Nutrition Course Materials

• Dairy • Nuts, seeds, beans and tofu • Fish, poultry and eggs • Vegetables and fruits • Healthy fats/oils • Whole grains • Daily exercise and weight control What does a healthy diet consist of? Organic nutrients that we digest and store in our bodies contain a certain amount of energy that is released during metabolism What is the energetic value of food? 1000 cal (calories) = 1 kcal= 4.18 kJ 1g fat = 8.8 kcal 1g carbohydrate = 3.8 kcal 1g protein = 4 kcal 1g alcohol = 7 kcal The amount of energy your body burns at rest on a daily basis The BMR for a 65kg male is 1800kcal and the BMR for a 55kg female is 1424kcal (approximately 1.1kcal/min) We need to multiply the BMR value by a certain factor depending on physical What is BMR (Basal Metabolic Rate)? activity level (PAL): x1.4 if you are inactive x1.6 if you are a female and have a moderate activity level x1.7 if you are a male and have a moderate activity level x1.8 if you are a female and are active x1.9 if you are a male and are active The UK average PAL is 1.5-1.6 Carbohydrate- 300-350g Lipids- 70g Protein- 50g Dietary fibres- 70g 1 / 22 Vitamins and minerals- depends Trace elements eg iodine, selenium- deWhat is the average daily intake of differpends ent food contents in adults? - the thyroid hormones contain iodine so iodine is required for thyroid function What are DRVs? Dietary Reference Values They are not recommendations but are the amounts needed for good health DRVs for carbohydrates and fats are given as a percentage of energy intake, eg the amount of carbohydrates consumed should be 50%, and not more than 5% of this should be free sugars, however the daily % consumed in average by the adult population is 60-80% and 20-30% of this is sugar What are carbohydrates? • Carbohydrates that we consume are 60% starch and 40% sugar (30% sucrose and 10% lactose + small amount of free sugars eg glucose, galactose and fructose) • Carbohydrates make up 60-80% of our daily calorie intakes, and 10% of this is lactose • Starch, sucrose and lactose are main glycaemic carbohydrates in the human diet • A single monomer of a carbohydrate is a monosaccharide eg glucose, galactose, fructose • Two monosaccharides joined together via a glycosidic bond in a condensation reaction form a disaccharide eg maltose, sucrose, lactose • Many glucose molecules joined together form a polysaccharide eg glycogen, starch, cellulose - Glycogen is the main energy storage 2 / 22 molecule in animals and is formed from alpha glucose molecules joined together by 1-4 and 1-6 glycosidic bonds - Starch stores energy in plants - Cellulose is in cell walls of plants and is composed of long, unbranched chains of beta glucose joined by glycosidic bonds How do we classify dietary carbohydrates? • Free sugars - mono/disaccharides - rapidly digested but some escape digestion • Sugar alcohols - poorly absorbed in the small intestine - some forms eg in a polyol structure can be used as food additives or as a sweetener - some can be toxic • Short-chain carbohydrates - starch is partially hydrolysed to create dextrins, which are short glucose chains - made of up to 17 glucose molecules - rapidly digested • Polysaccharides - eg starch, which is the most abundant dietary carbohydrate - can be made of 100s of glucose molecules - some types of starch are rapidly digested by enzymes in the small intestine eg amylopectin - some types of starch are slowly digested in the small intestine eg amylose • Non-starch polysaccharides - escape digestion in small intestine and are fermented to different extents - eg cellulose, which encapsulates and slows absorption of nutrients - good marker for naturally fibre-rich diets for which health may benefit 3 / 22 • Glucose, fructose and galactose are monosaccharides • Sucrose is a disaccharide made up of What are the different sugars that make glucose and fructose up carbohydrates? • Lactose is a disaccharide made up of glucose and galactose • Maltose is a disaccharide made up of two glucose molecules • Glucose is in sugary drinks, highly processed foods and baked goods • Fructose is in fruit mainly but also vegetables, honey and sugar cane • Galactose is in dairy products, potato's, sugar beets and avocados From which foods can we consume car• Sucrose is in fruit and veg, fizzy drinks, bohydrates? sweeteners • Lactose is in dairy products, mainly milk • Maltose is in mainly fermented grain eg wheat, cornmeal and barley, cereals and syrups • Starch is in wheat, rice and corn What is the structure of starch? • Lightly branched polymer of ±glucose molecules - Linked at 1-4 glycosidic bonds - Branched with 1-6 glycosidic bonds • The more 1-6 bonds, the more branching, and the more the starch is soluble and rapidly digestible, because the amylase can bind to the starch much easier What are the two types of starch? • Amylopectin and amylose • Amylopectin is highly branched as it has a 1-6 bond at every 24-26 glucose units, making it very soluble and rapidly digestible • Amylose is a more linear molecule and has very few 1-6 bonds, making it poorly soluble and slowly digested • Starch with a high amylose/amylopectin 4 / 22 ratio is more resistant to digestion • Glycogen (polymer of glucose) is very highly branched with 1-6 bonds occurring every 8-10 glucose How is starch absorbed? • Only monosaccharides and some disaccharides can be absorbed. Polysaccharides have to be cut into glucose, galactose and fructose. • Pancreatic ±amylase acts on linear (amylose) and branched (amylopectin) starch to break it up, however it can only break the 1-4 bonds. The end products are: - maltose (2 glucoses) - maltotriose (3 glucoses) - ±limit dextrin (branched oligosaccharides). • There are specific enzymes on the brush border membrane of the intestinal cells which can hydrolyse these end products - sucrase and isomaltase cut the branched dextrin into maltotriose or maltose - maltase cuts the maltotriose and maltose into glucose. The glucose is then transported inside the intestinal cells via an active transporter eg sodium gradient • Rate and extent of digestion and absorption of starch in the small intestine varies - RDS rapidly digestible starch eg cornflakes, crackers, white bread, white rice, potato's - SDS slowly digestible starch eg some types of rice eg Basmati rice, pasta - RS resistant starch 5 / 22 What is lactose intolerance? • The inability to fully digest lactose in dairy products • It is not serious (but is uncomfortable) • It is very common in adults but very rare in children • It is more common in people of African or Asian origin (specifically prevalent in some Chinese communities) • It is caused when the lactase enzyme on the membrane of intestinal cells which cuts lactose into glucose, has reduced activity/ becomes inefficient as the person enters adulthood. Thus the lactose cannot be hydrolysed, accumulates in the intestinal lumen, ferments and causes discomfort • There are various degrees of lactose intolerance, some people are partially intolerant and some are completely intolerant • Patients with lactose intolerance mainly avoid milk, but some patients may have to avoid any food that contains even a small amount of lactose eg - yoghurt - then cream and cheese - if this is not enough then salad cream, salad dressing and mayonnaise - then biscuits, chocolate, sweets, cake, many processed food etc 6 / 22 How does consumption of carbohydrates cause tooth decay? • The presence of sugar in the mouth favours dental decay • This is mainly due to sucrose (not much maltose) • Sucrose can be hydrolysed into glucose and fructose by enzymes, and then polymerisation of glucose occurs. This glucose polymer will attach to the surface of the tooth and the bacteria residing in the mouth eg S.mutans will use this polymer and accumulate on top of it to adhere strongly onto the surface of the tooth, causing decay. • The bacteria will use fructose to get energy in the form of ATP, and the fructose will be metabolised into lactic acid which will accumulate and cause dental plaques and cavities. • Fizzy drinks contain a lot of sucrose, so the acidity as well as the sucrose is a major cause for tooth decay What is the glycaemic response? • The effect that food has on blood sugar (glucose) levels after consumption, and how quickly blood glucose rises and elicits an insulin response • Foods rich in sugar have a high glycaemic response • Foods rich in starch have a variable glycaemic response, depending on the nature of the starch eg amylose is very slowly absorbed so has a low glycaemic response, amylopectin has a high glycaemic response • Foods with a high glycaemic response means we get a higher concentration of glucose in the blood after consumption, which can cause a detrimental effect, therefore we need to be mindful when a patient has a deficit of the hormone 7 / 22 insulin in the blood which regulates glucose levels. This is a problem in pre-diabetic and diabetic patients. • Eating a lot of food with a high glycaemic response over a long period of time can cause glucose levels to rise sharply which can cause diabetes in the long-term. What is the glycaemic response of starch? • The effect that starch has on blood sugar levels after consumption • Diet containing large amounts of rapidly digested carbohydrates can elevate blood glucose too quickly, which can be detrimental to health • Diets rich in slow digested carbohydrates eg amylose, starch mixed with fibre, can have a beneficial effect as it results in a more modest glycaemic response, so may prevent very high levels of glucose in the blood and protect against chronic disease eg diabetes. • The rate and extent to which starch is digested and absorbed, and the resulting glucose and insulin response, varies considerably depending on the source and degree of food processing - in many plant foods eg legumes and minimally processed cereal grains (eg pearled barley), nutrients are encapsulated within cell walls (dietary fibres/ non-digestible carbohydrates) which delay the release and hence digestion and absorption of starch and sugars, and these foods have low RAG (rapidly available glucose) values. 8 / 22 High • White bread • Dates • Puffed rice cakes • Potatoes, particularly when they are hot • Biscuits • Some fruits What are foods with a high and low glycaemic responses? Low • Legumes • Lentils • Beans eg kidney beans • Pearled barley • Nuts eg peanuts, cashew nuts • Some pasta, particularly when they are al denté What is the glycaemic index? • Measure of change in blood glucose following ingestion of a particular food • It is calculated as the measured glycaemic response to a portion of test food that contains 50g of available carbohydrate (which includes fructose) expressed as a percentage of the glycaemic response to the same amount of available carbohydrate from a standard food eaten (usually white bread as it is quickly converted into glucose and absorbed, or just glucose can be used) by the same subject • The glycaemic index for selected foods can be displayed on a table as a percentage where glucose would be 100%. A polymer of alpha glucose (starch) can What is the difference between a poly- be absorbed mer of alpha glucose and a polymer of A polymer of beta glucose is not abbeta glucose? sorbable/ hydrolysable by amylase so will remain in the intestine 9 / 22 What are amyloplasts? What are dietary fibres? Amyloplasts are specialised organelles responsible for the synthesis and storage of starch in plants Non-digestible carbohydrates that are intrinsic and intact in plants and are resistant to digestion and absorption in the small intestine In plant cell walls, they can be defined as non-starch polysaccharides Examples of dietary fibres are pectin in plants, which are small carbohydrates attached to the cell wall How do we classify dietary fibres? Either soluble (viscous) or insoluble What are soluble fibres? • They have some metabolic advantages eg they can modulate inflammation, oxidation, insulin resistance and cholesterol metabolism so can reduce LDL cholesterol • They can reduce or slow down absorption of some fat in the body, as they may impair bile acids reabsorption, which causes hepatic conversion of cholesterol into bile acids, leading to more important disposal of cholesterol out of the body • They may mitigate the association between digestive enzymes and their substrate, thus slowing down the absorption of nutrients • They have prebiotic action so may stimulate growth of some specific strain of bacteria in the intestine and gut eg bifidobacteria and lactic acid bacteria. These bacteria feed on the soluble fibres and release product eg short chain fatty acids like acetate, butyrate, propionate, 10 / 22 which have beneficial metabolic effects eg insulin resistance What are examples of soluble fibres? • Pectin, which is a complex polysaccharide found in fruit and veg eg orange, broccoli, carrots, potato, and they can interfere with digestive enzymes • Beta-glucan, which is a polymer of glucose found in oats, barley and bran and can impair nutrient digestion • Inulin and fructan, which are polymers of fructose, they can be partially degraded and absorbed as fructose, and have strong prebiotic function • They may lower the pH of the colonic environment, thereby preventing the growth of some pathogenic bacterial species • They may upregulate the expression of gut peptides that are very beneficial for health eg glucago-like peptide 1 (GLP-1) which helps release of insulin hence why a derivative of this peptide can be used to treat diabetes, and peptide YY (PYY). These both can help to regulate appetite, What are biological effects of short chain glucose and insulin homeostasis fatty acids (released from bacteria feed• They can further promote the growth ing on soluble fibres)? of bifidobacterium and lactobacillus species (can act as prebiotics) • They have systemic effects so can penetrate in the blood and activate selective receptors mainly expressed by immune cells and adipose tissue, which can have important metabolic effects eg can act as insulin sensitisers (good for diabetic patients), and anti-inflammatory effects • A diet with too much fibre means excessive short chain fatty acids are produced 11 / 22 and this can disturb intestinal permeability and be harmful eg this is seen in IBS • Skin of vegetables eg carrots • Cellulose • Polyphenol (found on cell walls) • Lignin- complex structure which may What are examples of insoluble fibres? control intestinal transit and may improve absorption of important mineral ions • Found in foods eg bran, kidney beans, lentils, green peas What are benefits of insoluble fibres? • They promote intestinal transit by absorbing water and making the stool more watery, therefore is good for constipation • They can favour absorption of mineral ions What is insulin? • Insulin is a hormone released when glucose levels in the blood increase eg after a meal • Insulin prevents us from having a high level of glucose in the blood, and helps us to store glucose and fat after a meal • Insulin stimulates glucose uptake in the muscles through the blood, and most glucose ingested enters the muscles. The glucose is then either immediately converted into ATP or it is converted into glycogen in the muscles to be stored which can be later used to make ATP • 20% of glucose doesn't enter the musWhat happens to glucose after absorpcles, it goes to the liver and it is transporttion? ed in the hepatocytes, where majority is converted into glycogen through glycogenesis and a smaller portion undergoes glycolysis and is converted into ATP • 2-4% of glucose intake is converted into triglycerides/ fatty acids through de novo lipogenesis • If you are diabetic, you have a deficien12 / 22 cy of insulin so your levels of glucose remain high in the blood. The muscles cannot use glucose for energy so you may feel tired. • High levels of glucose in the blood can be toxic in high concentrations as it can attach to some proteins and lead to tissue damage • All ingested fructose ends up in the liver, it is not transported to the muscles. It undergoes fructolysis, and can synthesise lipids in the liver if over-consumed What happens to fructose after absorp- (this is de novo lipogenesis where new tion? lipids are synthesised in our bodies) • A very small amount can be converted into glycogen, as fructose can be converted into glucose and then glucose can be converted into glycogen • In a low carbohydrate diet, the glycogen pathway occurs preferentially, so most of the glucose that reaches the liver will become converted to glycogen • In a high carbohydrate diet, the storage of glucose as glycogen is saturable so you cannot synthesise more than around 100g. This means most of the glucose will undergo glycolysis and fatty acids What is the role of carbohydrates in the can be synthesised through lipogenesis. diet? Some of the product of glycolysis can be used to make up lipids, which is a problem in people who are obese as consuming a large amount of dietary carbohydrates can lead to more fat • If total carbohydrate energy intake exceeds total energy expenditure (exercise), the synthesis of new lipids can increase through de novo lipogenesis. • In obese patients, up to 10-26% of car13 / 22 bohydrates from the diet can be converted to fatty acids, compared to the normal 2-4%, as obese patients are more vulnerable and have metabolic disturbance What is the role of fat in the diet? • 90-95% of fat consumed is in the form of triglycerides, and 5-10% is in the form of phospholipids, vitamins and cholesterol • Average daily intake of fat is 60-100g, which is an average of 20-70g of fat per meal • Absorption of fat is slow (between 3-5 hours). We spend most of the time in the day digesting fat, with a continual fluctuation of lipid levels in the blood. Fat is not digested in the middle of the night and lipid levels do not fluctuate • Other nutrients may greatly contribute to inter-individual variability in postprandial lipidema (levels of lipids in the blood after a meal) How are fats used in the body? • Fats are an important source of energy • Fats ensure thermal insulation of major organs, through adipose tissue • Fats are a vehicle for the absorption of fat-soluble vitamins • Fatty acids like linoleic and alpha-linolenic acids, which are precursors of prostaglandins and eicosanoic acids and omega-3, are essential • Fats are essential components of cell membranes 14 / 22 How are fats absorbed? Pt1 • Various lipids eg fatty acids, triglycerides and cholesterol, are solubilised in micelles, which are transported from the site of absorption across the apical membrane of absorptive enterocytes and then are absorbed by intestinal epithelium cells, where biosynthesis of complex lipids take place. - Fatty acids should not remain in the cytoplasm as it can be very toxic • In the Golgi system of the epithelial cell, fatty acids and monoglycerides reassemble to triglycerides and are packaged up with protein, cholesterol and phospholipids, to form chylomicrons - chylomicrons consist of 90% triglycerides, 5% phospholipids, 4% free fatty acids, 1% cholesterol and <1% proteins (called apoproteins) How are fats absorbed? Pt2 • The chylomicron will be transported through the lymphatic system and then the blood, and bind on the adipose tissue to deposit their fat content, and they can also bind to skeletal and cardiac muscles too - they are removed rapidly from circulation, but are present for 3-6 hours as new chylomicrons are continuously appearing in the circulation, since digestion of fat is slow • The triglycerides are then hydrolysed back into fatty acids so it can be absorbed by the adipocyte 15 / 22 What happens to fats after it is absorbed? • Fats are used as an energy storage (as triacylglycerol) • They are released into the circulation and bind to serum albumin (SAB) • They are taken up by tissues and used for oxidation as a structural or functional lipid - they can act as structural lipids in the lipid membrane and also as surfactants - they can act as functional lipids as cell signalling agents and hormones • For example the function of certain lipids include - diacylglycerol and ceramide (from sphingolipid hydrolysis, pro inflammatory agent) are second messengers - some fatty acids can activate some receptors - steroids are cholesterol derivatives and act on receptors, and they can sometimes change gene transcription What are fatty acids? • Carboxylic acids, typically with a single long chain, although they can be branched • Fatty acids can be saturated (C-C) or unsaturated (C=C) • They have more than one C=C unsaturated bonds in their structure • There are two families of essential polyunsaturated fatty acids: C20 and C22. • There are omega-3 fatty acids, where the carbon-3 from the end of the fatty acid tail is involved in double bonding (C=C) • There are three types of omega-3 fatty acids: eicosapentanoic acids EPA, alpha-linolenic acids ALA and docosa16 / 22 hexaenoic acids DHA • There are also omega-6 fatty acids, where the carbon-6 from the end of the fatty acid tail is involved in double bonding (C=C) • There are four types of omega-6 fatty What are polyunsaturated fatty acids? acids: linoleic acid LA, arachidonic acid ARA, gamma linoleic GLA, and conjugated linoleic acids CLA • Ecosanoids are derivatives of arachidonic acid. They are precursors of prostaglandins but can also have pro-inflammatory effects themselves. What are benefits of omega-3 fatty acids? • Alpha-linolenic acid is a precursor of eicosapentanoic acids EPA and docosahexaenoic acids DHA • They can reduce inflammatory response, by weakening the inflammatory processes triggered by arachidonic acid derivatives. They compete with the arachidonic acid derivatives, however they only act as partial agonists so have a weak effect • They can have beneficial cardiovascular effects- DHA lowers blood pressure, reduces risk for heart disease, and decreases blood clots • They reduce cholesterol and triglyceride levels • They can improve cognitive function • They can incorporate in lipid membranes • They have a huge variety of metabolic effects eg interaction with some nuclear receptors, promoting gene transcription for important metabolic enzymes 17 / 22 What are sources of omega-3? • ALA is found in oils eg fish oils, algal oils, plant oils eg walnut, rap seed and flaxseed oils • DHA is found in fish, a small amount in egg yolks and very low/ absent in meat products - DHA is mainly only found in fish so it is a problem for vegans/ vegetarians. However a very small amount of EPA can be converted into DHA, therefore vegans who can only have oils can get the ALA, and the ALA converts into EPA (which is more biologically active) which can then convert into DHA, however this conversion is very limited. What are sources of omega-6? • Poultry • Eggs • Nuts • Cereals • Whole-grain breads • Most vegetable oils • Milk • Variable contents in dairy products and meat 18 / 22 • It is advised to consume about 500mg of combined DHA and EPA per day OR consume at least 1.1-1.4 grams of ALA per day • People with heart disease are advised to consume even more DHA and EPA (around 1000mg) daily • There should be a ratio of around 2-3 of omega3 - omega6 that is consumed (in some western diets, this ratio can reach 15-16.7, which can promote/worsen some diseases eg obesity, cancer, CVD, inflammatory and autoimmune diseases). You shouldn't consume more than 3x of omega6 than omega3 (omega6 is normally consumed in exWhat advice is related to consumption of cess compared to omega3) omega fatty acids? - this is to make sure that you don't consume too much of omega6 which are precursors of inflammatory agents, which can promote and favour more inflammatory processes • Overheating oils rich in polyunsaturated fatty acids can be harmful due to hydrogenation and epoxidation of fats. However oils rich in saturated fats eg palm oil, coconut oil, can be consumed in a reasonable amount as part of a balanced diet • Therefore eating unsaturated fat is beneficial for health, but only if it is not overheated and if the amount taken of omega3 and omega6 is not disproportionate 19 / 22 What is hydrogenation of fat? • Heating polyunsaturated fat (frying) can cause hydrogenation of the double bonds • This can form both saturated hydrogenated fatty acids and trans-fatty acids - saturated hydrogenated fatty acids are formed by breaking the double bond in the unsaturated fat - trans-fatty acids are formed by the unsaturated fat changing from the cis form to the trans form upon heating (double bond remains) • They can increase LDL/ cholesterol in the blood • They can be detrimental to the cardiovascular system, especially when consumed in excess • The trans-fats that are present in industrially processed food eg pastries (iTFindustrial trans-fats) are harmful as they are shown to increase LDL cholesterol and therefore increase risk of CVD. However some trans-fatty acids are not harmful, and these are the naturally occurring trans-fats (rTF- ruminant naturally occurring trans-fats), which are different to iTFs. They are found in ruminant meat and dairy products. rTFs can make up to 9% of the total fat content in a persons diet, and they do not seem to have the same toxic effect as iTFs, they may even have beneficial anti-inflammatory effects or may even lower the LDL (still under investigation). 20 / 22 What is epoxidation of fat? • A problem which can occur when overheating polyunsaturated fatty acids. During the process of heating at high temperature, polyunsaturated fatty acids can undergo epoxidation, which is where an oxygen atom binds to the carbons that were previously in a double bond. • Oils with high concentrations of polyunsaturated fatty acids eg some vegetable oils, are more susceptible to lipid oxidation when heated/fried, so these oils should be avoided when cooking. • The oxidised fats produced are toxic. Small amounts can be tolerated however when levels become too high, it is harmful and dangerous as it can produce free radicals, which can be involved in toxic processes eg promoting tumours. What is cholesterol? • A waxy, fat-like substance that's found in all cells of the body • Daily intake is around 200-500g • 30-80% of cholesterol consumed is absorbed • Bile acids are molecules synthesised from cholesterol in the liver. Bile acids facilitate fat digestion and absorption, as bile acid synthesis generates bile flow which is important for biliary secretion of free cholesterol. Bile acids are recycled through the enterohepatic circulation • Dietary cholesterol is absorbed by the brush border transmembrane protein NPC1L1 • Ezetimibe is a new cholesterol levering drug, which selectively inhibits NPC1L1 to reduce cholesterol absorption. This drug also has beneficial effects on metabolic syndrome and obesity. Ezetimibe 21 / 22 can be paired with a statin, to dramatically reduce the blood cholesterol level. What is phyosterol and stanol? • They are very similar to cholesterol • They are compounds isolated from plants and incorporated and concentrated into food products eg yoghurt, milk, margarine, and they are also present in some oils eg corn oil • This helps to lower cholesterol- they can reduce up to 5-8% of LDL cholesterol • They are very abundant in vegetarian diets • The mode of action is unknown but it could compete with cholesterol transport at the brush border • However, disadvantages are - they impair vitamin D absorption - a high dose may have questionable systemic effects - clinical efficiency against CVD hasn't been demonstrated 22 / 22