Comprehensive Notes: Proteins, Amino Acids, and Lipids
Proteins and Amino Acids
Sources of Protein in the Diet
Protein deficiency is rare in the U.S., with approximately two-thirds (\frac{2}{3}) of dietary protein sourced from meat, poultry, seafood, eggs, and dairy products.
Globally, the majority of the population relies on plant-based proteins, primarily from grains and vegetables.
As a country's economy improves, there's a trend towards increased consumption of animal foods. This rise in animal protein intake is often associated with a concurrent increase in total fat and saturated fat consumption.
Examples of Protein Content (grams per serving):
Beef (3 oz): 24 ext{ g}
Chicken breast (3 oz): 24 ext{ g}
Salmon (3 oz): 19 ext{ g}
Eggs (2): 12 ext{ g}
Cheddar cheese (1.5 oz): 10 ext{ g}
1% milk (1 c): 8 ext{ g}
Yogurt (1 c): 8 ext{ g}
Cottage cheese (\frac{1}{2} ext{ c}): 14 ext{ g}
Kidney beans (1 c): 13 ext{ g}
Tofu (1 c): 20 ext{ g}
Peanut butter (2T): 8 ext{ g}
Sunflower seeds (\frac{1}{4} ext{ c}): 7 ext{ g}
Whole-wheat bread (2 sl): 7 ext{ g}
Spaghetti (1 c): 7 ext{ g}
Oatmeal (1 c): 6 ext{ g}
Corn (1 c): 5 ext{ g}
Broccoli (1 c): 3 ext{ g}
Potato (1 med): 3 ext{ g}
Banana (1 med): 1 ext{ g}
Kiwi (2 med): 1 ext{ g}
Orange (1 med): 1 ext{ g}
Avocado (\frac{1}{2}): 2 ext{ g}
Olive oil (1 T), Corn oil (1 T): 0 ext{ g}
How Protein Source Impacts the Diet
Animal Products:
Provide protein, B vitamins, and minerals such as iron, zinc, and calcium.
Are typically low in fiber and can be high in fat (including saturated fat).
Plant-Based Sources:
Also good sources of B vitamins, iron, zinc, fiber, phytochemicals, and calcium.
The absorbable forms of iron, zinc, and calcium from plant sources may be less digestible compared to animal sources.
Amino Acids
Amino acids are the fundamental building blocks of proteins.
Each amino acid features a central carbon atom bonded to:
A hydrogen atom ( ext{H})
An amino group ( ext{H}_2 ext{N})
An acid group ( ext{COOH})
A side chain (R group), which is unique to each amino acid and determines its specific properties.
Essential Amino Acids:
Are those that the human body cannot synthesize in sufficient quantities to meet its physiological needs.
Therefore, they must be acquired through the diet.
Examples: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
Nonessential Amino Acids:
Can be synthesized by the body.
Examples: Alanine, Asparagine, Aspartic acid, Glutamic acid, Serine.
Conditionally Essential Amino Acids:
Normally nonessential, but become essential under specific physiological stress or disease states (e.g., Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine).
Amino Acids: Transamination
When a nonessential amino acid is unavailable from the diet, the body can synthesize it through a process called transamination.
Transamination involves transferring an amino group from one amino acid to a keto acid to form a new amino acid.
Protein Structure
Amino acids are interconnected by peptide bonds.
Peptide bonds form between the acid group of one amino acid and the nitrogen (amino) group of the subsequent amino acid.
Dipeptide bonds link two amino acids.
Polypeptides are formed by the linkage of many amino acids.
A complete protein is composed of one or more polypeptide chains meticulously folded into a specific three-dimensional (3D) shape.
This folding is crucial; for instance, polypeptide chains fold to form 3D shapes, and the final protein may consist of multiple folded polypeptide chains.
Protein Shape Determines Function
The specific 3D shape of a protein dictates its biological function.
For example:
Connective tissue proteins and collagen are elongated.
Hemoglobin, responsible for oxygen transport, is spherical.
If a protein's shape is altered, its function will likely be disrupted.
Sickle Cell Disease Example: A genetic mutation leads to an altered amino acid sequence in hemoglobin, causing it to form long chains and distort red blood cells into a sickle shape. This altered shape impairs oxygen transport and causes various health issues.
Protein Digestion and Absorption
Mouth: Mechanical breakdown of protein begins with chewing.
Stomach: Hydrochloric acid (HCl) denatures proteins and activates the enzyme pepsin. Pepsin initiates the chemical digestion of protein, breaking it into smaller polypeptides.
Small Intestine:
Protein-digesting enzymes (proteases) are secreted from the pancreas.
Enzymes embedded in the microvilli (brush border enzymes) further break down polypeptides.
This process yields amino acids, dipeptides, and tripeptides.
Transport into Mucosal Cell: A variety of transport proteins move these smaller units into the mucosal cells lining the small intestine.
Amino acids that share the same transport system compete for absorption.
Dipeptide and Tripeptide Breakdown: Dipeptides and tripeptides can enter the mucosal cell. Once inside, they are broken down into single amino acids.
Bloodstream Transport: Single amino acids pass from the mucosal cells into the blood and are transported to the liver.
Feces: Very little dietary protein is lost in the feces, indicating efficient digestion and absorption.
Protein Turnover
The body is in a continuous state of protein turnover, meaning it constantly synthesizes new proteins and breaks down existing ones.
This process can be thought of as recycling, ensuring a dynamic balance of proteins.
Protein Synthesis: Transcription and Translation
Transcription (in the nucleus):
The genetic code for a specific protein, stored in DNA, is copied into a molecule of messenger RNA (mRNA).
Translation (in the cytosol, at ribosomes):
The mRNA molecule leaves the nucleus, carrying the genetic information to ribosomes located in the cytosol, where proteins are manufactured.
At the ribosomes, transfer RNA (tRNA) molecules read the genetic code on the mRNA.
Each tRNA delivers the corresponding amino acid to the ribosome, sequentially joining them to form a polypeptide chain according to the mRNA's instructions.
The Limiting Amino Acid
The composition of amino acids available in the body's 'amino acid pool' is highly dependent on the amino acid profile of the diet.
If there is a shortage of a particular essential amino acid (the 'limiting amino acid'), the body's ability to synthesize a protein that requires a high amount of that specific amino acid will be hampered or limited.
Protein Functions
Provide Structure: Proteins like collagen and keratin form the structural components of tissues (e.g., skin, bones, hair, nails).
Enzymes: Act as biological catalysts, accelerating metabolic reactions within the body without being consumed in the process.
Transport Proteins: Facilitate the movement of substances (e.g., nutrients, oxygen, waste products) in and out of cells and throughout the body (e.g., hemoglobin, membrane transporters).
Antibodies: Crucial components of the immune system, antibodies identify and help neutralize foreign invaders like bacteria and viruses.
Contractile Proteins: Enable muscle movement (e.g., actin and myosin in muscle fibers).
Hormones: Act as chemical messengers, regulating physiological processes. Examples include insulin and glucagon, which regulate blood glucose levels.
Fluid and Acid-Base Balance: Proteins play a vital role in maintaining the proper distribution of fluids between body compartments and in buffering the blood to prevent drastic changes in pH.
Protein Deficiency
Protein-Energy Malnutrition (PEM): A broad term encompassing a spectrum of deficiency conditions, ranging from primarily protein deficiency to combined protein and energy deficiency.
Kwashiorkor: A condition primarily caused by pure protein deficiency, often seen in children recently weaned onto starchy, low-protein diets. Characterized by edema (swelling), a distended belly, and impaired growth.
Marasmus: A severe form of PEM resulting from a deficiency in both energy (calories) and protein. Characterized by extreme wasting of muscle and fat (emaciation).
Dietary Protein Requirement
Determined by Nitrogen Balance:
Nitrogen balance is calculated as: \text{Nitrogen Intake} - \text{Nitrogen Output (urine + fecal + sweat + miscellaneous)}.
Studies often performed in metabolic wards to precisely measure intake and output over 7-10 days.
Decreased energy intake leads to increased nitrogen loss.
Higher energy and/or carbohydrate intake decreases nitrogen loss.
Estimated Requirement: \text{0.6 g} \cdot \text{kg}^{-1} \cdot \text{d}^{-1}.
Recommended Dietary Allowance (RDA): \text{0.8 g} \cdot \text{kg}^{-1} \cdot \text{d}^{-1}.
This RDA is generally applied irrespective of age or activity level, although studies that established it were primarily on sedentary (mostly male) young students.
Protein Needs Under Specific Conditions:
Increase during periods of growth, pregnancy, and lactation.
Regular aerobic exercise increases the oxidation of essential amino acids as fuel, potentially increasing protein requirements for athletes. However, most athletes also have increased total food intake, which may naturally meet these needs.
Impact of Protein Intake in Older Adults:
A study showed that adherence to a eucaloric diet providing protein at the RDA (\text{0.8 g} \cdot ext{kg}^{-1} \cdot ext{d}^{-1}) for 3 months resulted in a significant reduction in thigh muscle cross-sectional area in older adults (Campbell WW, et al. J Gerontol A Biol Sci Med Sci. 2001).
Another study (Health, Aging, and Body Composition Study) linked higher dietary protein intake (\text{1.1 g} \cdot ext{kg}^{-1} \cdot ext{d}^{-1} compared to \text{0.7 g} \cdot ext{kg}^{-1} \cdot ext{d}^{-1}) with positive lean mass change in older community-dwelling adults, suggesting dietary protein as a modifiable risk factor for sarcopenia (Houston DK, et al. Am J Clin Nutr. 2008).
Evaluating Anabolic Stimulus of Protein:
Ingestion of either 40 ext{ g} of balanced amino acids or 18 ext{ g} of essential amino acids (EAAs) significantly improved net muscle protein balance by stimulating muscle protein synthesis. Deleting non-essential amino acids did not negatively affect this response (Volpi E, et al. Am J Clin Nutr. 2003).
Comparing sources: 15 ext{ g} of beef or 15 ext{ g} of whey protein yielded similar increases in protein synthesis (approx. 60-70%). Lower doses like 7 ext{ g} or 10 ext{ g} of EAAs or whey protein resulted in lower increases (approx. 30-45%).
Whey protein is noted as a highly effective intact protein for stimulating muscle protein synthesis.
Muscle protein synthesis is generally maximal at about 15 ext{ g} of EAAs and 20 ext{ g} of whey protein; excess intake beyond this point tends to be oxidized.
Protein Intake and Resistance Training: Resistance exercise combined with feeding leads to a greater increase in muscle protein synthetic rate and a smaller increase in muscle protein breakdown, contributing to muscle growth (Burd NA, et al. J Appl Physiol Bethesda Md 1985. 2009).
Protein Quality by Essential Amino Acid Content
Digestible Indispensable Amino Acid Score (DIAAS): This score is used to determine protein quality.
It is calculated based on three factors:
The profile of each essential amino acid (EAA) relative to the ideal profile.
The total amount of EAAs per gram of protein.
The digestibility of the protein.
A protein with a DIAAS of 100 or more is considered a high-quality protein.
Protein Complementation
Protein complementation involves combining two or more plant protein sources that, when eaten together, provide all the essential amino acids lacked by individual sources.
For example, rice is limited in Lysine (Lys) but rich in Methionine (Met) + Cysteine (Cys). Beans are limited in Met + Cys but rich in Lys. Eating rice and beans together provides a complete essential amino acid profile.
Types of Vegetarian Diets
Nonvegetarian: Includes all types of animal products.
Semivegetarian: Occasional red meat, fish, and poultry, as well as dairy products and eggs.
Pescetarian: Excludes all animal flesh except fish. Usually includes dairy and eggs.
Lacto-ovo Vegetarian: Excludes all animal flesh but includes eggs and dairy products (e.g., milk, cheese).
Lacto-Vegetarian: Excludes animal flesh and eggs but includes dairy products.
Vegan: Excludes all food of animal origin.
Meeting Protein Needs with a Vegan Diet
Vegans can meet their protein needs by combining complementary protein sources from grains, nuts, seeds, and legumes.
Examples of Complementary Pairings:
Rice and Beans
Rice and Lentils
Bread and Peanut butter
Cashew and Tofu stir-fry
Corn tortilla and Beans
Sesame seeds and Chick peas
Corn bread and Black-eyed peas
Sesame seeds and Peanut sauce
Nuts and Soy beans
Rice and Tofu
Female Athletes and Amenorrhea
Risk Factors: The risk of amenorrhea (absence of menstruation) increases with increased volume and intensity of training in female athletes.
Health Consequences: Amenorrheic women often experience low bone density and an increased risk of osteoporosis.
Dietary Causes: Likely linked to a protein/calorie imbalance.
**Study Profile (Example):
Eumenorrheic runners (regular menstruation): Age 29.8 \pm 4.6 years, 19.7\% fat, \text{VO}_2 ext{max } 60.1, 39.9 miles/week.
Amenorrheic runners: Age 25.0 \pm 5.0 years, 21.6\% fat, \text{VO}_2 ext{max } 56.9, 34.7 miles/week.
Amenorrheic runners had significantly lower bone mineral content (\text{BMC L}1 \text{-L}4) (1.10 \pm 0.09 ext{ g/cm}^2) compared to eumenorrheic runners (1.20 \pm 0.12 ext{ g/cm}^2).
Amenorrheic runners had lower gynecological age, older age at menarche, and started running earlier relative to menarche compared to eumenorrheic runners.
**Hormone Status in Amenorrheic Women (compared to Eumenorrheic):
Lower estradiol levels.
Lower pulse frequency and amplitude for gonadotropins (LH, FSH).
Lower T3 (thyroid hormone).
Lower bone density.
Lower HDL cholesterol.
**Energy and Protein Intake Discrepancy:
Amenorrheic runners often show significantly lower energy intake compared to eumenorrheic runners, frequently falling below the RDA (e.g., Amenorrheic: approx. 1500 ext{ kcal}; Eumenorrheic: approx. 2200 ext{ kcal}; RDA higher than both).
Similarly, protein intake in amenorrheic individuals can be lower than in eumenorrheic individuals, sometimes falling below the 0.8 ext{ g} \cdot ext{kg}^{-1} \cdot ext{d}^{-1} RDA.
**Vegetarianism and Menstrual Disturbances (S. Barr, Am. J. Clin. Nutr. 1999):
Results from several cross-sectional studies suggest that clinical menstrual disturbances may be more common in vegetarians.
**Athletic Amenorrhea Management:
Is a multi-factorial problem, but likely strongly related to diet.
Addressing protein/calorie imbalance is key.
Focus on high-quality, low-fat protein sources.
Soy protein may be particularly effective due to its estrogen-like effects.
Many female athletes view amenorrhea as a desired side-effect of training, which is a dangerous misconception.
Greatly increased risk of stress fractures due to low bone density.
Amenorrhea should never be considered a form of contraception.
Calculating Protein Requirements
To calculate daily protein needs for an individual weighing 70 ext{ kg}, using the RDA of \text{0.8 g/kg/day}, the calculation is: \text{70 kg} \times \text{0.8 g/kg/day} = \text{56 g of protein/day}.
Lipids
Introduction to Lipids
Lipids are a diverse group of organic molecules of biological origin that are only marginally soluble in water (hydrophobic).
Fats are a type of lipid (specifically triacylglycerols, TAGs) that are solid at room temperature.
Oils are also TAGs but are liquid at room temperature.
Lipids significantly contribute to the texture, flavor, and aroma of food.
Fats and oils provide the highest energy density among macronutrients, yielding \text{9 kcal} per gram.
Triacylglycerols (TAGs) are the predominant form of lipid found both in food and stored in the human body.
The typical American diet derives approximately 34\% of its energy from TAGs and other lipids.
Triglycerides (TG) / Triacylglycerols (TAG)
TAGs are the major form of fat in both food and the body.
Structure: A TAG molecule consists of a glycerol backbone esterified to three fatty acid molecules.
Glycerol: A three-carbon alcohol molecule.
Fatty Acids: Long hydrocarbon chains with a carboxyl ($\text{COOH}$) acid group at one end and a methyl ($\text{CH}_3$) or omega ($\omega$) end at the other. Their length and saturation (number of double bonds) can vary.
Lipogenesis: The process of synthesizing TAGs.
Lipolysis: The process of breaking down TAGs into glycerol and fatty acids.
Lipids: Fasting/Feasting Cycles
**Feasting (Excess Energy Consumption):
When excess energy (calories) is consumed, it is stored as triglycerides within adipose (fat) tissue.
Chylomicrons (transport dietary fats) and Very-Low-Density Lipoproteins (VLDLs) deliver TAGs to adipose tissue.
Lipoprotein lipase (an enzyme) breaks down TAGs into fatty acids for uptake by adipocytes.
**Fasting (No Food Intake for a Period):
When the body requires energy and no food has been consumed, triglycerides stored in adipose tissue are broken down.
Hormone-sensitive lipase (an enzyme) breaks down TAGs into fatty acids and glycerol.
These released fatty acids can then be used by various tissues as an energy source through beta-oxidation to produce ATP.
Adipose Tissue
TAGs are stored within specialized cells called adipocytes (fat cells) as large lipid droplets.
White Adipose Tissue (WAT):
Primary function is energy storage.
Characterized by large, single lipid droplets per cell, with the nucleus and cytoplasm pushed to the periphery.
Two main types: visceral fat (surrounding organs) and subcutaneous fat (under the skin).
Brown Adipose Tissue (BAT):
Primary function is heat generation (non-shivering thermogenesis), especially in newborns and hibernating animals. Recent research shows adults also have BAT.
Distinguished by multiple smaller lipid droplets and a high number of mitochondria, giving it a brownish color.
Mitochondria in BAT can uncouple oxidative phosphorylation, generating heat instead of ATP.
Nature of Fatty Acids (FA)
Saturation:
Saturated Fatty Acids (SFA): Contain no double bonds between carbon atoms in their hydrocarbon chain; all carbons are saturated with hydrogen atoms, connected by single bonds. The omega carbon has three hydrogen atoms. Example: animal fats, tropical oils. Tend to be solid at room temperature and resist rancidity.
Unsaturated Fatty Acids: Contain one or more double bonds between carbon atoms.
Monounsaturated Fatty Acids (MUFA): Have only one double bond. In naturally occurring FAs, this double bond is in the cis configuration (hydrogen atoms on the same side of the double bond). Example: Oleic acid (in olive, canola, peanut oils). Often liquid at room temperature.
Polyunsaturated Fatty Acids (PUFA): Have more than one double bond. Naturally occurring PUFAs typically have cis double bonds. They are prone to rancidity. Example: Linoleic acid, linolenic acid (in most plant oils).
Cis vs. Trans Double Bonds:
Cis double bonds: The two hydrogen atoms are on the same side of the double bond (the typical, natural configuration).
Trans double bonds: The two hydrogen atoms are on opposite sides of the double bond. These are mostly introduced by human manipulation (e.g., hydrogenation) rather than found naturally in large amounts.
Chain Length:
Short-chain FA: 4 to 7 carbon atoms. Remain liquid at colder temperatures (e.g., TAGs in low-fat milk).
Medium-chain FA: 8 to 12 carbon atoms. Solidify when chilled but are liquid at room temperature (e.g., TAGs in coconut oil).
Long-chain FA: Greater than 12 carbon atoms. Usually solid at room temperature (e.g., beef fat).
Melting Point Factors: Both the number of carbon atoms (more carbons = higher melting point) and the number of double bonds (more double bonds = lower melting point) in the fatty acids of TAGs significantly affect the melting point of the fat.
Example: Lard (\text{~47°C}), Butter (\text{~37°C}), Coconut oil (\text{~25°C}), Olive oil (\text{-7°C}), Corn oil (\text{-14°C}).
Trans Fatty Acids
Trans fatty acids are rarely found in large quantities in nature.
They are primarily created industrially through the hydrogenation of oils, a process that converts liquid oils into semi-solid fats (e.g., margarine, shortening).
Hydrogenation introduces trans double bonds, which raise blood cholesterol levels and increase the risk of heart disease.
Commonly found in partially hydrogenated products like margarine, shortening, and many baked/fried goods.
Essential Fatty Acids (EFA)
EFAs are polyunsaturated fatty acids (PUFAs).
Mammals cannot synthesize EFAs and must obtain them through their diet.
The two primary EFAs are:
Linoleic acid (\omega-6 or omega-6): Found in vegetable oils (e.g., corn, soybean), and meat.
Linolenic acid (\omega-3 or omega-3): Found in canola oil, nuts (e.g., walnuts), and fish oils.
Functions: EFAs are crucial for growth, maintaining skin integrity, fertility, and the structure and essential functions of cell membranes.
Symptoms of Deficiency: A lack of EFAs can lead to symptoms such as a dry scaly rash, decreased growth, increased susceptibility to infection, and poor wound healing.
Associated Pathologies (linked to EFA deficiency or imbalance): Alzheimer’s, Atherosclerosis, Autoimmunity, Behavioral disorders, Cancer, Dementia, Diabetes, High blood pressure, Infection, Immune disorders, Inflammatory disorders, Intestinal disorders, Neurological diseases, Obesity, Vision disorders. Some estimates place the cost of treating diseases linked with EFA imbalances in the multi-trillions of dollars in the USA.
Omega-3 / Omega-6 Balance and Coronary Heart Disease (CHD)
The balance between \omega-3 and \omega-6 PUFAs in the diet, rather than genetics, is a significant factor in the proportion of \omega-6 PUFA in total bodily PUFAs.
This balance is increasingly recognized as important for cardiovascular health and may also be relevant for other conditions such as breast cancer.