The Impact of Portion Sizes on Weight Gain: A Review of Scientific Evidence, Study notes of Nutrition

Download The Impact of Portion Sizes on Weight Gain: A Review of Scientific Evidence and more Study notes Nutrition in PDF only on Docsity! Do Increased Portion Sizes Affect How Much We Eat? Portion Size: Then and Now It is no secret that portion sizes, as well as waistlines, in this country are expanding. The Dietary Guidelines for Americans 20051 urge Americans to pay special attention to portion sizes, which have increased significantly over the past 2 decades. Restaurant meals of all kinds have gotten larger with an emphasis on getting more food for the money. However, the rise of portion sizes is not limited to restaurants alone. Bags of snack foods or soft drinks in vending machines and the grocery store are offered in larger and larger sizes that contain multiple servings while a 1-ounce bag of snack food or an 8-ounce soft drink, which are the recommended single serving sizes, are very difficult to find. Americans are surrounded by larger portion sizes at relatively low prices, appealing to the consumer's economic sensibilities. However, the cost to America's health may be higher than most people realize. In the section on weight management, the Dietary Guidelines address portion sizes, stating that there are no empirical studies to show a causal relationship between increased portion sizes and obesity, but there are short- term studies showing that controlling portion sizes helps limit calorie intake, particularly when eating high-calorie foods. What is missing from the research is whether people monitor portion sizes and consistently choose to eat recommended serving sizes, thus consuming the appropriate amount of calories for maintaining or losing weight. The following research review examines what science underlies the notion that large portion sizes have contributed to weight gain among Americans. The research-to-practice section offers ideas to practitioners about how to counsel their patients or clients about portion size. Research Review Eat More Than You Use = Weight Gain The fundamental rule of weight management is that people gain weight when they eat more calories than they expend. Therefore, the number of calories in the amount of food consumed is integral to weight management. Portion size does not matter if the person chooses to eat only an appropriate serving or eats fewer calories in subsequent meals and snacks that allow them to stay within recommended calorie limits. But do people look at food that is offered and automatically assess how much is a normal serving size, and then actually eat only the normal serving size? Do they adjust what they eat after consuming large portion sizes? The research says they may not. Trends in Increasing Portion Sizes Restaurants Eating in restaurants offers many opportunities to encounter large portion sizes. The number of eating establishments in the United States increased by 75 percent between 1977 and 1991.2 While Americans have many choices in restaurants, the food (especially from fast food restaurants) is often very cheap and available in large quantities.3,4 A study shows that the frequency of eating out, particularly at fast-food restaurants, is associated with an increase in energy and fat intake and with a higher body mass index.5 Eating at Home and Snacks Even those who do not frequent restaurants are confronted with large portion sizes of prepackaged or convenience foods. Young and Nestle6 reported on a study examining the current weight of ready-to-eat foods and comparing them with past weights using data from manufacturers. Portion sizes of these foods began increasing in the 1970s and have continued to do so Research to Practice Series, No. 2 May 2006 National Center for Chronic Disease Prevention and Health Promotion Division of Nutrition and Physical Activity through today to the point where most exceed federal serving size standards. Nielsen and Popkin7 compared two cross-sectional surveys using Nationwide Food Consumption Survey data from 1977 and the Continuing Survey of Food Intake by Individuals (CSFII) data from 1989 and 1996-1998 to determine patterns and trends of portion sizes by type of food and eating location. "Key foods" (salty snacks, desserts, soft drinks, fruit drinks, French fries, hamburgers, cheeseburgers, pizza, and Mexican food) were chosen because they had the greatest percent change of energy in the U.S. diet. When data on the key foods were combined, they represented 18 percent of calories consumed in the United States in 1977-1978 and 27.7% of all calories in 1994-1996. The study found that between the survey years, portion sizes and energy intake increased for all key foods except pizza. The portion size increase resulted in an increased caloric intake for salty snacks (93 calories more), soft drinks (49 calories), hamburgers (97 calories), French fries (68 calories), and Mexican dishes (133 calories). A study by Smiciklas-Wright et al.8 illustrates the difficultly of assessing whether larger portion sizes are being consistently consumed. Their study of self-reported energy intake of food eaten at home compares quantities per eating occasion (portion size) using CSFII data from 1989-1991 and 1994-1996. The results showed that about one-third of the 107 commonly eaten foods showed significant differences in portion size. The majority of foods with significant differences were larger sizes in 1994 ­ 1996, including seven types of grains and cereals (e.g., oat rings, pasta, spaghetti with tomato sauce), and 11 beverages, such as orange juice, all soft drinks, beer, wine, and fruit drinks. Smaller portion sizes in 1994-1996 were reported for macaroni and cheese, pizza, chicken, bacon, margarine, and mayonnaise. No foods showed significant differences in portion sizes between the survey years for every age and sex category in this study. People can compensate for eating larger portions in one eating occasion by eating fewer calories during the rest of the day or the time period before or following the eating occasion. However, this is often difficult for many to do. In some of the studies in this brief,2,6 the researchers found that the people eating larger portion sizes did not notice the size difference and ate their normal amount of food at the following meal. Differences in Portion Size and Serving Size Portion size is the amount of a single food item served in a single eating occasion, such as a meal or a snack. Many people confuse portion size with serving size, which is a standardized unit of measuring foods—for example, a cup or ounce—used in dietary guidance, such as the Dietary Guidelines for Americans. Portion size is the amount offered to a person in a restaurant, the amount offered in the packaging of prepared foods, or the amount a person chooses to put on his or her plate. For example, bagels or muffins are often sold in sizes that constitute at least 2 servings, but consumers often eat the whole thing, thinking that they have eaten 1 serving. They do not realize that they have selected a large portion size that was more than 1 serving. Portion Size Affects How Much People Consume in an Eating Occasion. Short-term studies show that people eat more when they are confronted with larger portion sizes. The research studies described in the following cover only one or a few eating occasions in a short time frame. Research studies have yet to assess the impact of portion sizes over longer periods of time. However, the phenomenon of unknowingly eating larger amounts when presented with a large portion is an important aspect of weight management. A study by Rolls et al.9 tested how adults responded to meals on different days of four different portion sizes of macaroni and cheese. They found that the bigger the portion, the more participants ate. Participants consumed 30% more energy (162 cal) when offered the largest portion (1000g) compared to the smallest portion (500g). They also reported similar ratings of hunger and fullness after each meal despite the intake differences. After the study, only 45% of the subjects reported noticing that there were differences in the size of the portions served. Another study by Rolls et al.10 gave the same subjects different size sandwiches on several occasions to look at the effect on energy intake of increasing the portion size of a food served as a discrete unit (sandwich). Men and women who were offered different size (6-, 8-, 10-, and 12­ inch) sub sandwiches for lunch on four different days ate significantly more as the size of the sandwich offered became larger. A study by Diliberti et al.11 in a restaurant setting showed that when a pasta entrée was served in different portion sizes on different days, people ate larger amounts when they were given larger portions. This tendency to eat more when offered more was observed nearly 30 years ago when Pudel and Oetting12 conducted an observational study in which they served people soup from normal bowls to determine their 2