Plant foods and whole grains are not only powerhouses of vitamins and minerals, but contain phytochemicals (plant chemicals) that have been found to fight cancer and other diseases. A low carb diet that, in effect, limits vegetable, fruit and grain intake may have a very negative effect in the long run when the lack of key nutrients and phytochemicals impacts the body.
A diet low in carbohydrates may also increase the risk for developing diabetes. Many scientific studies show that a high fat/low carbohydrate diet worsens insulin resistance, which is a risk factor for diabetes. Basically, when the body is exposed to high amounts of fat, the hormone insulin that removes sugar from the bloodstream becomes less effective. Over time, this causes the body to secrete excess insulin, which can stress the pancreas and lead to problems controlling blood sugar.
Many of the popular low carb diets claim they'll actually improve insulin levels and lower diabetes risk. This could not be further from the truth. In one study published in a peer-reviewed medical journal, consuming a low-carbohydrate, high-fat diet for only three days results in a significant increase in post-meal insulin levels.
At least six scientific studies show that consuming a high fat, low carb diet for a period of several weeks to several months induces insulin resistance and increases insulin blood levels. Furthermore, population-based studies show that people who eat higher amounts of fat have a greater risk for diabetes.
Thus, the long-term effect of a low "carb diet is one of the greatest concerns, especially since diabetes rates in the United States are already at epidemic proportions.
Good carbs and bad carbs?
Some popular diets promote the concept of "good carbs" and "bad carbs." Typically, bad carbs, including refined sugar and products made from refined (white) flour such as white bread and refined pasta, cause a rapid rise in blood sugar. Good carbs, including whole grains, vegetables and fruits, cause a more delayed blood sugar response.
This differing effect of foods on blood sugar leads us to the concept of "glycemic index." This concept explains that low glycemic index foods cause a lower blood sugar rise than high glycemic index foods. It has been suggested that following a low glycemic index diet (similar to a low carb diet) may have weight loss benefits, but few scientific studies support this claim.
Unfortunately, the glycemic index concept is very challenging to grasp. Many popular diet books contain glycemic index lists. However, testing for glycemic index is based on consumption of a single food, not the mixed meals that people typically eat.
It is not clear that the glycemic "score" of a food holds true in the context of a mixed meal. Furthermore, a food's glycemic index depends on how it is prepared, how ripe a fruit or vegetable is, and many other factors. Thus, some nutrition experts feel that the glycemic index is really not very helpful.
The simple answer to the glycemic index conundrum is to eat mostly whole foods. Whole foods are those close to their natural state and minimally processed. This includes whole fruits and vegetables, whole grains like brown rice and quinoa, and naturally produced dairy and meat products. Most of these products have a relatively low glycemic index and, importantly, are rich in the nutrients and phytochemicals that protect against chronic disease.
What's a dieter to do?
Popular diets have always been part of American culture and probably always will be. In a population where nearly two-thirds of its adults are overweight or obese, weight loss is a national obsession.
If you're looking to lose weight or prevent weight gain, a low carb diet may not be your best bet. The National Weight Control Registry is a national database of more than 4,000 men and women who have lost weight and successfully kept it off for more than a year.
What does this database tell us about successful weight loss? The vast majority of participants in this registry of "successful losers" engage in three common behaviors: 1) they weigh themselves regularly; 2) they engage in daily, moderate exercise for 30 to 60 minutes a day, and 3) they eat a low fat, high carbohydrate diet (about 24 percent fat and 57 percent carbohydrate).
Less than one percent of the registry follows a low carb diet regimen. This suggests that it's unlikely that the current low carb diet craze is here to stay. People who want to succeed in their weight management goals might do better to follow a relatively low fat diet high in complex carbohydrates, fruits and vegetables.
Dr. Lovejoy is a professor and chair of the nutrition and exercise science department at Bastyr University. She maintains several research grants on obesity, menopause and Type 2 diabetes, and speaks regularly at national and international scientific conferences.