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Energy Metabolism Laboratory, Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111
3To whom correspondence and reprint requests should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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KEY WORDS: • dietary composition • energy intake • energy density • palatability • dietary variety
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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). The specific underlying causes of
adult weight gain are poorly understood, but can be traced to a
long-term imbalance between energy intake and expenditure. However,
factors leading to overeating and thus excess energy intake remain
controversial.
Of the many dietary factors that have been implicated, the percentage
of energy from dietary fat has received particular attention
(Bray and Popkin, 1998
, Willett, 1998
).
Recently, however, an expert panel concluded that dietary fat may not
be the sole dietary determinant of body fatness it has widely been
assumed to be (Roberts et al. 1998
), and new research
suggests that dietary components other than fat may be key contributors
to overeating and adult weight regulation (Ludwig et al. 1999
, McCrory et al. 1999a
, 1999b
and unpublished data, Saltzman et al. 1997
). These
dietary components, their relationship to one another and to energy
intake are shown in Figure 1
. In this review we will describe three recent studies from our
laboratory, which, when taken together with previous research, may help
to provide new explanations for dietary factors leading to adult
overweight and obesity (McCrory et al. 1999a
, 1999b
and unpublished data). These studies focus on the roles of energy
density, palatability and dietary variety as determinants of energy
intake and body fatness; dietary fiber and the glycemic index are
discussed elsewhere in this symposium (Burton-Freeman 2000
, Ludwig 2000
).
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The energy density-palatability interrelationship |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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). As discussed by Rolls in this symposium
(Rolls 2000
), one reason for this could be that humans may consume a
constant weight of food (Bell et al. 1998
,
Kendall et al. 1991
, Lissner et al. 1987
,
Rolls et al. 1998
, Stubbs et al. 1995
);
thus consumption of foods that contain a greater amount of energy per
unit weight may contribute to an overall greater energy intake. Another
reason for the higher energy intakes observed in consumption of a diet
higher in energy density may be that foods relatively higher in energy
density tend to be more palatable than foods lower in energy density
(Drewnowski and Greenwood 1983
). As reviewed by McCrory et al. (unpublished data), numerous studies have shown that palatability, or pleasantness of taste, is positively associated with the energy intake of single foods. However, these studies have the disadvantage that they take place over only a single session, and thus they provide no information on how palatability and energy intake may be related for different foods consumed in a normal diet. In addition, none of these studies have investigated palatability, energy intake, and energy density simultaneously; thus the relative influences of energy density and palatability on energy intake have not been determined previously.
We recently examined relationships among palatability, energy density
and energy intake as part of a metabolic unit–based study in which
seven monozygotic twin pairs were covertly fed ad libitum either high
fat or low fat foods (~40 and 20% of energy) over two 9-d study
phases (unpublished data). The primary purpose of this study was to
determine whether the effects of fat on energy intake are
independent of or mediated by the high energy
density of fat (Saltzman et al. 1997
). Diets were
matched for energy density, palatability and fiber content, all of
which are potentially confounding factors because high fat diets are
usually relatively low in fiber and high in palatability and energy
density. Mean daily energy intake over the 9 d did not differ
significantly between low and high fat phases (10.3 and 10.7 MJ/d,
respectively), suggesting that individuals do not consume more energy
from diets that are relatively high in fat when confounding dietary
factors are held constant.
Although the two diets described above were matched for average energy
density, palatability and fiber content, energy density varied among
the 22 foods from ~2 to 12 kJ/g, and there was no significant
relationship between energy density and dietary fat content either
within or between dietary phases. Examination of associations among
energy density, palatability and energy intake of the individual foods
showed that all three of these factors were highly interrelated, i.e.,
energy density and palatability were positively associated
(r = 0.46), and both energy density and palatability
were positively associated with energy intake (r = 0.56
and r = 0.73, respectively). Path analysis was
conducted to determine whether the effects of energy density and
palatability on energy intake were separable. This analysis showed that
the influence of energy density on energy intake was in part direct,
and in part indirect and mediated by palatability. In other words,
energy intake was higher from foods that were higher in energy density
not only because of their relatively high energy content per unit
weight, but also because of their greater palatability. As shown in
Figure 1
, this
interrelationship between energy density and palatability is one
important determinant of energy intake.
An important finding from this study was that subjects consumed more
energy from foods that were higher in energy density regardless of the
fat content. This result has implications with regard to weight control
and the increasing number of fat-modified food products in the U.S.
food market (Gallo 1997
) that are not necessarily lower
in energy density than the original, high fat product. Consumers may
mistakenly believe that, weight-for-weight, they are consuming less
energy from foods that are relatively lower in fat; however, our study
showed that this may not be the case if the energy densities of the
original and fat-modified foods are similar. This will be
particularly true if the fat-modified food also has the same
palatability as the original unmodified food because we found that
energy intake was greater from foods that were not only more energy
dense, but also more palatable.
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Dietary variety and body fatness |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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, Sclafani 1989
). Numerous single-meal
studies in humans also show this phenomenon (McCrory et al. 1999b
, Rolls 1985
). Although two longer-term
studies showed that laboratory rats have greater body weight and body
fat gains when fed a variety of foods compared with only a single food
(Louis-Sylvestre et al. 1984
, Rolls et al. 1983
), long-term studies in humans have been lacking until
recently.
We examined long-term dietary variety and its potential association
with energy intake and body fatness in adult men and women
(McCrory et al. 1999b
) by using accurate 6-mo dietary
intake reports from a food-frequency questionnaire (Block et al. 1986
) and accurate measurements of body fatness by
underwater weighing. We hypothesized that the relationship between
dietary variety and body fatness would depend critically on the types
of foods being consumed, specifically, that high variety would be
associated with fatness when it comes from high energy foods, but with
leanness when the variety comes from low energy foods. We also tested
the strength of dietary variety within food groups as a predictor of
body fatness relative to other putative dietary predictors.
The 10 food groups examined were breakfast foods; lunch and dinner entrees; sweets, snacks and carbohydrates; condiments; fruit; vegetables; energy-containing beverages; dairy products; breakfast food condiments; and beverage condiments. Dietary variety within each food group was calculated as the percentage of different foods consumed, regardless of the frequency of consumption. We found that within each of these food groups, dietary variety was positively associated with energy intake (r = 0.27–0.56). That is, individuals consuming a greater variety of foods within a group consumed more energy from that group, and this occurred within all 10 food groups.
In multiple regression analysis controlled for age and sex, dietary variety from a combined group of sweets, snacks, condiments, entrees and carbohydrates was positively associated with body fatness (partial r = 0.38), and in the same model, dietary variety from the vegetables group was negatively associated with body fatness (partial r = -0.31) (overall R2 = 0.46). In other words, individuals who consumed a high variety of sweets, snacks, condiments, entrees and carbohydrates, coupled with a low variety of vegetables were relatively fat. The opposite was also true, i.e., individuals who consumed a low variety of sweets, snacks, condiments, entrees and carbohydrates and a high variety of vegetables were relatively lean.
To evaluate dietary variety within food groups relative to other putative dietary predictors of body fatness, including the proportion of dietary energy from fat, energy density, fiber, and energy intake per kilogram body weight, we first computed a composite variety variable based on the previous regression analysis, which we termed the "variety ratio." The variety ratio was calculated as the ratio of the variety of vegetables consumed to the variety of sweets, snacks, condiments, entrees and carbohydrates consumed. The only dietary variables that were significantly associated with body fatness (in separate models controlled for age and sex) were the variety ratio and the percentage of energy from dietary fat. However, when these two variables were included in the same model, only the variety ratio remained a significant predictor of body fatness (partial r = -0.27, P = 0.02 for the variety ratio; partial r = 0.13, P = 0.28 for dietary fat). This indicates that not only was the variety ratio a better predictor of body fatness than the other dietary variables examined, but also that the association between the variety ratio and body fatness was independent of dietary fat.
These data, coupled with previous evidence from animal and short-term human studies, suggest that dietary variety may be an important tool in the prevention and treatment of disorders of weight regulation. In situations in which an increase in energy intake is mandated, such as in elderly adults experiencing unexplained weight loss, increasing the variety of high energy foods offered may accomplish the desired result without the necessity of intensive medical interventions. Concerning obesity, diets providing a high variety of vegetables and a low variety of sweets, snacks, condiments, entrees and carbohydrates may promote a long-term reduction in voluntary energy intake and body fatness without the need to resort to a conscious restriction of energy intake, thus preventing and perhaps helping to treat obesity and overweight.
These data may also help to explain the rising prevalence of obesity in
the U.S. (Flegal et al. 1998
) because the number of high
energy food products introduced into the U.S. food market in the past
30 years that are classified as condiments, candy, snacks and bakery
foods parallels the increasing prevalence of obesity and is strikingly
out of proportion to the number of new vegetable and fruit products
(Gallo 1997
).
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Restaurant food and body fatness |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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1994
),
and consumers are increasingly relying on them for daily meals and
snacks. From 1970 to 1996, the percentage of household food
expenditures spent on food away from home increased from 26 to 39%
(Putnam and Allshouse 1996
). In addition, a recent
national survey indicated that adult Americans consume a substantial
proportion of their food away from home. In 1995, 29% of meals and
snacks were consumed away from home, accounting for 31% of daily
energy intake (Lin and Frazao 1997
). Of the food
consumed away from home, 38% of meals and 24% of snacks were from
fast-food and other restaurants.
Adults who consume restaurant food more often have diets that are
higher in total energy, fat, and saturated fat and lower in fiber than
those who consume restaurant food less often (Eck Clemens et al. 1999
, Jeffery and French 1998
, McCrory et al. 1999a
). Because of this link between restaurant food
consumption and poor diet quality, we recently studied whether
consumption of restaurant food was associated with body fatness in 73
adult men and women who had participated in studies on diet and body
composition in our laboratory (McCrory et al. 1999a
).
Subjects were asked about their frequency of consumption of food from
seven types of restaurants, i.e., fried chicken, burger, pizza,
Chinese, Mexican, fried fish and "other." We found a positive
association between the frequency of consuming restaurant food and body
fatness (partial r = 0.36, controlled for age and sex).
This association remained significant after further controlling for
education level, smoking status, alcohol intake and physical activity.
There are a number of reasons why restaurant food consumption may
promote excess energy intake. First, restaurant meals tend to be higher
in fat and lower in fiber content, thus higher in energy density than
meals prepared at home (Lin and Frazao 1997
, Lin et al. 1999
). Recent studies have suggested that energy density
is a primary determinant of voluntary energy intake (Roberts et al. 1998
); thus, increased energy intake from restaurant meals
may be promoted by the high ratio of energy intake to food weight.
Second, restaurants usually serve large portions. For example, typical
meals from three different popular fast-food restaurants provide in
excess of 1000 kcal (Malouf and Colaguiri 1995
). Most
investigations of the effect of meal portion size on energy intake have
found that having large portion sizes increases energy intake
(Booth et al. 1981
, Edelman et al. 1986
,
Nisbett, 1968
, Shaw, 1973
), although one
study observed no effect (Stunkard et al. 1980
). Third,
restaurants typically serve food that is highly palatable. Consumers
say that taste is one of the major reasons why they choose particular
foods (Glanz et al. 1998
). In addition, as described
above, there is a positive association between taste ratings and the
intake of foods (unpublished data). Finally, restaurants serve a
variety of foods; not only are there a variety of foods offered within
and between restaurant types, but restaurants also provide food
varieties that differ from those prepared at home. Our recent study
suggests that long-term dietary variety may promote increased
energy intake, and the relative ratio of the variety of low to high
energy dense foods consumed may be an important contributor to body
fatness (McCrory et al. 1999b
).
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SUMMARY |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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FOOTNOTES |
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2 Supported in part by grants AG00209, AG12829, DK09747, DK46124 and DK46200 from the National Institutes of Health.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONThe energy density-palatability...Dietary variety and body...Restaurant food and body...SUMMARYREFERENCES |
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