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Questions and Answers
Which fatty acids are considered to have 4 to 6 carbons?
Which pathway leads to prostaglandin synthesis after membrane phospholipid cleavage?
What is the major fatty acid released after membrane phospholipid cleavage?
Arachidonic acid
What is the primary fatty acid found in the membranes of land animals?
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Sphingolipids are lipid esters attached to a __________ base.
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Humans can synthesize both omega-3 and omega-6 fatty acids.
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The body forms triglycerides by joining three ___________ to a glycerol side chain.
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Match the following lipids with their components:
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Who defined the glycemic index in 1981 to rank different dietary carbohydrates based on their ability to raise blood glucose levels?
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Which factors are the major regulators of blood glucose concentration after a meal?
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The Institute of Medicine (IOM) set an upper limit for the glycemic index in its 2002 recommendations.
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Fats and lipids constitute approximately ___% of the energy in the human diet.
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Match the following lipid structures with their characteristics:
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Which type of fat replacer is commonly used in dairy products, salads, baked goods, and confections?
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Fat substitutes can bind essential fatty acids and fat-soluble vitamins, leading to malabsorption.
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What is the recommended daily intake of saturated fatty acids according to the 2005 Dietary Guidelines for Americans?
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__________ acids are known to increase both LDL and HDL cholesterol levels.
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Study Notes
Nutrition Basics
- The human body regulates macronutrient levels to provide adequate supplies of fuel to body tissues.
- Insulin-dependent uptake of glucose by peripheral tissues is the major regulator of blood glucose concentration after a meal.
- The amount and digestibility of ingested carbohydrates, absorption and degree of liver uptake, and insulin secretion and sensitivity of peripheral tissues to insulin action are the key factors affecting blood glucose levels.
Glycemic Index
- The glycemic index (GI) ranks dietary carbohydrates based on their ability to raise blood glucose levels compared to a reference food.
- The GI of a diet has a predictable effect on blood glucose levels and may be useful in the dietary management of diabetes and hyperlipidemia.
- Slowly absorbed starchy foods with a low GI may have health advantages over those with a high GI.
Regulation of Blood Lipids
- Carbohydrate-induced hypertriglyceridemia can result from consuming a high-carbohydrate diet.
- Fat intake does not directly translate into blood lipid changes because the body regulates macronutrient levels to provide adequate supplies of fuel to body tissues.
Lipid Structure and Function
- Lipids comprise a heterogeneous group of compounds characterized by their insolubility in water.
- Lipids can be classified into three major groups: simple lipids, compound lipids, and miscellaneous lipids.
- Fatty acids occur primarily as unbranched hydrocarbon chains with an even number of carbons and are classified according to the number of carbons and double bonds.
Essential Fatty Acids and Omega-3/Omega-6 Ratio
- Omega-3 and omega-6 fatty acids cannot be synthesized by humans and are essential fatty acids.
- The omega-3/omega-6 ratio is important for optimal health, and an imbalance may contribute to chronic diseases.
- Sources of omega-3 fatty acids include marine organisms, such as sardines, mackerel, and salmon.
- The consequences of reduced availability of omega-3 fatty acids include impaired brain and neural function, learning problems, and increased risk of chronic diseases.### Lipid Structure and Function
- Triglycerides are the main form of lipids in the food supply, accounting for over 95% of lipids.
- They are composed of a glycerol molecule bound to three fatty acid chains.
- The fatty acid chains can be saturated or unsaturated, and their length and degree of unsaturation vary.
- Phospholipids are derivatives of phosphatidic acid, which is a triglyceride modified to contain a phosphate group.
- They are found in membranes and have a polar phosphate-containing head and two nonpolar fatty acid tails.
- They are amphipathic, meaning they have both hydrophilic and hydrophobic regions.
Types of Lipids
- Sphingolipids:
- Are lipid esters attached to a sphingosine base rather than a glycerol molecule.
- Are found in the nervous systems of animals and in plant membranes.
- Include sphingomyelin, which is a major component of the myelin sheath.
- Waxes:
- Are lipids composed of a long-chain fatty acid bound to a long-chain alcohol.
- Are almost completely water-insoluble and are used as water repellents.
- Isoprenoids:
- Are a large and diverse group of lipids built from five-carbon units.
- Include essential oils, plant pigments, and fat-soluble vitamins.
- Steroids:
- Are lipids derived from a four-ringed saturated ring.
- Include cholesterol, which is the basis for all steroid derivatives made in the body.
Fat Replacers
- Are structurally different from fats and do not provide readily absorbable nutrients.
- Are used to mimic the texture and sensation of fat in food products.
- Can be classified into several categories, including:
- Carbohydrate-based fat replacers (e.g. polydextrose, starch, maltodextrins)
- Protein-based fat replacers (e.g. microparticulated proteins)
- Fat-based fat replacers (e.g. caprenin, olean)
- Combinations of these categories (e.g. Prolestra, Nutrifat)### Fat Sources and Metabolism
- Modified fat sources can reduce GI absorption, thereby reducing caloric availability.
- Monoacylglycerides (monoglycerols) and diacylglycerols (diglycerides) are used as emulsifiers and contribute to the sensory properties of fat, but have fewer calories (approximately 5 kcal/g).
- Salatrim (an SFA and an LCFA triglyceride molecule) also contains approximately 5 kcal/g because of reduced absorbability.
Concerns about Fat Substitutes
- Long-term effects of fat substitutes center on their ability to bind essential fatty acids and fat-soluble vitamins, contributing to their malabsorption.
- However, under most circumstances, they appear to be safe, effective, and feasible alternatives for controlling fat and energy in diets.
Lipid Intake Recommendations
- Recommendations for lipid intake must take into account the documented physiologic and health effects of various lipid components.
- The 2005 Dietary Guidelines for Americans recommended the consumption of less than 10% of calories as SFA.
- PUFAs decrease the "bad" and "good" lipoproteins.
Saturated Fatty Acids (SFAs) vs. Polyunsaturated Fatty Acids (PUFAs)
- SFAs increase LDLs and HDLs, whereas PUFAs decrease them.
- PUFAs have double bonds that are highly reactive and bind oxygen to form peroxides when exposed to air or heat.
- SFAs and MEFAs, especially those in olive oil, do not produce toxic products when thermally stressed.
- Saturated fat and partially hydrogenated oils have fewer oxygen-binding sites and thereby have increased stability and a longer shelf life, but their intake is associated with greater risk of cardiovascular disease.
Cholesterol and Heart Disease
- The association between high serum cholesterol concentrations and risk for heart disease is well documented.
- Current guidelines recommend a dietary cholesterol intake of less than 300 mg/day.
Alcohol Metabolism
- Chronic alcohol consumption induces both ADH and certain enzymes in the MEOS system.
- The MEOS system is also responsible for the metabolism of many drugs, and chronic ingestion of large amounts of alcohol can alter drug responses in unpredictable ways.
- If the MEOS is saturated with alcohol, drugs are not metabolized at the expected rate, and a drug overdose can occur.
- The production of acetaldehyde in these pathways may be toxic in itself, leading to cirrhosis of the liver.
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Description
Learn about the regulation of blood glucose levels, insulin-dependent uptake, and counter-regulatory hormones in this nutrition basics quiz.
