Human Nutrition Notes

Introduction to Human Nutrition

• Human nutrition is a complex field with conflicting studies.
• It's hard to conduct experiments with humans to establish cause and effect.
• A lot of studies come from animal research or correlative human studies.
• Correlative studies aren't 100% sure of cause and effect.
• Human studies rely on self-reporting, which isn't very accurate.
• Around 25% of people underreport what they eat.
• The human nutrition industry is worth a lot of money, and manufacturers fund studies that might be biased.
• Be aware of who funded the research and if there's any independent research that shows the same outcome.
• The field of human nutrition is constantly evolving, and new information is becoming available all the time.

Course Overview

• The course will cover the underlying physiology of how nutrients power our bodies.
• It will also provide an overview of relevant topics in human nutrition.
• The first couple of lectures will be background information.
• The third lecture will be about digestion and where different nutrients are digested and absorbed.
• There will be a lecture looking at energy metabolism.
• There will also be lectures on obesity and appetite and whether there are genes that predispose some people to becoming overweight.
• Today's Lecture:
  • Nutritional research informs public health, some of the recent changes to nutritional recommendations, and the individual nutrients (carbohydrates and fats), specifically looking at glycemic index and the types and carriage of fats around the body.

Relevance of Nutrition

• Nutrition has huge impacts on human health.
• It affects immune function and growth.
• There have been huge improvements in human nutrition over the last 100 years.
• Simple public health messages, like making sure you get an adequate intake of protein and vitamins and minerals in your diet, have been key.
• Key nutritional deficiencies that used to be around aren't seen all that much in developed countries.
• Goiters: enlargement of the thyroid gland that comes from iodine deficiency.
• Kwashiorkor disease: inadequate intake of protein, even though someone's actually getting enough energy to meet their energy requirements. The main symptoms associated with that disease are swelling, and that's often due to fluid retention.
• These days, there's so much awareness of the importance of nutrition that we actually get bombarded with these really confusing nutritional messages.
• Manufacturers try to take advantage of that confusion.

Healthy Eating Guides

• Healthy eating pyramids are one of the main ways that Nutrition Australia tries to get simple, nutritional messages out to people about which sorts of foods they should eat.
• They're supposed to just basically be this easy visual guide to eating.
• You're supposed to eat most of the things that are down the bottom of the pyramid and least of the things that are at the top.
• They do change them as new nutritional advice becomes available.
  • In 2015, they moved the grains out of the bottom tier of the pyramid up into a tier of their own in recognition that you should be trying to eat more whole foods rather than processed foods.
  • They've also put healthy fats up the top of the pyramid in recognition that you do need some healthy fats in your diet each day and that not all fats are equal.
• The Australian government has a similar guide to healthy eating.
• They've got slices of a pie that represent what your serving sizes of different types of foods should be, but it's the same sort of idea.
• You can calculate your daily energy requirements based on your age and your body mass index.

Nutrients

• Nutrients: foods that are in a form that can actually be used by your body.
• Macronutrients: the main components of foods, and they're where we get our energy from.
  • Carbohydrates, proteins, and fats.
• Micronutrients: don't provide us with any energy, but they still play some really critical roles in our body.
  • Required in small amounts, and they're also present in small amounts in foods.
• Energy in food can be transferred to bonds of adenosine triphosphate or ATP, and that's what our cells actually use to power themselves.
• Different nutrients give us different amounts of energy.
  • Fat: nearly $38$ kilojoules per gram of fat.
  • Carbohydrates and protein: nearly $17$ kilojoules per gram.
  • Alcohol: not a nutrient, but we do get energy from it.
    • Low carb beers are a marketing ploy because the carbohydrate is providing a lot less energy than what the alcohol is.
• Total energy: burning that food in something called a bomb calorimeter and measuring how much heat is released.
• Digestible energy: gross energy minus any energy that we lose in our faeces.
• Metabolizable energy: digestible energy also minus any energy that's lost in urine or in gases as well.
• Nutrition panels on foods are talking about the metabolisable energy content of foods.
• If there are differences in how much energy you take in and how much energy you use, then that influences the amount of fat that your body is storing.

Energy Intake

• If you have zero excess intake, you have no change in body fat.
• If you change your energy intake so that now you're taking in about $55$ kilojoules per day in excess of your expenditure, and if you continue to have $55$ kilojoules extra per day, every day for a year, then you would increase your body fat mass by about half a kilo over that year.
• If you are taking in $550$ kilojoules extra per day this is per day over a year, then you would gain six kilograms of body fat mass per year.

Carbohydrate

• Carbohydrates can be classified in a couple of different ways.
  • Chemical classification.
    • Monosaccharides.
      • Simple sugars.
      • Most important ones in human nutrition are hexoses, which just means they've got six carbons.
      • $3$ important ones are glucose, galactose, and fructose.
    • Disaccharides.
      • Two monosaccharides linked together by something called a glycosidic bond.
      • $3$ main ones in human nutrition.
        • Lactose: milk sugar. A glucose and a galactose bonded together.
        • Sucrose: normal table sugar. A glucose and a fructose bonded together.
        • Maltose: mostly found in grains. Two glucoses bonded together.
    • Polysaccharides.
      • Long chains of monosaccharides, and they're actually the most common carbohydrates that we have in our diet.
      • Three forms.
        • Starch: main way that plants store glucose.
        • Cellulose: part of what we call dietary fibre, and that's the main part of plant cell walls.
        • Glycogen: way that animals store glucose.
• During digestion, our body breaks down any of the digestible carbohydrates and it breaks them down into their monosaccharide components.
• Monosaccharides get absorbed and they get converted to glucose in our liver.
  • Based on the effect that the carbohydrates have on our blood glucose concentration.
  • Different types of carbohydrates affect your blood glucose concentration in different ways, and that is known as the glycemic index.
  • Glycemic Index: ranking of foods from zero to $100$ for their immediate effects on your blood glucose concentration.
    • A value of $100$ is what's given to the effect of $50$ grams of pure glucose on your blood glucose levels over a two hour period.
    • Foods that have a high glycaemic index, you see that sharp, fast rise in your blood glucose concentration, whereas foods that have a low glycaemic index release glucose much more slowly into your blood and have a lower peak blood glucose concentration, and it's spread over a long time period.
    • Classifying carbohydrates as either simple or complex carbohydrates doesn't actually give us a very good understanding of what happens to your blood glucose concentrations.

Influences on Glycemic Index

• Type of sugar: foods that contain fructose have a lower glycemic index than foods that contain glucose because the fructose has to be converted to glucose by your liver.
• Type of starch: foods that have amylose starch have a lower glycaemic index than foods with amylopectin starch, and that's just to do with the shape of the starch molecules.
• Amount of processing particle size also influences glycemic index.
  • Foods with whole grains have a lower glycaemic index than really highly processed white breads.
• Cooking can change the glycemic index.
  • Raw carrots have a lower glycemic index than cooked carrots, and that's just because the cooking process changes the properties of the starch.
• Fat content of foods also modifies the glycaemic index.
  • Large amounts of fat in our stomach slows the rate at which our stomach is emptied, which means a slower release of glucose into the bloodstream.
  • Mars bars have a lower glycaemic index than corn flakes.
• When a large dose of glucose enters our bloodstream, the hormone insulin is required to remove that glucose from your bloodstream and get your blood glucose levels back down to normal.
  • People with diabetes, knowing the glycaemic index of foods can really help them to control the rate at which glucose is entering their bloodstream and reduce the risk of hypoglycemia.
    If you do continually eat foods with a really high glycaemic index, then your body can actually develop an, insensitivity to insulin, and that can lead to type two diabetes and then all sorts of other health risks, like coronary heart disease and weight control, appetite control, etcetera.
• There is not any scientific evidence that people can get addicted to sugar.
• Fructose is metabolised a little bit differently compared to other monosaccharides.

Fat

• Diets that are high in fat have been linked to things like obesity and heart disease.
• Not all fats are equally bad for us.
• The fats come from a completely different source, so they're mostly plant based fats, so things like olive oil.

Types of Fats

• Fatty acids: building blocks of most of the fats that we actually eat, and they're these long chains of carbons that are linked together, and they're flanked by hydrogens and some oxygen.
• Saturated Fat: no double bonds between any of the carbons.
• Unsaturated Fat: that does have double bonds between the carbons.
  • Monounsaturated fatty acid: one double bond.
  • Polyunsaturated fatty acid: more than one double bond.
• Saturated fats: solids at room temperature.
• Unsaturated fatty acids: normally liquid at room temperature.
• Omega three fatty acids: that refers to the first position of the double bond on the carbon. So if you count the carbons up from the bottom end at the omega end, then this is an omega three fatty acid because the third carbon is the first position of a double bond.
• We actually need fat in our diet for a whole range of different things.
  • Important in cell functioning, used to form various steroid hormones, and it's also used to transport fat soluble vitamins around our body.
• For most of the fatty acids that our body actually needs, we can eat other fatty acids, break those down, and then synthesise the ones that we do need.
• There are two particular fatty acids that our body needs that it can't actually synthesise.
  • Essential fatty acids.
• Trans fats.
  • Most naturally occurring unsaturated fatty acids are cis fats.
  • Trans fats have been shown to be really bad in terms of promoting heart disease.
  • Some trans fats do occur naturally in foods, but most of the time they're actually chemically induced by this process called hydrogenation.
  • If you want to reduce your risk of coronary heart disease, the World Health Organization recommends that trans fats should make up less than one per cent of our total energy intake.

Fats and Heart Disease

• If you removed saturated fats from your diet and you replaced them with carbohydrates, then you would see a reduced risk of heart disease.
• If you replaced monounsaturated fats in your diet with carbohydrates, you actually get an increase in the risk of heart disease.
• If you get rid of saturated fats and you replace them with mono or polyunsaturated fats, you reduce your risk of heart disease.
• Triglycerides: molecule of glycerol that's bonded to three fatty acids, and triglycerides are really important in your body.
  • Important source of energy, they store energy in your adipose tissues, they provide insulation and protection for your body, and they also transport fat soluble vitamins around your body.
• Consider the types of fats you're eating rather than just the total amount that you're eating.

Sterols and Cholesterol

• Cholesterol is a really important part of various hormones in your body, so testosterone, oestrogen and your stress hormones, the corticosteroids.
• Important precursor for bile acids.
• Important component of cell membranes, and it's also part of the process for transporting fats around your body so it can deliver fats as a source of energy to your cells.
• Dietary cholesterol doesn't actually have a very big influence on your blood cholesterol levels.
• It's the amount of saturated fat that's in your diet that has the biggest influence on your blood cholesterol levels.

Lipoproteins

• Fats are not water soluble, but our blood is mostly water.
• If we want to transport fat around our body, then the fats actually need to be packaged up in a way that lets them be transported around in this aqueous medium.
• Fatty acids get absorbed, they get repackaged again, mostly again as triglycerides, and then they get assembled into these particles that are called chylomicrons.
• Lipoproteins: They've got triglycerides in the middle, and then on the outside of that, they've got some cholesterol, some phospholipids, and then various types of proteins, on the surface, and that's what enables the fats to be transported around in your blood.
• The chylomicron residues get broken down in the liver, and these new fatty acids and cholesterol are synthesized, and then they get repackaged again as these new packages called very low density lipoproteins.
• Very low density lipoproteins: deliver the triglycerides to cells. Once they have delivered the triglycerides to cells, they have less triglycerides in them, so that means they get a little bit denser, and they become what's called low density lipoproteins.
• Low density lipoproteins deliver cholesterol to cells so that cholesterol can be incorporated into cell membranes, and it can be used to synthesise various steroid hormones.
  • Can actually interact with other byproducts of cell metabolism and be oxidised, and then they start to build up and form clots in your arteries. So, it's these low density lipoproteins that are called bad cholesterol.
• High density lipoproteins are synthesised in the liver, and their job is to go around and collect excess fat and cholesterol and bring it back to the liver so it can be disposed of from the body.
  • If you get high ratios of these high density lipoproteins relative to the low density lipoproteins, they can actually help to reduce your blood cholesterol level. So these high density lipoproteins are what is called the good cholesterol.