1
Building a system dynamics model of body
weight regulation and obesity

• Özge Karanfil
• PhD student
• Department of Biomedical Physiology and
  Kinesiology, SFU
• ozgekaranfil_at_gmail.com
• 19th MIT-UAlbany-WPI System Dynamics PhD
  Colloquium
• October 30, 2009

2
Outline

• Introduction
• Methodology
• Problem Description and Research Objectives
• Data
• Weight cycling, or yo-yo dieting, as a common
  feature

3
Introduction

• Obesity is a problem..
• Factors influencing obesity and the regulation of
  body weight have been under intensive
  investigation
• Growing interest has culminated in the growth of
  simulation models
• as a tool to investigate this complex system
• as a means for evaluating hypotheses concerning
  the underlying pathology
• Obesity is a dynamically complex problem..
• A dynamic problem is one that necessitates
  continuous monitoring and action (management).
  Chronic problems.
• Internal structure is the main cause of dynamic
  behavior

4
Introduction

• Body weight regulation constitutes a suitable
  area for simulation modeling
• Feedback complexity of the underlying structure
• Different levels of factors involved (genetic,
  dietary, life-style ..)
• Time delays, interplay of factors make it
  difficult to make quantitative predictions of
  dynamic patterns
• System dynamics is appropriate for quantitative
  analysis of chronic problems

5
Methodology- Steps of System Dynamics
methodology

• Problem identification (a dynamic feedback
  problem is selected)
• Model conceptualization (causal loop diagramming)
• Model construction (mathematical, numeric)
• Simulation verification testing
• Validation (is my model appropriate for real
  life?)
• Analysis and results
• Implementation

6
Problem Description and Research Objectives

• Most adults maintain a stable body weight and
  composition, in spite substantial deviations in
  their daily food intakes, physical activity
  levels, resulting energy balances.. (Flatt, 1995)
• Energy In and Energy Out tend to remain adjusted,
  and protein, carbohydrate, fat balances are
  achieved
• As long as this is not the case, body composition
  keeps changing
• Deviations from energy balance trigger bodys
  homeostatic mechanisms
• 1-Regulating food intake
• 2-Altering rate of energy expenditure
• 3-Altering the composition of fuel mix used for
  energy generation

7
Some of core physiological mechanisms Energy
Intake-Expenditure
adapted from Abdel-Hamid TK. 2002. Modeling
the dynamics of human energy regulation and its
implications for obesity treatment. System
Dynamics Review 18431-71
8
Some of core physiological mechanisms Energy
Intake-Expenditure
Adapted from Abdel-Hamid TK. 2002. Modeling
the dynamics of human energy regulation and its
implications for obesity treatment. System
Dynamics Review 18431-71
9
Carbohydrate-fat interactions and obesity
examined by a two-compartment computer model
Adapted from Flatt JP. 2004. Carbohydrate-fat
interactions and obesity examined by a
two-compartment computer model. Obesity Research
122013-22
10
CarbohydrateFat Interactions and Obesity
Examined by a Two-Compartment Model- J. P. Flatt

• A systems dynamics computer model to examine how
  interactions between CHO and Fat metabolism
  influence BW regulation
• Environmental factors such as food palatability,
  and availability can be expected to raise the
  range within which glycogen levels are habitually
  maintained.
• This restrains fat oxidation, until expansion of
  the fat mass is sufficient to promote fat
  oxidation to a rate commensurate with dietary fat
  intake. This metabolic leverage can explain why
  increased food offerings tend to raise the
  prevalence of obesity.

Flatt JP. 2004. Carbohydrate-fat interactions
and obesity examined by a two-compartment
computer model. Obesity Research 122013-22
11
Two-compartment model showing the impact of
circumstantial, lifestyle genetic factors on
glycogen levels and fat stores

• Reflects operation of a 2 reservoir-system
  representing the bodys limited glycogen, and its
  large fat reserves. Outflows from reservoirs
  correspond to oxidation of glucose and fat, whose
  relative contributions are affected by size of
  prevailing glycogen and fat reserves.
• Flow through the small turbine reflects the
  exclusive use of glucose by the brain
• Flow through large turbine represents EE in the
  rest of the body to which 2 reservoirs contribute
  in proportion to levels of fuel they contain at a
  given time

The conduit from the small to the large
reservoir lipogenesis
Flatt JP. 1995. McCollum Award Lecture, 1995
diet, lifestyle, and weight maintenance. Am. J.
Clin. Nutr. 62820-36
12
The two main issues for body weight maintenance
in obesity
Well known and less well known variables
affecting adiposity and their potential role in
explaining the increased incidence of obesity
revealed by the (NHANES Ill) data.
Flatt JP. 1995. McCollum Award Lecture, 1995
diet, lifestyle, and weight maintenance. Am. J.
Clin. Nutr. 62820-36
13
Computational model of in vivo human energy
metabolism during semistarvation and refeeding

• A general simulation model for body weight change
  based on the Minnesota Starvation Experiment
  (1940).
• Model consists of 3 macronutrient compartments
  (stocks) fat, glycogen and protein, and fluxes
  (flows) between them.
• Energy intake of fat, carbohydrate and protein
  are external inputs.

Hall KD. 2006. Computational model of in vivo
human energy metabolism during semistarvation
and refeeding. Am. J. Physiol Endocrinol. Metab
291E23-E37
14
Hall Model- Stock-Flow diagram
Adapted from Hall KD. 2006. Computational model
of in vivo human energy metabolism during
semistarvation and refeeding. Am. J. Physiol
Endocrinol. Metab 291E23-E37
15
Hall Model- Stock-Flow diagram
Body Composition
Fat Metabolism
Protein Metabolism
Resting Metabolic Rate
Physical Activity Expenditure
Macronutrient Oxidation Rates
Adapted from Hall KD. 2006. Computational model
of in vivo human energy metabolism during
semistarvation and refeeding. Am. J. Physiol
Endocrinol. Metab 291E23-E37
16
Why to model food intake regulation?

• Diet induced changes in energy expenditure help
  to slightly attenuate the gap between energy
  intake expenditure, but does not have power to
  offset energy imbalances. (Flatt, 1978)
• Regulation of energy intake appears to be a
  more important phenomenon to help achieving a
  steady-state body weight.
• Less well explored by previous models, also less
  is known
• We aim to explore interactions between food
  intake regulation and body composition.

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Different levels

• Human energy and weight regulation is a complex
  of nested feedback processes at multiple levels.
• 1- Physiological aspects
• 2- Aspects between the physiology and the
  behaviour Combination of voluntary and
  involuntary effects
• 3- Environmental, cognitive aspects

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Research Questions

• How does food intake- diet composition effect the
  effort of losing weight?
• How does our body composition change when we lose
  and gain weight, and what are the implications of
  this for further attempts of weight loss?
• Can we simulate this model to examine a common
  feature weight cycling?

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Weight cycling, or yo-yo dieting

• Unintended consequences of dieting Detrimental
  effects on body composition? Both supportive and
  counter-arguments can be found
• Cannon G, Einzig H. 1983. Dieting makes you fat.
  London Century Publishing.
• We believe that drivers for natural weight
  cycles and todays yo-yo dieting and consequent
  body weight trajectories are different in nature.
• First dynamic behavior is mainly driven by an
  externally imposed function.
• We cannot observe a similar external data source
  for the latter.

Fig. Seasonal fluctuations in body weight in
rural Gambian women. Plot derived from over 20
000 measurements in women of child-bearing age.
(Prentice et. al, 1992)
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When does weight cycling become more interesting
for us?

• We see a pattern in the real data, but cannot
  observe any externally imposed functions to
  explain this cyclic behavior .
• No simple, straight answers, no
  straight-line-thinking!
• Endogenous (structural) explanation for weight
  cycles
• Without having external data force, peoples
  weights are oscillating, i.e. problem
  structural.
• Structure Complex interactions between our
  physiology, psychology, behavior social factors
• Problem is quantitative and dynamic well suited
  to computer simulation.
• Iteratively develop models of varying size
  scope along with the development of hypotheses
  supporting evidence.

21
A model for the dynamics of human weight cycling
Adapted from Goldbeter A. 2006. A model for the
dynamics of human weight cycling. Journal of
Biosciences 31129-36
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Summary

• Previous models on body weight regulation are
  explored in detail, reproduced and simulated when
  possible
• The application of system dynamics is appropriate
  because of the central role homeostatic
  (feedback) processes play in human energy
  regulation.
• Regulation of food intake behavior appears to be
  an important phenomenon to help achieving a
  steady-state body weight, needs to be further
  explored
• Investigating the weight cycling phenomenon would
  be a good application of SD modeling.       

23
References

• Abbott, WGH, Howard BV, Christin L. et al. 1988.
  Short-term energy balance relationship with
  protein, carbohydrate, and fat balances A. J.
  Physiology. 255 E 332-7.
• Abdel-Hamid TK. 2002. Modeling the dynamics of
  human energy regulation and its implications for
  obesity treatment. System Dynamics Review
  18431-71
• Abdel-Hamid TK. 2003. Exercise and diet in
  obesity treatment an integrative system dynamics
  perspective. Med. Sci. Sports Exerc. 35400-13
• Bjöntorp P, Sjöström L. 1978. Carbohydrate
  storage in man speculations and some
  quantitative considerations. Metabolism. 271853-
  65.
• Cannon G, Einzig H. 1983. Dieting makes you fat.
  London Century Publishing.
• Chow CC, Hall KD. 2008. The dynamics of human
  body weight change. PLoS. Comput. Biol.
  4e1000045
• Christakis NA, Fowler JH. 2007. The spread of
  obesity in a large social network over 32 years.
  N. Engl. J. Med. 357370-9
• Christiansen E, Swann A, Sorensen TIA. 2008.
  Feedback models allowing estimation of thresholds
  for self-promoting body weight gain. Journal of
  Theoretical Biology 254731-6
• Flatt, JP. 1978. The biochemistry of energy
  expenditure. In Bray GA, ed. Recent advances in
  obesity research. Vol 2. London Newman,
  1978211-28.
• Flatt, JP. 1987. Dietary fat, carbohydrate
  balance, and weight maintenance effects of
  exercise. Am J Clin Nutr. 45 296-306
• Flatt JP. 1995. McCollum Award Lecture, 1995
  diet, lifestyle, and weight maintenance. Am. J.
  Clin. Nutr. 62820-36
• Flatt JP. 2004. Carbohydrate-fat interactions and
  obesity examined by a two-compartment computer
  model. Obes. Res. 122013-22
• Goldbeter A. 2006. A model for the dynamics of
  human weight cycling. Journal of Biosciences
  31129-36
• Hall KD. 2006. Computational model of in vivo
  human energy metabolism during semistarvation and
  refeeding. Am. J. Physiol Endocrinol. Metab
  291E23-E37
• Homer J, Milstein B, Dietz W, Buchner D, Majestic
  D. 2006. Obesity population dynamics exploring
  historical growth and plausible futures in the
  U.S. 24th International System Dynamics
  Conference Nijmegen, The Netherlands.
• Keys A, Brozek J, Henschel A, Mickelsen 0, Taylor
  HL. 1950. The biology of human starvation.
  Minneapolis University of Minnesota Press.
• Mayer J, Thomas DW. 1967. Regulation of food
  intake and obesity. Science.156328-37.
• Oga H, Uehara T. 2003. An Application of System
  Dynamics to an Obesity Prevention Program
  Simulation of the Risk Reduction of
  Cardiovascular Disease and the Savable Medical
  Expenses. Proceedings of the 21st International
  Conference of the System Dynamics Society.
• Prentice AM, Jebb SA, Goldberg GR, Coward WA,
  Murgatroyd PR, Poppitt SD, Cole TJ. 1992. Effects
  of weight cycling on body composition, American
  Journal of Clinical Nutrition. 56 209S-216S

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(My) Questions

• One shortcoming of the second part of this
  proposal
• It may be hard to find empirical data in these
  areas, so that we can test our theories (i.e
  models).
• Do you know any good datasets to study..
• How dieting affects body composition, and vice
  versa?
• Weight cycling/ yo-yo dieting, its effect on
  further attempts of losing weight?
• Contact ozgekaranfil_at_gmail.com