L2 - Energy and Nutrients

What are essential nutrients?

Ones that the body cannot produce (or not produce to an adequate quantity) and must be provided by the diet

What are the macronutrients and what quantities are they needed in?

• Carbohydrates, fats, protein, and water

• Large quantities

What are the micronutrients and what quantities are they needed in?

• Vitamins and minerals

• Small quantities

What is adequate nutrition?

All essential nutrients are consumed in adequate quantities and in the correct proportions

What is adequate nutrition needed for?

• Providing energy

• Regulating metabolism

• Promotion of growth and development

• Maintenance of good health

Is the digestive (gastrointestinal) system metabolically active?

Yes and it accounts for about 10% of energy expenditure

What are the 6 components of the digestive system and what is their function in digestion?

• Mouth - Food is pushed into the throat by the tongue

• Oesophagus - peristalsis (muscular contractions to move food) begins, moving food automatically into the stomach

• Stomach - food is mixed with digestive juices, forming chyme

• Small intestine - digestion continues, nutrients and water are absorbed

• Large intestine - absorbs water, forms and removes stool (faeces) to the rectum

• Anus - remaining waste is expelled as stool

What are the 4 accessory organs of the digestive system and what is their function?

• Salivary glands - produce saliva to moisten food, aiding in easy movement to the stomach

• Pancreas - produces enzymes that break down carbohydrates, fats and proteins

• Liver - creates bile, which helps in the breakdown of fats

• Gallbladder - stores bile from the liver and releases it into the small intestine when needed

Describe the 3 main processes that ingested food undergoes

• Digestion - breakdown of food into smaller molecules

• Absorption - nutrients enter the bloodstream via the gastrointestinal tract

• Elimination - removal of undigested waste

Describe the 2 types of digestion

Mechanical - breaks up the food molecules into smaller pieces

Chemical - food is broken down by the action of chemical agents

Describe the 2 forms of mechanical digestion in the gastrointestinal tract

• Peristalsis - contraction of longitudinal muscle to move through the gastrointestinal tract

• Segmentation - contraction of circular muscle in the small intestine alternating forwards and backwards movements to mix enzymes and chyme and to ensure chyme touches the intestinal wall for absorption

Describe the function of the 5 main stomach regions and structures

• Fundus - acts as a food reservoir

• Body - responsible for gastric secretions, including hydrochloric acid

• Antrum - mechanically grinds food to produce chyme

• Gastroesophageal sphincter - prevents backflow of food into the oesophagus

• Pyloric gland area - controls the release of chyme from the antrum of the stomach into the duodenum of the small intestine

What is the function of the small intestine?

Absorption and digestion of nutrients

How is the small intestine adapted to its function?

• It is lined with villi to increase the surface area where absorption takes place

• Villi are lined with microvilli to further increase surface area

What enzymes are involved in chemical digestion of carbohydrates and where does each one act?

• Salivary amylase - mouth

• Pancreatic amylase - duodenum

• Disaccharidases - small intestine

What enzymes are involved in chemical digestion of lipids and where does each one act?

• Bile salts - duodenum

• Lipase - duodenum

What enzymes are involved in chemical digestion of protein and where does each one act?

• Pepsin - stomach

• Trypsin - duodenum

• Chymotrypsin - duodenum

• Carboxypeptidases - duodenum

• Peptidases - small intestine

What is gastric emptying?

The process by which the contents of the stomach are transferred to the duodenum

Describe how gastric emptying is measured

• Gastric emptying scintigraphy

• A radioactive element is digested with food so that it can. betracked through the digestive system

How is gastric emptying regulated?

Coordinated motor activity in the stomach and proximal intestine, involving smooth muscle, neural and hormonal factors

Why is regulation of gastric emptying necessary?

• It ensured appropriate passage of nutrients and indigestible particles into the duodenum

• It facilitates the absorption of digested food into the bloodstream, supporting efficient nutrient uptake and metabolic health

Name the 4 hormones involved in controlling gastric emptying, their production site, and their effects

• Ghrelin - stomach - accelerates gastric emptying and stimulates appetite (hunger hormone)

• Gastric inhibitory polypeptide (GIP) - intestinal K-cells - delays gastric emptying and reduces appetite

• Glucagon-like peptide-1 (GLP-1) - intestinal L-cells - delays gastric emptying and reduces appetite

• Peptide tyrosine-tyrosine (PYY) - intestinal L-cells - delays gastric emptying and reduces appetite

Describe the 2 phases of gastric emptying for solids

• Lag phase - initial delay of about 30 minutes before emptying begins. This is due to food needing to be broken down into particles less than 2mm in diameter

• Linear phase - a linear pattern of emptying that lasts about 3-4 hours based on volume, consistency, and nutrient content

What is gastric emptying of liquids like?

Generally exponential - initial linear emptying before slower emptying from about 45 minutes

Why is knowledge of gastric emptying of liquids important for nutrition?

• Rapid glucose absorption impacts glycaemic control, which is important for managing metabolic health and diabetes

• Less satiety (feeling of fullness) may affect energy intake and weight

What factors slow down gastric emptying? (6)

• Solids - in comparison to liquids

• Fats - in comparison to carbohydrates and proteins

• Indigestible foods - in comparison to digestible foods

• Higher energy content or density

• Acidity

• Larger volumes and particle sizes

What does food composition and structure affect and what are these important for?

Digestion, satiety and energy intake, which are key for dietary strategies, metabolic health, and sports performance

How does the rate of gastric emptying affect metabolism and health?

• A slower gastric emptying rate leads to a lower postprandial (period after a meal) rise in nutrients

• Reduced absorption leads to a lower metabolic burden due to lower circulation

• Impaired postprandial triacylglycerol (TG) clearance increases cardiovascular disease risk as an independent factor because high fat meals increase the rate of gastric emptying and therefore increase the metabolic burden

Describe TG levels after a meal

• TG levels peak 3-4 hours after a meal

• TG levels return to normal 6-8 hours after a meal

How does the rate of gastric emptying affect glycaemic control?

Small differences in gastric emptying can significantly affect glycaemia (blood sugar levels after eating) in both healthy individuals and those with type 2 diabetes

How does the rate of gastric emptying impact food intake?

Gastric emptying is associated with satiety, appetite and hunger, impacting overall food intake

Why is gastrointestinal distress caused during prolonged exercise?

• The gastrointestinal tract delivers carbohydrates and fluids during prolonged exercise

• Gastrointestinal functions are often compromised during endurance conditions due to a lack of blood flow

• This can cause gastrointestinal distress and is influenced by the nutrient type and previous diet history

How is energy obtained by the body?

Oxidation of macronutrients provided by food and drink consumption

Describe Estimated Average Requirement (EAR) for energy

• EAR for energy is a value intended for healthy adults in the general UK population to prevent nutrition-related diseases

• It is based on average physical activity levels and aims to maintain energy balance and healthy BMI, so it isn’t tailored for individuals

What is the recommended macronutrient distribution for adults?

• 50% carbohydrates

• 35% fat

• 15% protein

How do energy requirements change with age?

• Infants, young children, and teenagers need proportionally more energy for their size to grow and be active

• There is an increase in EAR between ages 11 and 18 due to rapid growth. This increase is greater in boys than girls to support hormonal and muscle mass development

• Energy requirements decrease in adulthood, but actual needs depend on an individual’s physical activity levels

• For older adults, energy requirements decrease as activity levels fall and there is a reduction in basal metabolic rate

How do energy requirements change during pregnancy?

An increase of 200 kcal/day in the final trimester

What is energy intake?

The energy content of food and drink ingested as provided by the major sources of dietary energy

What is food energy determined by?

Macronutrient content

How is food energy measured?

The Atwater system

How does measuring energy content work with the Atwater system?

• The Atwater system measures heat released by burning the food in a bomb calirometer

• This gives average energy values for macronutrients

• Adjustments are then made for digestion, absorption, and excretion losses

• The Atwater general factor system calculates the net metabolisable energy of foods based on their macronutrient content, via chemical analysis

What are the average energy values for carbohydrates, fats, and proteins?

Carbohydrates - 4 kcal/g

Fats - 9 kcal/g

Proteins - 4 kcal/g

Convert 1 kcal into kJ

1 kcal = 4.184 kJ

Describe the 3 components of Total Daily Energy Expenditure (TDEE)

• Resting metabolic rate (RMR) - energy for basic functions, e.g., breathing and circulation

• Activity energy expenditure (AEE) - energy for movement and exercise

• Thermic effect of food (TEF) - energy to digest and process food

What is resting metabolic rate determined by? (5)

• Sex

• Age

• Genetic traits

• Fat-free mass

• Fat mass

How much of TDEE does resting metabolic rate account for in sedentary individuals?

70%

How does resting metabolic rate change with age?

• Infants and younger children have proportionally higher resting metabolic rates due to repid growth and development

• Older adults have lower resting metabolic rates due to a decrease in muscle mass with age

How does resting metabolic rate vary between males and females?

Males generally have a higher resting metabolic rate than females due to a higher muscle mass

Which factor of energy expenditure is the most variable?

Active energy expenditure

What can active energy expenditure range from and to?

15% of TDEE in very sedentary individuals to 50% in very active individuals

What is the energy cost of physical activity determined by?

• Activity level - more active = higher energy expenditure

• Energetic efficiency - how efficiently activities are preformed. An experienced runner will have a lower energy expenditure than novice runners if all else is equal

What is active energy expenditure determined by? (4)

• Genetic traits

• Age

• Sex

• Environment stimuli, e.g., weather, availability, and cultural norms

What is the thermic effect of food influenced by? (2)

• Type of nutrients

• Diet composition

How much of TDEE does thermic effect of food account for in Western diets?

10%

What determined thermic effect of food? (5)

• Diet composition

• Age

• Physical activity

• Obesity

• Insulin resistance

How does age affect thermic effect of food?

TEF decreases with age

How does physical activity affect TEF?

TEF may be enhanced with physical activity

What is an altered TEF associated with?

Obesity

Wjhat is energy balance?

• The balance between how much energy is consumed (energy intake) and how much is expended (energy expenditure)

• A state of energy balance is seen in individuals who maintain their body weight over a sustained period

What is positive energy balance and what can it lead to?

• Energy intake is greater than energy expenditure

• Results in weight gain and can lead to obesity or being overweight if it is sustained

What is negative energy balance and what can it lead to?

• Energy consumed is less than energy expended

• Results in weight loss and can lead to osteoporosis, infertility, and heart failure if sustained

What percentage of adults in England were obese in 2015?

63%

Why is obesity becomign such a big problem?

• 5th on a list of 19 risk factors for death

• Accounts for around 3 million deaths per year

• There are rising obesity rates in children

• More money is spent each year on obesity treatments than what is spent on the fire service, police service and justice service comboned

How is BMI calculated?

BMI = weight (kg) / height (m²)

What is BMI?

A surrogate measure of body fatness that reflects excess weight, not body fat directly

How do BMI thresholds vary with cultures?

Lower BMI threshold values are used for Asian populations due to ethnic differences in body fat distribution and related health risks

What are the advantages of using BMI? (3)

• Simple, cheap, and non-invasive

• Predictive - high BMI is linked to increased risk of cardiovascular disease and diabetes

• Useful in population-level analysis - effective for tracking trends across time, countries, and subgroups

What are the limitations of BMI? (3)

• Not a direct measure of body fat - can’t distinguish between fat, muscle, and bone

• Interpretation varies - influenced by age, sex, ethnicity, and muscle mass

• Doesn’t show fat distribution - can’t distinguish between abdominal and peripheral fat

What is obesity?

Excess accumulation of body energy in the form of fat (adipose tissue)

Describe the 2 types of adipose tissues

• Subcutaneous adipose tissue (SAT) - located just below the skin and is generally considered neutral or protective for health

• Visceral adipose tissue (VAT) - located deep within the abdominal cavity and is associated with increased cardiometabolic risk

Describe the process of fat storage progression and give the metabolic risks at each stage

Stage 1 - Subcutaneous fat storage

• Initial storage of energy in subcutaneous adipose tissue

• Generally considered neutral or positive for health, although abdominal subcutaneous adipose tissue, especially its deep component, can contribute to cardiometabolic risk

Stage 2 - visceral fat accumulation

• Overflow into visceral adipose tissue when SAT is saturated

• Increased risk of metabolic diseases

Stage 3 - ectopic fat deposition

• Fat stored in organs like the liver, muscles, and pancreas, which are normally lean organs

  • Linked to insulin resistance, inflammation, organ dysfunction, cardiovascular disease, obesity, and type 2 diabetes

What is a cheaper alternative to CT scans and why?

• Waist circumference measurements

• Intra-abdominal adiposity (IAA) is closely correlated with abdominal obesity

What are the environmental drivers of obesity? (8)

• It is generally cheaper to eat unhealthy foods than healthy foods, so it’s easier for people on a low income to become obese

• Food is more available in obesogenic environments

• Less healthy food options are readily available

• High fat and salty foods are generally cheaper

• Frequent price promotions on unhealthy foods

• Packaging of unhealthy foods has become more appealing, misleading, and changed portion sizes

• Lots of brands sell unhealthy foods - lots of choice

• If large portion sizes are offered, more food will be eaten than if small portion sizes are offered