Physiology & Behavior, Vol. 19, pp. 7 5 - 7 8 . Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A.
Set Points, Settling Points, and
the Control of Body W e i g h t 1
D A V I D W I R T S H A F T E R 2 A N D J O H N D. D A V I S
Department o f Psychology, University o f Illinois, Chicago, Chicago, IL 60680
(Received 9 March 1977)
WIRTSHAFTER, D. AND J D. DAVIS. Set pomts, settling pomts, and the control of body weight. PHYSIOL. BEHAV.
19(1) 7 5 - 7 8 , 1977. - - Recently a number of investigators have suggested that body weight may be controlled by a
neural system which contains a reference or set point. The finding that the body weight level which animals wall defend can
be altered by lesions of the lateral and ventromedlal hypothalamus has been taken to provide support for this claim. In this
paper we describe a simple feedback control model which contains no set point, and yet is able to account m full for these
and other data which have been cited m support of the existence of a body weight set point Since such a model exists It
appears to be unnecessary and unparsimomous to introduce the concept of a neural set point to explain the fact that body
weight maintains a relatively constant value which is defended against certain types of challenges. Constancy and defense
of body weight are at best only prima facte evidence for the existence of a neural set point controlling body weight
Set point
Body weight control theory
VMH
LH
Llpostat hypothesis
Feeding
Homeostasis
such a c o n t r o l s y s t e m IS to m a i n t a i n a relatively c o n s t a n t
value of t h e c o n t r o l l e d q u a n t i t y in t h e face o f d i s t u r b a n c e s
i m p o s e d b y t h e e n v i r o n m e n t . It is n a t u r a l , t h e r e f o r e , w h e n
investigating h o m e o s t a t i c s y s t e m s w h i c h adjust t o disturb a n c e s b y m a k i n g a p p r o p r i a t e correctxons, t o assume t h a t
t h e a d j u s t m e n t s are m a d e in r e s p o n s e t o an e r r o r signal
i n d i c a t i n g d e v i a t i o n f r o m t h e desired state.
In spite o f t h e p o p u l a r i t y o f this n o t i o n t h e r e is a real
q u e s t i o n c o n c e r n i n g its value in f u r t h e r i n g o u r u n d e r s t a n d i n g of t h e c o n t r o l o f b o d y weight. P o s t u l a t i n g t h e
e x i s t e n c e of a n e u r a l set p o i n t in o r d e r to a c c o u n t for t h e
c o n s t a n c y of b o d y weight does n o t explain m u c h ; it m e r e l y
assigns t o t h e c e n t r a l n e r v o u s s y s t e m ( C N S ) j u s t
that
p r o p e r t y n e e d e d to a c c o u n t for b o d y weight c o n s t a n c y
F u r t h e r m o r e , t h e i n t r o d u c t i o n of s u c h a c o n c e p t t o explain
one set o f facts creates difficulty i n e x p l a i n i n g others. F o r
e x a m p l e , it is well k n o w n t h a t r o d e n t s gain an e n o r m o u s
a m o u n t of weight b e f o r e h i b e r n a t i o n [ 1 1 ] , t h a t m a n y
species of bards increase t h e i r fat stores t r e m e n d o u s l y prior
t o m i g r a t i o n [ 2 1 ] , a n d t h a t females of m a n y species
increase b o d y weight d u r i n g p r e g n a n c y [32] It is also well
k n o w n t h a t rats can be m a d e obese s i m p l y b y o f f e r i n g t h e m
a very p a l a t a b l e h i g h fat diet [ 1 6 ] . T o e x p l a i n these
p h e n o m e n a w i t h i n t h e c o n t e x t o f a set p o i n t m o d e l , o n e
w o u l d have to a s s u m e e i t h e r t h a t t h e set p o i n t is labile or
t h a t it b e c o m e s i n o p e r a t i v e u n d e r c e r t a i n c o n d i t i o n s s u c h as
t h e availability o f a h i g h fat diet Of course, e i t h e r or b o t h
o f these a s s u m p t i o n s m a y be c o r r e c t , b u t s k e p t i c i s m begins
IN R E C E N T Y E A R S t h e a p p h c a t l o n of c o n t r o l t h e o r y to
t h e p r o b l e m s o f p h y s i o l o g y [27] a n d b e h a v i o r [1, 6, 15,
18, 24, 30] h a s b e c o m e widespread. T h e increasing i n t e r e s t
in t h e use o f f e e d b a c k m o d e l s t o f u r t h e r o u r u n d e r s t a n d i n g
of t h e c o n t r o l o f b e h a v i o r seems to be justified in part
because t h e c o n s e q u e n c e s of w h a t organisms do very o f t e n
feed b a c k m s o m e sense to alter o n g o i n g or s u b s e q u e n t
b e h a v i o r F u r t h e r m o r e , c o n t r o l t h e o r y p r o v i d e s a way t o
a n a l y z e m a logical a n d q u a n t i t a t i v e m a n n e r c o m p l i c a t e d
s y s t e m s t h a t are i n f l u e n c e d b y a m u l t i p l i c i t y o f variables
o f t e n o p e r a t i n g s i m u l t a n e o u s l y . T o a t e s a n d O a t l e y [31]
have s h o w n , f o r e x a m p l e , t h a t t h e m a n y variables w h i c h
c o n t r o l w a t e r i n t a k e m t h e rat can be o r g a n i z e d b y c o n t r o l
t h e o r y t e c h n i q u e s to provide a r e m a r k a b l y c o m p r e h e n s i v e
m o d e l of thirst m this animal.
T h e g r o w i n g i n t e r e s t m t h e use of c o n t r o l t h e o r y t o
a n a l y z e t h e physiological s u b s t r a t e s of b e h a v i o r has
p r o m p t e d a n u m b e r of investigators t o suggest t h a t b o d y
weight, or s o m e closely r e l a t e d q u a n t i t y s u c h as t o t a l lipid
v o l u m e , m a y be r e g u l a t e d b y m e a n s o f a set p o i n t c o n t r o l
s y s t e m [4, 7, 13, 19, 20, 22, 28, 3 2 ] . I n d e e d , this
c o n c e p t i o n of w e i g h t c o n t r o l has b e c o m e so p o p u l a r t h a t it
is p u t f o r w a r d as e s t a b l i s h e d fact m at least o n e i n t r o d u c t o r y p s y c h o l o g y t e x t [ 2 ] . T h e d e f i n i n g c h a r a c t e r i s t i c of
s u c h a s y s t e m is t h a t it c o n t a i n s a r e f e r e n c e p o i n t against
w h i c h t h e c u r r e n t s t a t e of t h e o r g a n i s m is c o m p a r e d
D e v i a t i o n s f r o m t h e set p o i n t g e n e r a t e a n e r r o r signal w h i c h
activates t h e s y s t e m t o c o r r e c t t h e deviation. T h e effect of
i This paper was supported in part by National Science Foundation Grant B M S - 7 5 - 1 7 0 9 1 .
2 Requests for reprints should be sent to David Wlrtshafter, Department of Psychology, Umverslty of Ilhnols, Chicago, Box 4348, Chicago,
IL 60680
75
76
to develop as the h y p o t h e t i c a l set p o i n t begins to acquire
j u s t t h a t a m o u n t of v a r i a b l h t y w h i c h is necessary to explain
o b s e r v e d variability m b o d y weight
One r e a s o n for the p o p u l a r i t y of the set p o i n t c o n c e p t is
t h a t ~t is a plausible a n d c o n v e n i e n t way to a c c o u n t for the
m a i n t e n a n c e of a stable b o d y w e i g h t Plausabthty and
c o n v e m e n c e aside, h o w e v e r , t h e f u n d a m e n t a l q u e s t i o n is is
a set p o i n t necessary to explain t h e evidence w h i c h has
b e e n cxted m s u p p o r t of its existence9 We show here t h a t it
IS not, since a simple f e e d b a c k m o d e l w i t h o u t a set p o i n t
can be c o n s t r u c t e d w h i c h can explain all o f the evidence
w h i c h has b e e n cited m s u p p o r t of t h e existence of a CNS
b o d y weight set p o i n t Given t h a t this is the case. it
b e c o m e s clear t h a t t h o s e w h o w~sh to argue for the
existence of a n e u r a l set p o i n t for b o d y weight will have to
provide m o r e direct e v i d e n c e for it t h a n b o d y weight
s t a b i l i t y Let us n o w c o n s i d e r evidence w h i c h has b e e n used
to s u p p o r t the idea of a set p o i n t a n d see how well an
a l t e r n a t e m o d e l can explain ~t
S o m e o f t h e m o s t c o n v i n c i n g s u p p o r t for the e x i s t e n c e
of a set p o i n t for b o d y weight c o m e s f r o m t h e studies of
Keesey a n d his associates [3, 12, 13, 17, 25] P o w l e y and
Keesey [25] f o u n d t h a t f o l l o w i n g small lateral h y p o t h a l a m l c I LH) lesions rats m a i n t a i n t h e i r b o d y weight at
l o w e r t h a n n o r m a l levels and will d e f e n d this l o w e r e d
weight m the face of various r e g u l a t o r y challenges F o r
e x a m p l e , ff t h e b o d y w m g h t ot lesioned a m m a l s is f u r t h e r
decreased t h r o u g h d e p r i v a t i o n or increased t h r o u g h force
feeding, these s u b j e c t s will, u p o n b e i n g r e t u r n e d to ad h b
feeding c o n d i t i o n s , adjust thexr f o o d i n t a k e so as to regain
t h e i r p r e w o u s , p o s t l e s m n i n g w e i g h t [13]
In the m o s t
d r a m a t i c o f these e x p e r i m e n t s , Powley and Keesey [251
f o u n d t h a t rats starved p r e o p e r a t l v e l y to weights below
t h o s e m a i n t a i n e d by LH lesioned a m m a l s actually b e c a m e
h y p e r p h a g l c a n d g a m e d weight w h e n allowed to feed freely
following surgery The h y p e r p h a g l a c o n t i n u e d until the
animals r e a c h e d a n e w s u b n o r m a l b o d y weight, at w h i c h
they stabilized W h e n dietary p a l a t a b i l i t y a n d caloric
d e n s i t y were m a n i p u l a t e d , LH and c o n t r o l rats s h o w e d
a l m o s t parallel a d j u s t m e n t s m b o d y weight
F r o m these elegant studies Keesey et a l [ 13 ] c o n c l u d e d ,
"a direct or p r i m a r y effect of LH lesions is to l o w e r the set
p o i n t for b o d y weight " If it is g r a n t e d t h a t h y p o t h a l a m i c
lesions can adjust a neural set p o i n t , it follows t h a t t h e r e
m u s t n o r m a l l y exist such a set p o i n t a n d t h a t the LH m u s t
s o m e h o w be revolved in its f u n c t i o n i n g A l t h o u g h these
a u t h o r s a d m i t t h a t damage to s e n s o r y or m o t o r systems
m i g h t r e s u l t in l o w e r e d weight, it is a p p a r e n t l y a s s u m e d
t h a t such damage c o u l d n o t alter t h e level at w h i c h b o d y
weight is actively d e f e n d e d T h u s the c o n c l u s i o n o f Keesey
and his c o w o r k e r s rests on the a s s u m p t m n t h a t o n l y by
a l t e r i n g t h e set p o i n t of t h e b o d y weight c o n t r o l s y s t e m
could t h e weight level w h i c h t h e s y s t e m d e f e n d s be altered
Let us n o w c o n s i d e r an a l t e r n a t i v e m o d e l for b o d y
weight c o n t r o l Let us s u p p o s e t h a t an a n i m a l ' s feeding
m e c h a n i s m is a c t i v a t e d by s e n s o r y stimuli arising f r o m
available f o o d , w h i c h we will r e p r e s e n t by t h e l e t t e r S, a n d
i n h i b i t e d b y a f e e d b a c k signal w h i c h is p r o p o r t i o n a l to
b o d y weight w h i c h we will r e p r e s e n t by t h e l e t t e r W A
c o n t r o l s y s t e m i n c o r p o r a t i n g these a s s u m p t m n s is s h o w n in
Fig 1
In this m o d e l . H a n d G ale b o t h c o n s t a n t s ol p r o p o r t l o n a h t y H r e p r e s e n t s t h e w e i g h t i n g given to a signal w h i c h
d e p e n d s u p o n the h p l d c o n t e n t of t h e a n i m a l ' s b o d y
A l t h o u g h s u c h a signal has n o t yet b e e n Identified, any t y p e
WIRISHAI-It
R X\l) l)\\I,'-
+g g
S
FIG. 1. Diagram ot a simple body weight control model which
contains no set point See text for detalb
of s y s t e m c a p a b l e of c o n t r o l h n g b o d y h p l d c o n t e n t m u s t be
able to m o m t o r this variable G reflects t h e f u n c t i o n i n g o f a
s y s t e m w h i c h p r o d u c e s , m p r o p o r t i o n to ~ts i n p u t , c, a
p a r t i c u l a r b o d y weight The e l e m e n t s s y m b o l i z e d by G
w o u l d t h u s have to c o n t a i n , a m o n g o t h e r things, mecha m s m s for the s h o r t a n d l o n g - t e r m c o n t r o l of r e t a k e G is
s o m e t i m e s k n o w n as the f o r w a r d gain of the system, and H
the f e e d b a c k gain
F o r s l m p h c l t y we will Ignore the possible dynamlc~ ol
the m o d e l a n d c o n s i d e r only p r e d i c t i o n s it m a k e s as to
steady state weight Since the o u t p u t from an.~ box in the
dmgram is simply the i n p u t to it m u l t l p h e d by the
p a r a m e t e r m the b o x , we can derwe a t r a n s f e r f u n c t i o n for
this s y s t e m w h i c h will describe h o w the o u t p u t of the
system, W ( b o d y welght), changes with t h e changes in the
input S and changes m the parameters G and H
Thus
e = S-WH
W=eG
by s u b s t i t u t i o n ,
W = (S-
WH) G
rearranging,
W + GWH = GS
and,
W-
GS
1 + GH
(I)
This last f o r m u l a [ 1 ] is the t r a n s f e r f u n c t i o n for the m o d e l
i l l u s t r a t e d in Fig 1 It describes the c o n t r o l of b o d y weight
(W) in t e r m s of sensory i n p u t a n d the f o r w a r d a n d f e e d b a c k
gain o f the s y s t e m
T h e value w h i c h W assumes is the p r e d i c t e d s t e a d y state
weight of an o r g a n i s m w h o s e b o d y w e i g h t is c o n t r o l l e d by
the s y s t e m s h o w n in Fig. 1. We call t h e value w h i c h W takes
t h e settling p o i n t of t h e o u t p u t o f t h e system, since a n y
change in t h e values of S, G, or H will alter t h e value of W
a n d cause it to settle at a n e w value. Some a u t h o r s ( e . g ,
[32] ) have used t h e t e r m set p o i n t to describe the level at
w h i c h a n i m a l s m a m t a m t h e i r b o d y weights. We p r e f e r the
t e r m settling p o i n t since this t e r m as n e u t r a l w i t h respect to
C O N T R O L O F BODY W E I G H T
the way in which stability is achieved, which m a y or may
not be by means of a neural set point. F u r t h e r m o r e , it
avoids the possible c o n f u s i o n which can result f r o m using
the same term to refer to b o t h the input and the o u t p u t of
the same system
Consider now the l m p h c a t l o n s of this formula for the
control of b o d y weight; it indicates that given c o n s t a n t
sensory properties of a diet and constant gains G and H,
b o d y weight will be constant Suppose that the effect of a
LH lesion were either to lower G or raise H. T h e n b o d y
weight would be lowered to a new settling point. If a
lesioned animal which had achieved this new level were
deprived, causing W to fall below its settling p o i n t
d e t e r m i n e d by the current values o f G and H, then were
f o o d to be offered again, the animal's weight would
increase to predeprlvatlon levels. This is exactly the result
which has been r e p o r t e d by Keesey e t at [ 13]
The m o d e l predicts that increasing the palatability of the
food, 1 e , increasing S, will increase b o d y weight, and
decreasing S will decrease b o d y weight. F u r t h e r m o r e it
predicts that, if LH lesions affect G and H as discussed
above, percentage weight changes for lesioned and control
animals in response to altered dietary palatability should be
identical This is the effect which Powley and Keesey [25]
and Keesey and Boyle [ 12] have r e p o r t e d
Suppose further that v e n t r o m e d l a l h y p o t h a l a m l c (VMH)
lesions have opposite effects on the system, that is t h e y
either increase G or decrease H U n d e r either or b o t h of
these conditions b o d y weight would settle at a new and
higher level If a lesioned animal which had achieved this
new level were deprived, causing W to fall below its settling
point, then were f o o d to be offered again, the animal's
weight would increase until the original high level was
reached This is the result r e p o r t e d by Hoebel and
T e i t e l b a u m [8] using VMH lesioned animals. Again,
w i t h o u t the i n t r o d u c t i o n of a set point notion, the m o d e l
predicts that b o d y weight would be defended. It Is
interesting to n o t e that the m o d e l also predicts that as
dietary palatability is lowered towards zero, the weight of
normal and VMH lesioned animals will converge to a
c o m m o n value, a result r e p o r t e d by Sclafani, Springer and
Kluge [ 2 0 ] , and one which is difficult to predict based on a
simple set point model.
These considerations suggest, therefore, that were
animals' b o d y weights controlled by a system similar to
that of Fig 1, Keesey's results, as well as those of Hoebel
and T e i t e l b a u m [8] and m a n y others, could be a c c o u n t e d
for w i t h o u t recourse to the n o t i o n of a set point. In o t h e r
words changes in the set p o i n t of a system cannot be
inferred from changes in its setthng point, nor can the
existence of an internal set point be inferred f r o m the fact
that a system appears to have a settling p o i n t which it will
defend. It should be p o i n t e d out that if the f u n c t i o n s we
have s y m b o l i z e d by G were p e r f o r m e d in the LH, and those
s y m b o l i z e d by H In the VMH, we would have a m o d e l for
the control of feeding very similar to that of Panksepp
[22], except for the additional presence of a set point in
his model.
This m o d e l suggests that the defense of b o d y weight by
animals may reflect n o t the existence of an internal
standard, but rather a constant m a g n i t u d e of stimuli which
drive the intake mechanism. Were this so, one w o u l d e x p e c t
that changes in dietary palatability should greatly effect
77
b o d y weight This has often been shown to be the case
(e g., [ 5 , 1 4 ] ) , and in fact the fattest rat on record to our
knowledge, 1,445 g, was p r o d u c e d not by h y p o t h a l a m l c
lesion, but by m a i n t e n a n c e on an e x t r e m e l y palatable high
fat diet [ 16 ]
If b o d y weight is regulated by an internal set point
mechanism one naturally wonders why it is singularly
ineffective in maintaining a constant weight in the face of
altered dietary palatability. Rejection of a set point concept
of weight regulation would have the additional advantage of
resolving the problem of an animal's willingness to ingest
highly palatable substances in the absence of deprivation
[10, 23, 16] The i m p o r t a n c e of external stimulation in
controlling feeding behavior is further emphasized by the
m a b l h t y of rats to maintain f o o d intake and c o n s e q u e n t l y
b o d y weight in the absence of oropharyngeal stimulation
[9].
It has been argued that because individuals of the same
species defend different weights, there must exist individual
differences in set points [20]. It should be obvious,
however, that such differences b e t w e e n individuals, as well
as b e t w e e n sexes and age groups, could as well reflect
disparate gains (G and H of our model) s o m e w h e r e within
the weight control system In like manner, the changes in
b o d y weight associated with hibernation [ 11 ], migration
[ 2 1 ] , and pregnancy [ 3 2 ] , could be p r o d u c e d by alteratlons of gains or by the presence of biasing signals which
might, for example, add to S and GW at the summing point
shown in Fig 1 In all of these cases, given our current
knowledge, it seems to be unnecessary and unparsimonious
to i n t r o d u c e the n o t i o n of a set point to explain data which
can as well be a c c o u n t e d for w i t h o u t its i n t r o d u c t i o n
A l t h o u g h the model which we have proposed here is
m u c h t o o simple to provide an explanation of the control
of b o d y weight, it may serve to draw a t t e n t i o n to several
points. First, it suggests a way in which the effects of
variations in the sensory properties of f o o d might be
i n c o r p o r a t e d into a m o d e l of body weight control Since it
is widely recognized that input from gustatory, olfactory
and o t h e r sensory systems play a central role in the control
of ingestion, and hence b o d y weight, it seems essential that
such an input should be an i m p o r t a n t element of any
weight control model. Using sensory stimulation rather
than a set point as the input to the system takes account of
this point It also emphasizes the fact that the a m o u n t of
food which an animal ingests may be more d e p e n d e n t on
sensory factors than upon an internal drive which arises
when nutritional reserves fall below some reference point
Secondly, it demonstrates that the postulation of a neural
set point is not necessary to account for weight control
The fact that a given q u a n t i t y Is, under some circumstances,
maintained at a relatively constant value does not by ~tself
imply the existence of an internal reference value This
point is relevant not only to the issue of the control of
b o d y weight but to any biological control system. F o r
example, regulation and defense of body temperature, or
body water does not necessarily imply the existence of set
points controlling these variables
In summary, this m o d e l suggests a new way of looking at
the problem of the control b o d y weight. Alternative ways
of looking at problems sometimes lead to progress in a
field
78
W1RtSHAI
Ii R \ X [ ) 1~,~\ I ~,
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