Lecture 1 - Energy Balance: Concepts, Behaviour, Physiology & the Environment

Transcription

1 PHY3102 THEME 1 (Lectures 1-5) - Nutrition, Digestion and Absorption Lecture 1 - Energy Balance: Concepts, Behaviour, Physiology & the Environment Overview: - Lectures 1 and 2 will introduce the concept of being able to sustain energy balance and body weight within the framework of living in an obesogenic (causing obesity) environment - Students will begin to explore the complex interactions between physiological drivers of appetite and satiety and how these intersect with behavioural responses to food and eating. - An overview of digestion and absorption of nutrients will be given Objectives: - Understand the complex interactions between human energy intake and energy expenditure - Describe how human's maintain energy balance and the consequences of any imbalance on body weight in adults - Define the interactions between physiological drivers, environmental and behavioural cues that influence appetite and satiety - Calculate basal metabolic rate for a healthy adult - Calculate the energy value of a food item Do we eat to live or live to eat? - At a physiological level = hunger drives us to seek food - Availability of food influences what we may select to eat - Eating behaviour is complex - conceptualised by our culture, food preferences, availability, and frequency of eating episodes Food choices: - What stops you from eating more of your favourite foods? Weight concerns, high cholesterol, bad for you, 'treat foods' - What stops you from eating more of the top 4 healthy foods? Cost, availability, hard to prepare, don't know how to cook them, don't like them very much - Factors affecting food choice: Cultural factors Socioeconomic factors

2 Psychological factors Environmental factors Physiological factors The individual Characteristics of food - An ecological framework depicting the multiple influences on what people eat >> Control over food (energy) intake - Food (energy) intake can be regulated by many different mechanisms: Psychological factors (appetite, cravings, mood) Behavioural (e.g. peer influence, habitual behaviour), social and environmental factors Physiological factors (hunger, stretch receptors and peripheral mediators and metabolic events) Neurotransmitters (chemical messengers generated in neurons in the CNS) Some definitions: - Hunger - a physiological response to a need for food (general motivation to eat). Unpleasant sensation - Appetite - the psychological desire to eat specific foods. Pleasant sensation - Satiation - a feeling of satisfaction and fullness that occurs during a meal and halts eating - Satiety - a feeling of satisfaction and fullness that occurs after a meal and inhibits eating until the next meal Stages in eating: - Initiation of feeding: Physiological sense of hunger Habituation and/or visual and other cues e.g. smell, taste - Choice of foods > innate responses Salt is pleasurable > there is a physiological need for Na+ Sweet is pleasurable > detection of carbohydrate as energy source Savoury is pleasurable > detection of protein Sourness is aversive > under-ripe fruits are generally sour Bitterness is aversive > many toxic alkaloids are bitter - Cessation of feeding Physiological sense of satiety - Psychological factors can over-ride physiological control Variety of foods may stimulate eating despite satiety Gourmet > tempted by choice foods despite satiety Glutton > cues can over-ride physiological sense of satiety Anorexia > loss of appetite

3 REGULATION OF ENERGY BALANCE: 1. Mechanisms that control endogenous energy substrate storage and mobilisation 2. Anorexigenic & Orexigenic mechanisms + nutrients - Nutrients? Which foods keep you hungry after consumption? Which foods satisfy you for longer after consumption? - " Appetite is regulated by an interplay of peripheral and neural mechanisms"

4 Example: Obesity Epidemic 'Obesogenic' Environment: - "Coca Colarisation" - "Nintendisation" - "Computerisation" - " Never in the history of man has food been so readily available to so many populations, presented in such an energy dense fashion (high sugar, high fat) and in such a manner designed to appeal to all the senses"

5 Some global comparisons: - Obesity is a problem both in developed and developing nations - Obesity: BMI > 30kg/m2 Energy balance: - To maintain weight: Energy equilibrium Energy IN = Energy OUT - Intake > Expenditure = weight gain (POSITIVE energy balance) - Intake = Expenditure = weight steady (Energy balance achieved) - Intake < Expenditure = weight loss (NEGATIVE energy balance) Measuring energy intake & expenditure:

6 Components of total energy expenditure: 1. Basal Metabolic Rate (BMR) - The energy needed for the work of vital functions, e.g. maintaining electrolyte balance across cell membranes, cell and protein turnover, respiratory and cardiovascular functions, thermoregulation etc. - BMR is the largest component of energy expenditure - 75% of the variability in BMR is predicted by Lean Body Mass - Obese individuals have a higher BMR They have higher Lean Body Mass as well as Fat Mass (individual variation) - Factors affecting BMR:

7 2. Diet Induced Thermogenesis - The energy expended processing food (i.e. to digest, absorb, transport, metabolise and store ingested nutrients) - Smallest component of daily Total Energy Expenditure (10-15%) 3. Physical Activity - Exercise-related activity thermogenesis - Non-Exercise Activity Thermogenesis (NEAT) NEAT = energy expended on activities of normal daily living (e.g. job-related, housework, laundry, walking ) Accounts for 15% (sedentary) to 50% (highly active) of daily Total Energy Expenditure - The extra physical activity involved in daily living 50 years ago compared with today, has been estimated to be the equivalent of running a marathon a week (NAO 2001)

8 basal - Can use predictive equations, with weight being the main variable Estimating metabolic rate Measurement of activity: - It is very hard to measure accurately in free living individuals - Different types of monitors Energy estimates - Reference male 76kg years 10.8 MJ/ day if sedentary 13.8 MJ for moderate activity - Reference female 61kg years 8.8mH MJ/day

9 - Measure of energy: 1 Joule = the energy used when a mass of 1kg is moved through 1metre by a force of 1 neutron (N) 1 Calorie = the energy required to raise the temperature of 1g of water (H2O) from 14.5 to 15.5 degrees celsius - Conversion factors: 1 kjoule = 0.239kcal 1 kcalorie = 4.18kJ

10 Lecture 2 - Digestion and Major Nutrients, Absorption Objectives: - Understand how food is converted to nutrients within the digestive system - Understand the concept of glycaemic index - Understand how the properties of carbohydrates affect digestion - Understand how macronutrient content of food affects satiety (prescribed reading) Energy in food (recap from lecture 1): - Protein - 1 gram = 17kJ = 4 kcal - Carbohydrate - 1 gram = 17kJ = 4 kcal - Fat - 1 gram = 37kJ = 9 kcal - Alcohol - 1 gram = 29kJ = 7 kcal - Water = 0kJ or 0 kcal NUTRIENTS: Essential and non-essential - 'Essential' suggests cannot be made by the body > required from the diet - There are about nutrients known to be essential to human health - Some metabolic conditions can affect the body's ability to make a nutrient. In these circumstances a non-essential can become essential. This is also known as 'conditionally essential' What are nutrients essential to human health?

11 Major nutrients, digestion and absorption (revise 2nd year physiology) 5 PATHWAYS TO NUTRIENT ASSIMILATION: Nutrient assimilation 1. No digestion required e.g. Glucose 2. Luminal hydrolysis: breaking polymers to monomers e.g. Proteins to amino acids 3. Brush border hydrolysis: CHO oligomers to monomers e.g. sucrose to (glucose + fructose)

12 4. Intracellular hydrolysis e.g. peptides to amino acids 5. Luminal hydrolysis plus intracellular resynthesis e.g. triglycerides to free fatty acids to new triglycerides Amino acids, peptides, sugars enter the portal blood and then to the liver - Liver: 75% of blood flow to the liver comes from the portal vein 25% comes from the hepatic artery All blood OUT of the liver leaves via the hepatic vein

13 - Insulin and glucagon from the pancreas are released into the portal blood Fatty acids and triglycerides enter the lymphatics CARBOHYDRATES:

14 Classification of the major dietary carbohydrates: Functions of carbohydrates: - Energy source: Glucose is the body's main fuel Only source of energy for the brain, red blood cells and part of the kidney Adequate glucose spares protein Adequate glucose prevents ketosis Allows for complete fat breakdown - Excess glucose is stored as glycogen Stored in muscle and liver Glycaemic and non-glycaemic CHO - Rate of glucose uptake from the gut is determined by the rate of hydrolysis of oligo- & polysaccharides - Glycaemic CHO - Mono- & Disaccharides, starch - Non-glycaemic CHO - certain oligosaccharides & non-start polysaccharides (not absorbed in the small intestine and enter the large intestine) - Glycaemic Index (GI) - method of ranking CHO based on their immediate effect on blood glucose levels (glucose - GI of 100; Low GI = 55; High = 70) Effects of the rate of CHO absorption on plasma (glucose) - Concept of the glycaemic index: High GI foods break down quickly during digestion and cause a rapid rise in plasma glucose Low GI foods break down slowly and cause a slow rise in plasma glucose Glycaemic Index:

15 - GI is affect by many physical characteristics of food that affect how readily starch digestion can proceed (physical and chemical properties) - Depends on the ratio of amylose : amylopectin High ratio = low GI = slow absorption GI for glucose = 100% GI is affect by many physical characteristics of food that affect how readily starch digestion can proceed: Comparing the GI of sugars: - Glucose = Sucrose = 61 - Lactose = 46 - Fructose = 19 What is the significance of the GI? - Low GI foods: Have a smaller and more gradual rise in blood glucose - help control diabetes Can help people to lose weight and lower blood cholesterol (however watch out for fat content, as high fat foods can have a low GI) Low GI diets improve the body's sensitivity to insulin - High GI foods:

16 Help re-fuel carbohydrate stores after exercise Benefits of a low GI diet: - Decreased BG levels - Decreased cholesterol levels - Decreased weight - Decreased heart disease risk - Decreased type 2 DM risk Factors influencing GI ranking: Glycaemic load: - Considers the effect that the GI and the amount of CHO have on postprandial blood glucose levels - Incorporates both the quantity and quality of the dietary CHO consumed - E.g. some fruits may have a high GI and be perceived as 'bad' however when consider quantity of CHO/portion the GL is low - GI of 55 is low; GL of 10 is low

17 High GI foods are not highly satiating - and can lead to excess intake Non-starch polysaccharides: definitions - Dietary fibre The proportion of food which is derived from the cellular walls of plants which is digested very poorly in human beings - Non-starch polysaccharides: The major component of the plant cell wall and the best single index of the 'dietary fibre' concept - Total fibre: Dietary fibre = non-digestible carbohydrates and lignin that are intrinsic and intact in plants Added fibre = isolated, non-digestible carbohydrates which have beneficial physiological effects in humans

18 Physiological effects of certain properties of dietary fibre: Dietary fibre and disease - CVD Soluble NSP Cholesterol and bile acid absorption Blood insulin Total fat intake - Diabetes mellitus: Slow glucose absorption Decreased postprandial insulin Decreased blood cholesterol Glycaemic index - Cancer: Potential carcinogens Diluted by fluid attracted to NSP Bound to NSP More rapidly excreted DIETARY FATS: - Appetite and satiety: Longer to eat Stomach emptying Nutrient absorption Dietary sources of fats: - Most fats are sources of saturated, monounsaturated and polyunsaturated fatty acids

19 Fat digestion and absorption: - Absorbed dietary fats - lipids - Hydrolysis (lipase), emulsification - Most fat digestion occurs in the small intestine - Bile breaks large fat globules into small globules DIETARY PROTEIN: Dietary protein: - Best sources: "complete" or "high quality" Eggs Fish Meat, poultry Milk and milk products Soy

20 All essential amino acids In the right amounts 90-99% digestibility - Other sources: "incomplete" or "low quality" Nuts and seeds Legumes Cereals and grain foods Do not contain all essential amino acids 70-90% digestibility Macronutrient uses - CHO and fat are used for energy production - carry micronutrients and essential fatty acids - Fats and proteins are used to make cell structures - Proteins - also used as carriers for minerals, vitamins, signalling molecules (in plasma) - Functional proteins: Collagen Enzymes Transporters Alcohol a non-core energy source! - Quickly absorbed in the stomach Needs no digestion - alcohol dehydrogenase - Broken down for use as energy, primarily in the liver Alcohol and the liver and brain - Fatty liver - B complex - Thiamin and niacin are coenzymes - Sedates inhibitory nerves Acts as central nervous system depressant - Blood alcohol levels and brain responses Death of liver and brain cells Depression of antidiuretic hormone (ADH) Loss of body water and loss of important minerals

21 After this lecture you should be able to: - Define what a nutrient is - Describe dietary sources of macronutrients - Describe nutrient assimilation - Describe glycaemic and non-glycaemic carbohydrates - Describe factors that affect the glycaemic index of carbohydrates - Describe factors that affect the glycaemic index of foods - Explain the physiological significance of glycaemic index - Describe the properties of dietary fibre