Lecture Notes on Nutrition

Macronutrients: Proteins

• Proteins are amino acids linked by peptide bonds into long chains.

• Excess protein is not stored significantly; intake depends on diet.

• Proteins are broken down into amino acids for absorption by peptidases or proteases.

• Peptidases break peptide bonds through hydrolysis, adding water molecules.

• Amino acids have an amino group on one side and a carboxyl group on the other; the middle part differs.

• There are 21 different amino acids available from dietary proteins.

• The body recombines these into needed proteins and enzymes, constantly breaking down and synthesizing proteins in a process called protein turnover.

• Essential amino acids cannot be synthesized by the body and must be obtained from the diet. Humans require nine essential amino acids.

• Animal foods (meat, eggs, milk) contain all amino acids, while plant foods may be deficient in some essential ones. Vegetarians need to eat a variety of plants to ensure intake of all essential amino acids.

• Protein deficiencies and excesses can cause health problems.

• Kwashiorkor: A severe protein deficiency despite adequate energy intake. Common in children weaned onto high-carbohydrate diets in developing countries. Treatment involves adding protein to the diet, but long-term mental and physical development issues may persist.

• Excess protein is more common in Western cultures due to high-protein supplements. Excess protein is converted to fat or excreted, producing acidic waste products that can damage kidneys. The body uses calcium to buffer the acid load, potentially reducing bone mass.

• Rabbit Starvation: Eating only lean rabbit meat (protein) leads to nausea, and potentially death from protein poisoning. When protein is the primary energy source, ammonia is produced as a byproduct, overwhelming the pathway that converts ammonia to urea for excretion. This leads to ammonia buildup in the blood.

• Normal protein intake is 10-25% of energy intake; protein poisoning typically occurs above 40%.

Macronutrient Balancing

• Macronutrient balancing involves optimizing the proportion of energy from protein, carbohydrates, and fats.

• Research on wild and lab animals shows they converge on a specific macronutrient intake when given a range of food options.

• Animals prioritize meeting protein requirements over other macronutrients.

• Hypothetical example:

  • An animal needs a certain amount of dietary protein (x-axis) and non-protein energy (carbohydrate and fat, y-axis), represented by an intake target.

  • A balanced food meets these requirements (green line). Unbalanced foods do not (pink lines).

  • Animals adjust intake of different unbalanced foods to meet their intake target.

• Studies on humans and cockroaches show they can correct nutritional imbalances when given access to a variety of foods.

  • Cockroaches converge on a diet of 1 to 2.5 protein to carbohydrate and fat.

  • Humans, whether previously suffering from kwashiorkor or marasmus (energy deficiency), converge on a diet of approximately 15% protein.

  • Important point: humans and other animals seem to innately be able to choose diets that meet their macronutrient requirements as long as they are given access to foods that actually allow them to do that.

• If foods don't allow meeting the intake target, humans prioritize protein.

• Two human studies:

  • Sydney University students restricted to 28 foods with varying protein content (10%, 15%, 28%).

  • On low-protein weeks (10%), subjects ate 14% more food to meet protein needs, overeating carbohydrate and fat.

  • Analysis of 9,000 US diets showed that high consumption of ultra-processed foods (biscuits, lollies, cakes) led to eating more overall to meet protein requirements, resulting in weight gain.

• Ultra-processed foods are often high in carbohydrates and fats, and low in protein.

• High-protein diets may promote weight loss because individuals eat less overall when protein intake is higher.

• High-carbohydrate, low-protein diets may be healthier, especially in later life. Studies in mice have indicated that they tend to be in better health than mice that are fed a high protein, low carb diet.

• Australian Dietary Guidelines recommend:

  • Wide variety of foods.

  • 15-25% of energy from protein.

  • 45-65% of energy from carbohydrate.

  • 20-35% of energy from fat.

• Book Recommendation: Eat Like the Animals by Sydney Uni scientists explains animal nutritional ecology applied to humans.

Micronutrients: Vitamins

• Micronutrients are needed in small amounts, provide no energy, but are essential for health.

• Vitamins act as coenzymes.

• 13 known vitamins are needed by humans.

• Except for vitamin D, vitamins must be obtained from the diet.

• Vitamins are divided into water-soluble and fat-soluble.

  • Water-soluble vitamins enter blood directly after absorption, excesses are excreted in urine, requiring constant dietary supply. Overdoses tend to cause short term effects, such as nausea, cramps, and vomiting.

  • Fat-soluble vitamins enter the lymph first, need protein carriers for transport, and excesses are stored in the body.

• Eight B vitamins:

  • Water-soluble, act as coenzymes in cell energy metabolism, found in meats, eggs, dairy, legumes, and grains.

• Vitamin C (ascorbic acid):

  • Water-soluble, involved in tissue repair, immune function, and iron absorption. Found in fruits and vegetables.

• Vitamin deficiencies can cause severe health issues:

  • Scurvy: Vitamin C deficiency causing weakness, weight loss, gum disease, bleeding from the skin, and tiredness. James Lind's experiment with sailors showed citrus fruits could prevent and cure scurvy.

  • B vitamin deficiencies: Pregnant women and those with digestive conditions like celiac disease are most at risk. Celiac disease reduces B vitamin absorption.

  • Alcohol interferes with B vitamin absorption, leading to vitamin B1 deficiency and beriberi (damages muscles and nerves, causes swollen legs and muscle weakness).

    • It can also cause swollen legs and muscle weakness too.

  • Folate (Vitamin B9) Deficiency: Adults experience fatigue, irritability, headaches, and anemia. Critical during pregnancy for neural tube formation. Deficiency causes neural tube defects. Mandatory fortification of bread with folic acid in Australia.

Fat-Soluble Vitamins

• Vitamin A:

  • Essential for vision, particularly in low light and color vision.

  • Found as retinol in animal foods and carotenes in plant foods. Carotenes are converted into the active form of vitamin A by the human body.

• Vitamin D:

  • Assists with the absorption of minerals like calcium.

  • Main source is sunlight-induced reaction in the skin. Dietary sources include fatty fish and fortified foods.

• Vitamin E:

  • Maintains cell membranes, prevents oxidation of fatty acids and low-density lipoproteins (LDL).

  • Found in whole grains, dark green vegetables, nuts, and seeds.

• Vitamin K:

  • Forms proteins involved in blood clotting and binds calcium to make bones.

  • Used in photosynthesis by plants. Dietary sources include green, leafy vegetables. Synthesized by gut bacteria, providing about half of daily vitamin K requirements.

• Deficiencies in fat-soluble vitamins can have serious consequences:

  • Vitamin A deficiency: Leading cause of preventable blindness in malnourished children. Early stages reversible with supplementation.

  • Vitamin K deficiency: Uncontrolled bleeding due to lack of clotting factors.

  • Vitamin D deficiency: Bone deformities due to impaired calcium absorption.

• Overdosing on fat-soluble vitamins can cause severe consequences due to accumulation in the body:

  • Excess vitamin A: Builds up in the liver, causing nausea, dizziness, coma, and death.

  • Excess vitamin K: Liver damage.

  • Excess vitamin D: Build up of calcium in the kidney, damaging the kidneys.

  • Excess vitamin E: Blood thinning and strokes.

• Vitamin availability depends on the amount in food, absorption rate, nutritional state, other foods consumed, food preparation, and whether the vitamin is in natural or fortified form.

Minerals

• Perform important roles (e.g. calcium in bones).

• Sodium:

  • Regulates water balance, creates nerve impulses, and triggers muscle contractions.

  • Humans have a specific appetite for sodium. Deficiency triggers animals to search for sodium sources and eat unusual items.

• Iron: Needed to make hemoglobin for oxygen transport.

• Mineral availability is affected by other compounds.

  • Phytic acid in grains binds to minerals and blocks their absorption.

  • Oxalic acid in spinach blocks calcium absorption.

  • High intake of iron or zinc can block copper absorption.

• Common mineral deficiencies in Australia:

  • Calcium: 50-70% of people don't meet requirements.

  • Iron: 23% of females don't meet requirements.

  • Iodine: 2-8% of people don't meet requirements.

• Excesses:

  • Excess sodium is common, associated with hypertension and increased risk of heart disease. 42-76% of Australians exceed the upper limit of sodium ingestion.
    There is a wide variety of minerals that a person can be deficient in. It's important to be aware of which food groups are the best source of which minerals so you can have a balanced diet.

Phytochemicals and Nutraceuticals

• Phytochemicals: Naturally occurring compounds in foods that don't fit into other nutrient categories.

• Nutraceuticals: Phytochemicals added to foods with claimed health benefits.

• Phytochemicals:

  • Minor components of foods, don't give energy, but impact color and flavor.

  • Examples: sulfur compounds in garlic, capsaicin in chili, lycopene in tomatoes.

• Animal ecology research: Understanding which phytochemicals are toxic or deter animals.

• Human health perspective: Interest in potential health benefits and new medicines. Results are often inconclusive.

  • Antioxidants: Protect from aging, cancer, heart disease. Manufacturers promote foods containing antioxidants (chocolate, tea, wine). Benefits from supplementation are uncertain. Can be toxic at high doses.

  • Caffeine: Has both positive and negative health benefits depending on the study.

  • Salicin: Metabolized to salicylic acid in the body. Originally from willows and used to develop aspirin.

• Nutraceuticals:

  • Foods with added phytochemicals and claimed health benefits. The nutraceutical industry is poorly regulated, requiring caution about claims.

  • Herbal Remedies: Can have dubious health claims.

• Turmeric (curcumin):

  • Very low bioavailability in humans when taken orally.

  • Supplements: Often poorly labeled and have poor quality control, and there can be a marked difference batch to batch the amount of active ingredient.

  • Natural does not equal safe: Can be toxic at high doses.

  • Prebiotics: A substrate that stimulates growth of beneficial bacteria in your colon.

  • Probiotics: Contain live bacteria and can improve the balance of microorganisms in your gut, but this is hard to achieve.

• Prebiotics and Probiotics:

  • Attractiveness: Microbiome can impact allergies, immune system, and mental health.

  • Influence on Gut: Studies show it's hard to change the gut microbiome, especially if already healthy. Most people have a relatively stable microbiome.

  • Influencing Factors: Diet, genotype, and environment. One of the biggest things that can actually alter your microbiome is taking antibiotics or some other medications as well.

• Effects of these ingredients: Changes in the gut microbiome tend to be short-term because it is capable of reasserting itself.