Family Practice
© Oxford University Press 2000
Vol. 17, No. 4
Printed in Great Britain
Selections from Current Literature
What is the ideal body weight?
Samuel A Sandowski
Sandowski SA. What is the ideal body weight? Family Practice 2000; 17: 348–351.
The question of ideal body weight historically is a subject
of study that raises more questions than answers, i.e. is
obesity linked with morbidity and/or mortality and, thus,
important in medical management? What, if any, is the
lower healthy limit for body mass index? In 1869, Quetelet
described a formula for body mass index (weight in kg/
height m2) which assumes a certain amount of body fat,
based on weight, and corrected for height. In a 1959 study,
the Metropolitan Life Insurance Company suggested
‘ideal’ body weights for men and women based on weight,
height and body frame.1 However, co-morbid factors
such as smoking and family history for diseases are not
considered as variables in this survey. Today, ideal body
weight remains controversial.
Body mass index (BMI) is categorized by the International Obesity Task Force as:
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hydrodensitometry, underwater weighing via a submersion tank, is being replaced by dual-energy X-ray
absorptiometry (DEXA) scans because of its simplicity
and high precision. These, however, currently are used
more for research, while anthropomorphic methods,
such as waist circumference, waist-to-hip ratios and skin
fold thickness are utilized clinically due to availability.
Bioimpedance, a painless measure of resistance to an
electrical current, is also readily accessible. It is based
on the premise that muscle conducts better than fat.
Thus, electrical current through fat is slower, permitting
quantification of the percentage of body fat. Bioimpedance, though more objective than anthropomorphic
measures, is not better at measuring fat nor predicting
biological outcomes.
The authors note that clinical measurement of ideal
body weight is filled with methodological problems. The
ideal body weight is described typically as the weight
associated with the lowest mortality, and its U-shaped
graph suggests that ideal body weight has a lower as well
as an upper limit. Conditions causing weight loss, however, may not be detected for months to years, creating
an apparent higher mortality at lower weights, which
may be confounding. Secondly, most large studies analysing weight loss do not account for smoking, alcohol
use, physical activity nor genetic susceptibility to disease.
The authors address several studies, including the
Nurses’ Health Study,2 which show a U-shaped graph
implying increased mortality for lower as well as higher
body weights. However, when adjusted for cigarette
smoking, age and reverse causation (morbid conditions
causing the low weight), the relationship between BMI
and mortality becomes a positive linear one. This suggests that a low BMI, if not associated with cigarette
smoking or a co-morbid condition causing the weight
loss, may not necessarily be associated with increased
mortality. In fact, a BMI as low as 17 in women from
the Nurses’ Health Study2 is not associated with excess
mortality. Notably, the US Dietary Guidelines Advisory
Committee does not include a lower limit for the BMI as
‘healthy’ or ‘unhealthy’.
Concomitantly, another issue raised in the manuscript
is whether mortality, as a single parameter, is sufficient
18.5–24.9 = Healthy weight,
25.0–29.9 = Overweight,
30.0–34.9 = Class I obesity,
35.0–39.9 = Class II obesity,
.40.0 = Class III obesity.
It is generally acknowledged that obesity increases the
risk of many diseases, including diabetes mellitus, hypertension, coronary heart disease, cholelithiasis, cancer
of the endometrium, colon and kidney, stroke, osteoarthritis and infertility. These risks become more evident
as the BMI increases. Yet the definition of obesity has
varied by age and sex and is generally unclear. Additionally, the risk of obesity has increased significantly in
western countries over the years. The purpose of this
article is to review current literature pertaining to ideal
body weight and its association with morbidity and
mortality.
Willet WE, Dietz W, Colditz GA. Guidelines for healthy
weight. N Engl J Med 1999; 341: 427–434.
In this review article, the authors concisely describe
methods of body fat assessment. The gold standard of
Department of Family Practice, South Nassau Communities
Hospital, 2445 Oceanside Road, Oceanside, NY 11572, USA.
348
What is the ideal body weight?
to determine an ‘ideal’ weight, as morbidity correlates
positively with increased BMI as well. The risk of hypertension, cholelithiasis and coronary heart disease
increased significantly with a BMI .30, but the risk of
diabetes increased significantly with a BMI .25. Of
additional importance, the risk of most of these conditions begins to increase at BMIs .22–23, a value often
considered ‘normal’ or ‘ideal’.
Comments
This well written review identifies the difficulties in
determining ideal body weight. Though studies demonstrate that a BMI .30 is associated with increased morbidity and mortality, there exists a ‘grey zone’ between
25 and 30. This ‘grey zone’ is influenced by co-morbid
conditions, such as coronary artery disease and diabetes.
Furthermore, there is no consistent BMI at which the
relative risk for disease, in general, increases, i.e. relative
risk increases at a BMI of 25 for some diseases, and at a
BMI of 30 for others.
While most studies cite BMI to determine association
between weight and morbidity/mortality, it is generally
accepted that it is the increase in percentage of body fat,
and not the absolute weight and height, that correlates
positively with morbidity and mortality. Additionally,
current studies do not address athletes, who may have an
elevated BMI, but a higher percentage of lean muscle
mass and a lower percentage of body fat. Future studies
using DEXA scans and body fat analysis may be able to
produce more definitive results between body fat (not
body weight) and increased morbidity and mortality.
Stevens J, Cai J, Pamuk ER et al. The effect of age on
the association between body-mass index and mortality.
N Engl J Med 1998; 338: 1–7.
This survey investigates ideal body weight as a function
of age through information gathered from October 1959
to March 1960,3 in a cohort of over 1 million men and
women. Included in this study are Caucasians who had
never smoked. Excluded are people with involuntary
weight loss of .10 lb in the past 2 years, a history of
cancer (other than skin cancer), death in the first year
of the study, a history of heart disease, stroke or selfcategorization as sick or in poor health. A total of
262 019 women and 62 116 men completed a four-page
questionnaire including height, weight, the exclusion
criteria, queries on alcohol use, educational levels and
physical activity. Vital status for 93% of the cohort was
followed for 12 years. Death from all causes and deaths
from cardiovascular disease were assessed and compared
with BMI.
These data indicate that men and women between
30 and 74 years old with BMIs .25 have an increased
relative risk of death from any cause, as well as from
cardiovascular disease specifically. Risk is more evident
with BMI .32. However, the relative risk of death
related to increased BMI (in both categories) decreases
349
as age increases. An increased risk of death does not
correlate for those over 74 years of age.
Comments
The 1995 Dietary Guidelines for Americans4 states that
a BMI of ,25 is ‘healthy’. This study is confirmatory,
but only for those below age 74. In older subjects, an
elevated BMI is not associated with increased mortality.
The 1990 Dietary Guidelines for Americans do imply
that as one ages, increased weight may be beneficial,
suggesting an ideal BMI of 19–25 for those below age
35 years. Over age 35, the ideal BMI increases to 21–27.5
However, this age-specific recommendation is controversial, and in the 1995 Dietary Guidelines for Americans
the age-specific recommendations are omitted.4 This current study quotes others supporting the hypothesis that
the relative risk of death associated with an elevated
BMI decreases with age. These quotes include two studies.
The first is of 17 159 Finnish women in whom mortality
varies little with respect to BMI.6 The second is of 12 576
Seventh-Day Adventist women in whom the relative
risk of death associated with elevated BMI is lower for
women aged 55–74 than for those 30–54 years old.7
The current study is limited by focusing on mortality
only. The association between morbidity and BMI is
not investigated. Additionally, information, except for
data regarding death, is self-reported. This raises concern regarding under-reporting of body weight. Another
limitation in the methodology is that weight parameters
provided are only collected at the beginning of the study.
Weight changes over the 12 years are not noted. Moreover, information from data collected 30–40 years ago
may “limit the generalizability of the results”. However,
the number of study participants yields significant power,
and should not be ignored.
Finally, there is no discussion of those patients with a
BMI of ,19. The results and the graphs, however, imply
that a low BMI is not associated with increased mortality. Moreover, these subjects are non-smokers, and
exclusion criteria such as cancer, self-reported sickness
or poor health, or death in the first year of the study help
to reduce reverse causation, a common confounding
variable, as a possible cause of association between low
BMI and mortality.
Heini AF and Weinsier RL. Divergent trends in obesity
and fat intake patterns: the American paradox. Am J
Med 1997; 102: 259–264.
While quantification of the term ‘obesity’ is controversial, there is little debate that obesity increases morbidity
and mortality. Obesity in the US population, however, is
increasing despite this knowledge. In the article cited
above, the authors review the databases of NHANES II
and III, USDA Nationwide Food Consumption Survey,
the Behavioral Risk Factor Survey System and the
Calorie Control Council Report, in total analysing
150 000 patients.
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Family Practice—an international journal
The data imply that the prevalence of ‘overweight’
rises from 25.4% (1976–1980) to 33.3% (1988–1991), a
rise of 31–32%. Paradoxically, the total calorie intake
decreases by 3% in men and 6% in women during these
time periods. This is accompanied by a decrease in fat
intake and becomes most obvious in women aged 50–59
years, who show a 43% increase in the prevalence of
‘overweight’, but the largest reduction in fat intake
(a decrease of 13%). Black men demonstrate less of a
decrease in total fat calorie intake (9%) than white men
(11%), but without concomitant weight gain. In fact,
they demonstrate less of an increase in the prevalence of
obesity than white men (21% versus 32%).
Comment
If fat intake is decreasing and the amount of calories
consumed is decreasing, why is the prevalence of ‘overweight’ and obesity in the USA increasing? The authors
postulate two hypotheses. The first suggests that there is
an increase in efficiency of energy expenditure. The
body now requires fewer calories to perform the same
amount of work as before. This is unlikely. The second
hypothesis holds that the total daily energy expenditure
has decreased. The authors suggest that the decrease in
energy expenditure is secondary to ‘non-exercise-related’
activities, such as housework, occupational activities and
routine daily activities. Goran and Poehlan8 demonstrate
that energy expenditure does not increase with regimented exercise training in the USA, because of nonexercise-related activities. By way of an example,
Gortmaker et al.9 demonstrate that increased television
viewing affects ‘overweight’ among youth, supporting
the concept that Americans, as a society, are less active.
The paradox is evident. In spite of decreasing fat and
calorie intake in American diets, there is an increased
prevalence of ‘overweight’. In spite of an increasing
number of ‘fitness centres’ in the USA, and an increasing
awareness of the importance of activity, total activity is
declining. Healthy People 200010 sets goals to lower fat
intake and increase exercise to “moderate activity for at
least 30 min, at least 3 times per week”. However, will
these goals, even if reached, be enough for Americans to
achieve ideal body weight in the face of a decrease in
general activity levels secondary to passive entertainment and introduction of modern conveniences?
Lusky A, Barell V, Lubin F et al. Relationship between
morbidity and extreme values of body mass index in
adolescents. Int J Epidemiol 1996; 25: 829–834.
We have commented previously on the significance of
undetected co-morbid conditions as confounding variables that may falsely increase the apparent risk of low
BMI on mortality. Lusky et al. examine morbidity and its
association with BMI in this article, finding morbidity to
be increased in adolescents that were either underweight
or overweight.
In Israel, at age 17 years, almost all males are assessed
for mandatory army service. Lusky reviews the files of
~110 000 17-year-old Jewish, Israeli males for height and
weight as well as any medical condition that lowers their
health profile. A condition that precludes a recruit from
serving in the combat unit of the army is considered
significant morbidity.
These data indicate that overweight adolescents are
found to have a higher risk of hypertension and pathological joint conditions. The risk for each of these increases
as weight increases, and is higher for ‘severe’ overweight
persons than ‘mild’ overweight persons. (‘severe’ overweight is defined as a BMI .95th percentile). The odds
ratio (OR) is 13.1 (prevalence of 14.9 per 1000) to develop
hypertension in the severely overweight adolescent.
Joint conditions that are serious enough to cause army
exemption (significant morbidities) include:
• hip joint limitations (OR = 3.4 with severe overweight),
• meniscus tear (OR = 2.4 with severe obesity),
• ankle trauma, recurrent sprain or arthrodesis of the
ankle (OR = 2.3 with severe obesity),
• knee limitations with radiological changes and
objective physical findings following trauma or
surgery (OR = 2.6).
‘Severe’ underweight is defined as a BMI of ,5th percentile. Morbidity associated with severe underweight
included:
• bronchial lung conditions, including asthma (OR 1.8),
• intestinal disease (OR = 5.0) and neurosis (OR = 1.3).
Intestinal diseases are predominately coeliac and
Crohn’s disease.
Comments
The authors conclude that both overweight and underweight classifications, especially when severe, increase
morbidity significantly. Each classification is associated
with different pathologies. The morbidity is severe
enough to exclude a recruit from the military’s combat
units. However, the question remains whether the morbidity is a reason for the deviation of weight from the
norm, or is the fact that the recruit is underweight or
overweight the cause (at least in part) of the morbidity.
In other words, does the neurosis and the malabsorption
associated with the ‘underweight’ cause the underweight,
or does the fact that the recruits are underweight put
them at risk for neurosis and intestinal disease? Conversely, does an extremity injury of the hip, knee or ankle
significantly impede activity, resulting in increased weight,
or does the increased weight predispose to joint injuries?
Summary
An ideal body weight is as difficult to define as it is
elusive to achieve. Should it be based on mortality alone
What is the ideal body weight?
(as in some studies) or should morbidity be included as
an end point? If morbidity is included, how do we define
the degree of severity and what degree is considered significant? Furthermore, studies tend to agree that there is
an ideal upper limit for BMI, but this limit is debated. Is
it a BMI of 22, when there is a trend toward obesityrelated morbidity, or a BMI of 25? The literature is also
unclear about ageing. Should we encourage patients to
gain weight as they age, or should we just not discourage
it if it occurs?
Does the ideal BMI have a lower limit? The debate for
this continues. Some studies using mortality end points
say yes, while others say no. To date, studies using morbidity as an end point remain flawed through neglect of
reverse causation.
While there are no definitive answers and multiple
unanswered questions, many authorities generalize the
ideal body weight as a BMI of 19–25. This seems practical in that it takes into account morbidity and not
mortality alone. Achieving this goal via recommendations regarding diet and exercise has yet to prove fruitful.
Abnormal BMI, either too high or too low, remains a
‘disease of civilization’, and, as such, is multifaceted in
formulation and elusive to resolve.
351
References
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3
4
5
6
7
8
9
10
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Colditz GA. The Nurses’ Health Study: a cohort of U.S. women
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