SUMMARY: Nutrition researchers are uncovering new evidence that plant foods have a long-overlooked but surprisingly central role in managing weight. The key, according to this new spate of research, is energy density. Vegetables, fruits, whole grains and beans are high in water and fiber, but comparatively low in energy – that is, in calories. As a result, they promote a feeling of fullness without the hefty calorie payload that accompany foods higher in fat. At the same time, they provide the body with an array of vitamins, minerals and phytochemicals that promote health and protect against chronic disease.
Research that has provided food for subjects conclusively shows that diets high in unrefined carbohydrates promote weight loss. In trials where subjects are merely advised to increase plant foods consumption, marked differences in compliance and methodology make results more difficult to interpret. Reviews of the current scientific literature on the subject conclude that diets high in plants food are not a “magic bullet,” but are an important part of any effort to lose or maintain weight, particularly when they are combined with increased physical activity.
Consumers Beginning to Tire of Low-Carb Diets
According to a July 2004 consumer survey conducted by InsightExpress, an online research firm, half of Americans who have tried low-carb diets have given them up, and the number of people trying them for the first time is shrinking. The August 26 issue of Forbes magazine profiled a recently launched low-carb food store in Southern California now facing deep debt because its inventory of “carb-free” products is languishing on the shelves.
Many find low-carb eating to be too restrictive, or simply tire of meals that revolve around the same high-fat, high-calorie foods.
New Research Puts Energy Density to Work
Before the low-carb craze began, people turned to plant foods to help them manage weight: dinner salads, an extra helping of vegetables, or a piece of fruit. It wasn’t until the 1980s that nutritional scientists began to examine how plant foods could affect weight management efforts under rigorous clinical conditions. They began by testing how diets with different proportions of fat and carbohydrates affected two basic factors that, in turn, determine how many calories an individual consumes.
During a meal: How long it takes for people to feel full and stop eating.
After a meal: How long it takes for people to feel hungry again.
Researchers theorized that diets high in fat, which is energy dense (meaning that a small amount comes with a high caloric payload) would be associated with weight gain. They suspected that diets high in carbohydrates would not be associated with weight gain, because vegetables and whole grains are energy dilute, meaning that most of the “bulk” of these foods comes from water and fiber, and thus promote a feeling of fullness without delivering as many calories.
Different researchers conducted clinical trials that provided food for small groups of individuals over a relatively short time – as long as 11 weeks1-8. The food looked and tasted the same, and subjects were allowed to eat as much as they wished, but researchers had secretly manipulated the food’s fat and complex carbohydrate content. Thus, over the course of the trials, some groups ate diets that were higher in fat while other groups ate diets higher in carbohydrates.
As predicted, the results of these trials consistently showed that the subjects’ body weight increased on high-fat diets, and decreased on high-carbohydrate diets. What these trials (and subsequent studies that have asked subjects to rate how full they feel after eating different foods)9,10 have found seems surprising: foods high in fat promote passive overconsumption because they don’t “fill you up” as effectively as foods higher in “complex” or unrefined carbohydrates.
There’s a common sense reason for this: the key thing that makes people feel full and stop eating is the sheer volume of food consumed, the researchers concluded. Simply put, it will take just as long for you to feel full eating a meal that’s high in fat as it will for you to feel full eating a meal that’s high in vegetables and whole grains. The difference is, by the time you start to feel full eating the high-fat meal, you will have consumed many more calories than you would have if you ate a meal higher in unrefined carbohydrates.
This was an important finding, because it conclusively demonstrated that differences in fat and complex carbohydrate proportions can have a significant and measurable impact on total caloric intake. This simple fact became the foundation of a new branch of nutritional science focusing on energy density and weight management.
Energy Density Proves to be an Effective Clinical Tool . . . .
Barbara J. Rolls is a researcher at the forefront of efforts to employ energy density in a targeted way to help manage weight. She and her colleagues at Pennsylvania State University have conducted many studies to investigate how adding vegetables, fruit, whole grains and other plant foods to a meal lowers its energy density. They have consistently found that diets high in vegetables and fruits result in lower caloric intake – without feelings of hunger. In one study, increasing the amounts of fruits and vegetables in meals reduced subjects’ total calorie consumption by more than 400 calories per day.11
Researchers have also found that vegetables and fruits seem to have the advantage after a meal, because the fiber and water in meals high in plant foods helps keep hunger from returning. In a series of Swedish studies, simply adding spinach, carrots, and other high-fiber vegetables to a meal significantly lengthened the time it took for subjects to feel hungry again. (It was necessary to add at least 200 grams – or two standard servings – of these vegetables to be effective.)12-15
Two long-term clinical studies provided meals for small groups of overweight subjects over the course of three weeks. At the end of the both trials, during which they ate meals that were much higher in vegetables and fruits and lower in fat than their habitual diet, subjects showed significant weight loss – in one trial, an average of 17.2 pounds.16,17
…But One That Must Be Adapted to “Real World” Situations
Lowering the energy density in the diet consistently leads to weight loss in studies where food is provided to subjects. In studies where subjects are merely advised to increase the amount of fruits and vegetables in their meals, however, the strategy has produced promising – but not yet consistent – results.18-29
There are several possible reasons for this. Complications arise because of differences in study design and methodology. Some studies that advise subjects to increase fruit and vegetable consumption explicitly tell the subjects that weight loss is a goal. Others provide the same advice in another context, such as heart health or changes in cholesterol levels; in such studies, weight tends to remain constant or decline only slightly.26,27,29 Some studies combine advice to increase fruit and vegetable consumption with advice to lower fat intake.22,25,28 Not all of the studies that have investigated energy density ask subjects to rate their between-meal hunger, or measure how full they feel immediately after a meal. Compiling results that arise from such different methodologies and comparing them in a meaningful way is difficult.
In addition, there are many pertinent factors that can be controlled for in the clinic, but that become more pronounced in the “real world.” Clinical trials measure how different kind of foods affect the basic physiological aspects of hunger, but people eat what they eat for many other reasons. There are emotional and psychological factors that cannot be easily measured or controlled for, and these also serve to cloud the scientific picture.
Scientific Reviews Assess the Current State of Research
One recent paper that reviewed the available evidence from 34 intervention studies on the relationship between plant food consumption and weight loss concluded that “fruits and vegetables may play an important role in weight management.”30
Another recent review examined the role of overall diet and nutrition in obesity prevention in a more comprehensive way. The authors of that review, which was originally prepared for the World Health Organization, examined over 200 studies, 70 of which were specifically related to the role of dietary fat, dietary fiber (i.e., plant foods) and energy density. They concluded that “evidence is convincing that a high dietary [fiber] intake helps to protect against weight gain.”31
Research is ongoing, but one thing has clearly emerged from the current scientific evidence: adding vegetables, fruits and other energy dilute plant foods to the diet is one important component of a successful weight management program.
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