Question

Why might certain foods be more satiating/have a higher satiety index than others, regardless of time of day eaten? Give your overall conclusion on the information presented regarding time of day (and day of the week, such as changes on weekends) on satiety and an overall healthy diet/eating plan.

Answer 1

A number of highly controlled laboratory-based studies have tested whether modifications either to foods or meals can increase energy intake. One approach is to choose foods that are not very satiating, that is, calorie-for-calorie they reduce hunger less than other foods. Another approach is to alter the way the food is served so that portions are bigger or the meals are more varied. These studies will be reviewed and ways that foods might be altered to influence energy intake will be considered.

Satiety and Food Intake

The main reason that individuals choose particular foods is because they like the taste. While palatability of foods was found to be a major determinant of intake in an extensive review of the literature on human food intake (Spitzer and Rodin, 1981), it is possible that other characteristics of foods such as the energy density or nutrient composition could have an impact on energy intake.

Energy Density and Food Intake

Various types of foods satisfy hunger differently (Kissileff, 1984; Rolls et al., 1990). The reasons for these differences in satiating effects are not clear. Among the causative factors that have been suggested are rate of consumption of a food, the sensory properties of the food, and beliefs about the satiety value of a food (Kissileff, 1984)/

Soup is an example of a food that is highly satiating. In a clinical survey in which intakes were analyzed from food diaries, Jordan et al. (1981) found that meals that included soup were associated both with lower caloric intakes within the meals and with lower daily caloric intakes than those meals without soup. Several studies have confirmed that soup is a highly satiating food (Kissileff et al., 1984; Rolls et al., 1990). At least part of the explanation is the low energy density of soup. Energy density, that is the calories in a given weight of food, could affect satiety by influencing the rate at which nutrients reach receptors involved in satiety (Kissileff, 1985). Foods with a low energy density require that a greater bulk of food be consumed for a given level of energy intake. The bulk of food to be consumed affects eating rate, gastric distension, and intestinal stimulation. Also, it is likely that individuals have learned the appropriate portion sizes that they should eat to experience satiety.

Energy density of foods can affect daily energy intake and body weight. In one study (Duncan et al., 1983), obese and normal-weight subjects had access to one of two different diets, for 5 days each. One diet had twice the energy density of the other; the low-energy-density diet was low in fats and sugars and high in fiber. The subjects consumed three meals a day and were allowed to eat as much of the available foods as they liked at each meal. Subjects on the high-energy-density diet. consumed nearly twice as many calories as those on the low-energy-density diet. Subjects on the low-energy-density diet were slightly hungrier at mealtimes but found the meals to be satiating. The diets in this study differed not only in energy density, but also in the fat content and in the amount of fiber, both of which could affect the amount of food consumed.

Dietary Fat and Food Intake

Foods high in fat can be readily overeaten, not only because fat increases the energy density of foods, but also because it contributes to the palatability of foods (Drewnowski, 1988). A key question is whether fat and carbohydrate, the principal macronutrients of most diets, have similar effects on hunger and satiety since, if the proportion of fat in the diet is reduced, the proportion of carbohydrate will increase. Dietary fat may be overconsumed because it is not as satiating as other nutrients, which would relate to postabsorptive factors such as nutrient absorption, hormonal release, and oxidation of nutrients (Rolls and Shide, 1992).

Rolls et al. (1992) have conducted a number of studies that compared the effects on subsequent food intake of eating equicaloric amounts of foods that varied in fat and carbohydrate content. Results indicated that in normal-weight men who were unconcerned with their body weight, foods differing in fat and carbohydrate content were equally satiating in that they similarly affected subsequent food intake. However, in individuals who were overweight or who were concerned with their body weight, yogurt that was high in fat suppressed subsequent intake significantly less than did yogurt that was high in carbohydrate. This result suggests that in some individuals there could be a relative insensitivity to the satiety value of fat in foods. Future studies will determine whether this insensitivity plays a role in the etiology or maintenance of obesity.

Two studies conducted at Cornell University suggest that it is difficult to maintain body weight when consuming a diet composed only of low-fat foods. In the first study (Lissner et al., 1987), females of varying body weight consumed sequentially three 2-wk dietary treatments in which the energy consumed as fat was 15–20 percent, 30–35 percent, and 45–50 percent, respectively, of the diet. Relative to their energy consumption on the medium-fat diet, which resulted in significant changes in body weight. In the second study (Kendall et al., 1991), female subjects consumed a low-fat diet (20–25 percent fat) or a control diet (35–40 percent fat) for 11 weeks each, with a 7-wk washout period (when they ate their usual diets) between conditions. Results showed that subjects ate less total energy on the low-fat diet and lost twice as much weight as on the control diet. In both of these studies, subjects ate the same weight of food in the various conditions, which suggests that the weight or volume of food consumed is an important determinant of energy intake.

In another recent study, Prewitt et al. (1991) lend support to the suggestion that the fat content of the diet can have marked effects on body weight maintenance. Over a 20-wk study in lean and obese premenopausal women, a significantly greater energy intake was required to maintain subjects' body weight on a 21 percent fat diet than on the 37 percent fat maintenance diet. The authors concluded that macronutrient composition may play a role in energy requirements for weight maintenance.

When underconsumption is a problem, it is clear that the fat content or energy density of available foods can significantly affect food intake and body weight. High-fat foods are easier to consume in quantity because they are often highly palatable, a smaller volume is required to achieve a given caloric intake, and possibly they are less satiating. To determine the optimal percentage of calories from fat in the diets of military personnel, concerns about underconsumption of energy will need to be balanced against problems that may be associated with a high-fat diet.

Dietary Fiber and Food Intake

Dietary fiber could reduce food intake for a number of reasons (Levine and Billington, 1994):

• high-fiber foods take longer to eat,
• some fibers such as guar gum and pectin slow gastric emptying,
• fiber may reduce the digestibility of food,
• increased fecal loss of energy may occur on high-fiber diets, and
• fiber may affect some gastrointestinal hormones that influence food intake.

Because of the different types and doses of fiber that have been tested and the wide variety of experimental protocols, the literature on this topic is complex. Nevertheless, a number of studies have shown that high-fiber foods consumed either at breakfast (Burley et al., 1993a; Levine et al., 1989; Turconi et al., 1993) or lunch (Burley et al., 1993b; Turnbull et al., 1993) significantly reduce intake at the next meal when compared to low-fiber foods. A number of studies have also investigated the effects on weight loss of supplementing of the diet with fiber over longer periods of time (Levine and Billington, 1994; Stevens, 1988). While several of these studies show a slightly greater weight loss for subjects on a fiber-supplemented diet, the results of the studies are variable and difficult to interpret. More well-controlled studies are required that compare the effects of different types of fiber in various doses to the effects from diets of equivalent energy and protein without added fiber.