Obesity and Energy Balance

Key Skills• Define obesity and body mass index (BMI), and explain the health

consequences of obesity and underweight.• Discuss contributing factors that have led to an increase in obesity

in the U.S.• Use the “calories in/calories out” model to explain how positive

and negative energy balance lead to weight increase.• Distinguish between hunger and appetite, and explain how various

hormones affect these sensations.• Discuss the metabolic set point hypothesis of obesity, and how it

relates to both hunger and satiety, as well as the growth/division of fat cells.

• Understand factors that affect caloric input, and how we harvest energy from sugar and fat.

• Explain how multiple factors affect a person’s calorie “output,” including their basal metabolic rate, the thermic effect of food, and their physical activity.

• Explain the major components of a sustainable weight maintenance program.

Comparing WeightsSuppose one person weighs 175 pounds and another weighs 120 pounds.

Is this enough information to tell whether either person is overweight?

Does directly comparing the weights of two people tell you which one is “fatter?”

What other information might help a doctor work out what a person’s healthy weight is?

BMIMany factors determine a healthy weight, such as sex, age, ethnicity, and height.

BMI takes height into account to estimate how heavy a person is for their size. BMI < 18.5 = underweight; BMI > 25 = overweight; BMI > 30 = obese

BMI is not for making a diagnosis! Muscular athlete with compact frame may have BMI over 25 (“overweight”).

(Img source:vertex42.com)

UnderweightBeing underweight (BMI < 19) can be dangerous, especially in age.

Factors that cause underweight include genetics, eating disorders, depression, digestive problems, and thyroid conditions.

As people get older, it becomes harder to heal if the body doesn’t have some fat reserves. Especially in older women, being very thin can be associated with osteoporosis and brittle bones.

(Img source:Medguidance.com)

Obesity in the USThe rate of obesity in the United States has nearly tripled since the 1960’s.

Between 1980 and 2012, the percentage of adolescents (age 12-19) who are obese increased from 5% to 21%!

Even children as young as 6-11 years old are now showing obesity (18% of children in this age group in 2012).

Hypothesize some factors that could be worsening this epidemic of obesity.

(Img src: Centers for Disease Control)

Causes of ObesityObesity is caused by caloric imbalance. The body takes in more dietary calories than it releases through exercise and body functions, and stores the excess energy by making fat.

A simplified model:

Calories in > calories out = Weight gain

Calories in < calories out = Weight loss

Be careful! Losing weight safely is NOT as simple as just reducing your caloric intake or increasing your caloric output with exercise. As we’ll see, a combination of calorie in/calorie out/appetite-based approaches with healthy meals is needed to lose weight in a maintainable way!

(Img credit: Centers for Disease Control)

Factors in the Obesity EpidemicFactors raising the Calorie Input of Americans:• Soda and juice consumption.• Snacking• Bigger portion sizes• “Value-sized” empty calories• Advertising• Fast food

Factors lowering the Calorie Output of Americans:• Limited time for exercise• Sedentary lifestyle/jobs• Increased use of TV and video games• Lack of safe parks and walking areas

Portion SizesThe average portion sizes for a burger and fries have tripled since 1960.

Large portion sizes have a psychological effect on appetite (the want to eat). People are more likely to overeat when more food is put in a container.

Why have portion sizes increased so drastically, and how can consumers cope with this?

Hunger vs. AppetiteAppetite is psychological and physiological. It determines how much you eat.

Psychological: Portion size, food platingPhysiological: Leptin, dopamine

Hunger is physiological - it makes you feel like you need to eat something.

Low blood sugarMetabolic set point hypothesis

Satiety is the sense of “fullness.”

Metabolic Set PointOne factor that affects both hunger and appetite is how much fat a person already has!

Fat cells (adipocytes) make hormones that regulate satiety. When you lose fat, your body compensates to increase calories in and reduce calories out by making you hungrier.

This is the metabolic set point hypothesis, and helps explain why it’s so hard to sustainably lose weight.

The brain has a built in adipostat, which is like a “thermostat” for fat. Weight loss is possible, but it effectively requires you to reprogram your brain.

Fat Cell Hyperplasia vs. Hypertrophy

When we make more fat in our bodies, fat cells need to either expand or divide to store it.

Hypertrophy is when fat cells individually get larger to store more fat. This is the most common way fat grows in adults.

The division of fat cells, which increases the number of fat cells, is called hyperplasia. This is especially common in childhood and adolescent obesity. These fat cells all secrete hormones and tend to set the metabolic set point towards more calorie storage. This is one reason why it’s harder to reverse obesity that begins early in life.

Caloric Input: Metabolism of NutrientsLast lecture, we discussed how the digestive system breaks starch, triglycerides, and proteins into sugars, fatty acids, and amino acids.

But how does the body actually get usable energy (calories) from these macronutrients?

There are three steps needed to harvest energy from sugar: glycolysis, the Krebs cycle (Citric acid cycle), and oxidative phosphorylation ( electron transport chain). These steps produce the main “energy storage” molecule of cells: adenosine triphosphate (ATP).

ATP ATP is used by most cell processes for energy. It consists of adenosine (part of DNA), and three phosphate groups.

Breaking off one phosphate and bonding it to oxygen releases energy for use.

Releasing that phosphate turns ATP into ADP (adenosine diphosphate). We need to eat and breathe to “recharge” ADP back to ATP.

Glycolysis: First Step in Glucose Metabolism

Whoa! That’s a lot of steps!

Can we simplify this a little?

Glycolysis: First Step in Glucose Metabolism Breaking down one glucose (6-carbon sugar) gives us two pyruvate (3-carbon) molecules, a little ATP, and some NADH.

NADH is a carrier of high-energy electrons we’ll use in a later step!

Pyruvate gets turned into acetyl CoA

We lose one carbon from each pyruvate. This makes a little more NADH and releases carbon dioxide waste.

Acetyl CoA can then go into the mitochondria, parts of the cell specialized for harvesting energy.

The Krebs Cycle (citric acid cycle) harvests energy from acetyl CoA The Krebs cycle gives us a little

ATP, makes more NADH and FADH2 (a different electron carrier), and finishes breaking down the sugar.

At this point, glucose has been broken down into CO2. But what do we do with the NADH and FADH2?

Organisms that don’t use oxygen perform fermentation to get rid of the extra NADH/FADH2. But we can use it more efficiently.

The Electron Transport Chain (oxidative phosphorylation) uses oxygen and

NADH/FADH2 to make ATPThis step produces the most ATP. Oxygen is turned into water, and NADH is recycled for later use.

We aren’t ready to fully understand what is happening here… but we’ll revisit it after we’ve studied electrolytes.

Glucoseglycolysis

Pyruvate

Acetyl CoA

Fat

Beta

oxid

atio

nA little ATPNADH

Mitochondrion

Kreb’scycle

CO2

Some ATPNADHFADH2Electron transport

chain

Carbon chain processing is in red.Electron carriers in blue

Lots of ATPWater

Oxygen

Another Diagram (Same Pathway, without Fat)

Oxygen and Human LifeWhile we can live for days without water and weeks without food (though it’s not recommended), we can only survive for minutes without oxygen.

Given everything we’ve seen here so far…

Why is a constant supply of oxygen so critical for survival? Why do we die so quickly if we don’t breathe?

Brain Oxygen DemandThe brain uses 20% of the body’s oxygen, roughly, because it needs a huge amount of ATP.

The cells that make up the brain, called neurons, need ATP to send messages back and forth. Without these messages, a person is brain-dead. When part of the brain loses oxygen because of lack of blood flow from a clot, a stroke results.

Calorie OutputThere are three major ways we spend and release energy, burning calories:

1. Basal metabolism (Resting Energy Expenditure): The energy we burn just to maintain our body temperature and keep us alive.

2. Physical activity: Exercise and daily movement. In addition, physical activity increases our Resting Energy Expenditure!

3. Digestion + Metabolism of Food: It takes energy to break down food and harvest energy from it. Some foods take more energy to break down than others. We call this the Thermic Effect of Food.

Resting Energy ExpenditureResting Energy Expenditure consists of the calories you burn

even when you are not physically active or digesting food. Basal Metabolic Rate specifically refers to the REE after waking up in the morning.

About 2/3rds of the calories you burn in a day are REE! Keeping your body warm takes a lot of energy.

Having more muscle can increase REE. On the other hand, crash diets and heavy food restriction lower REE, making it harder to lose weight and maintaining the metabolic set point.

Aging also decreases REE.

Physical Activity

Regular physical activity, which raises REE, is better than occasional fits of exercise.

As the diagram shows, it takes a long time for exercise alone to burn off calories!

Physical

activity

Digestion/Thermic Effect of Food

About ten percent of the calories you eat are burned in the process of digestion.

This is the Thermic Effect of Food. While it may be slightly higher for protein, research remains ambivalent on whether this effect is sufficient to explain weight loss on high-protein diets.

A Maintainable Diet

A good diet is not one that a person uses for six months, then drops. Many dieters regain the lost weight! (The often-quoted figure of “95% of dieters regain lost weight” may be too pessimistic, but the regain rate is high.)

To avoid this, long-term lifestyle change is needed. If you eat fewer calories, you may feel less energetic. If you exercise more, you may feel hungrier.

The only way to sustainably, safely lose weight is to eat healthily and exercise regularly in a way you can maintain.