Nutrition and Metabolism

Calories

• A Calorie is a unit of energy– Measures how much heat

could be released by completely burning a given amount of material

• The average adult requires around 2000 Calories a day– The exact rate at which we

burn calories is called the metabolic rate

Calories in Food

• Carbohydrates and Proteins have 4 Calories per gram

• Fats have 9 Calories per gram– This is why our body uses fat as a

long-term energy storage material

– This is also why fat tastes delicious: it’s a very calorie-rich food!

• Fun fact: alcohol has 7 Calories per gram!

Metabolism• Metabolism is a combination of

two processes: anabolism (building up of materials) and catabolism (breaking down of materials)– Growing new cells and proteins is

anabolism– Burning glucose for energy is

catabolism• When not active, you are at

your basal metabolic rate (BMR)

Basal Metabolic Rate• BMR is affected by:– Age (decreases with every

decade past 30s)– Muscle mass (more skeletal

muscle, more calories burned)– Thyroid activity and iodine

intake (thyroxine increases BMR)

• Activity above basal levels contributes substantially to your overall calorie use– Exercise burns calories!

Exercise and Metabolism

• Exercise is very healthy to be able to do regularly– Increases muscle mass, which increases BMR– Increases use of cholesterol and saturated fats, so they

don’t clog your arteries– Sweating helps balance your sodium

• Blood cholesterol is measured as HDL and LDL levels, often called “good cholesterol” and “bad cholesterol”– HDL (High density lipoproteins) transports lipids like

saturated fats and cholesterol to liver for processing• Exercise increases HDL levels

– LDL (Low density lipoproteins) transports lipids to fat cells for storage

Anabolism and Catabolism• A major example of both anabolism and catabolism is a

process called gluconeogenesis– The production of blood glucose

• Glycogen is made of glucose, so breaking it down to glucose is pure catabolism

• Proteins and lipids can be disassembled and reassembled into glucose through anabolism– Amino acids release ammonia when this happens

Excess Nutrition• An excess of nutrients above

what is needed to make more cells and perform cellular respiration causes the body to store extra materials– Excess glucose stored as

glycogen (glycogenesis)– Excess glucose and fat also

stored as fat (lipogenesis)– Protein cannot be stored

Lipid Metabolism

Nutrient Storage• People have genetic differences

in how they form and store fat– Some people break down fat

more easily than others (lipolysis)– Fat is also stored in different

places in the body• Due to its high calorie density,

storing fat is vital for life– Pictured is Lizzie Velasquez, who

has a genetic condition where her body literally cannot form fat

– If she were to stop eating for more than a few hours she will die (once her glycogen runs out)

Nutrient Deficiencies• If insufficient carbohydrates or

lipids are eaten, the body digs into its reserves

• The body also lowers the metabolic rate to try and slow the loss of nutrients– Prevents starvation

• Insufficient protein poses serious health risks because protein cannot be made from fat or sugar, so protein is taken out of the muscles

Nutrients and Dieting• Due to sociocultural forces, being

“thin” is currently conflated with being “attractive” by many people– This has led to fats and “carbs”

having a negative connotation since they may result in fat buildup• Can often lead to eating disorders

– Please note that having a high metabolic rate is a better indicator of health than body weight or BMI (body mass index)

• Ironically an increasing proportion of people in the US are also obese due to high-sugar diets

Hunger Effect• Hunger is a complex effect in the body mediated by

the hypothalamus, and affected by ghrelin, leptin, cortisol, glucagon and other hormones, as well as psychological factors– “7PM? Well it’s dinnertime then.”

• Diabetics who inject insulin after a meal often report being very hungry again soon, because their blood sugar has crashed– Processed sugars are unhealthy often because they cause

a large insulin release, a blood sugar crash, and a new feeling of hunger

– The large amount of insulin also ensures that fat cells absorb a lot of the glucose

Hunger and Stress

• Stress (Sympathetic nervous system and cortisol) both block hunger

• Ghrelin is released by the parasympathetic nervous system in response to food stimuli (even thoughts)– This allows you to eat even

when you’re stressed– Also, mmm, donuts…

Who’s Hungry?

• Next week: last week of new material!– Monday: Kidneys– Wednesday: Reproductive Systems– Friday: Review day!