Dietary metabolism at the biochemical level

Home » Molecular Biology and Biochemistry » Dietary metabolism at the biochemical level
Headshot of Ted Chauvin, PhD · Associate Professor
Ted Chauvin
PhD · Associate Professor
envelope icon phone icon
Table of Contents

The chapter from Lippincott is Chapter 27: Nutrition. The reading and the class session will go into more depth and detail. This material is very high-level, but there are some important concepts that you need to remember and it will hopefully help you integrate all of metabolism you were introduced to this term.

Nutrition: Overview and Macronutrients. Lippincott® Illustrated Reviews: Biochemistry, 8e. Medical Education: Health Library.

  • Relationship of CHD and cholesterol
  • Saturated fat versus monounsaturated fat versus trans-fat
  • Benefits of MUFA
  • Benefits of fiber
  • High-quality versus low-quality protein
  • Dietary reference intakes
  • Acceptable Macronutrient Distribution Ranges (AMDR) as related to fat, carbs, and protein
  • Hypoglycemia
  • Kwashiorkor
  • Marasmus
  • Ethanol metabolism (enzymes) and how it relates to gluconeogenesis and hypoglycemia

Describe the role of carbohydrates, fats, and protein in the diet

The major fuels we obtain from our diet are the macronutrients, namely carbohydrates, proteins, and fats. When these fuels are oxidized to CO2 and H2O in our cells (the process of catabolism, aka, most of 501), energy is released by transferring electrons to O2. The energy from this oxidation process generates heat and adenosine triphosphate (ATP). Carbon dioxide travels in the blood to the lungs, where it is expired, and water is excreted in urine, sweat, and other secretions. Although the heat generated by fuel oxidation is used to maintain body temperature, the primary purpose of fuel oxidation is to generate ATP. 

The energy content of food. Average energy available from the macronutrients and alcohol. Figure 27.5 from Lippincott Illustrated Reviews: Biochemistry, 8e.

Terms you should recognize and understand

Components of the Dietary Reference Intakes (DRI). Figure 27.2 from Lippincott Illustrated Reviews: Biochemistry, 8e.
Comparison of the components of the Dietary Reference Intakes.
EAR = estimated average requirement
RDA = recommended dietary allowance
AI = adequate intake
UL = tolerable upper intake level.
Figure 27.3 from Lippincott Illustrated Reviews: Biochemistry, 8e.

Dietary Reference Intakes (DRI) are estimates of the nutrients required to prevent deficiencies and maintain optimal health and growth.

The average daily nutrient intake level estimated to meet the requirement of 50% of the healthy individuals in a particular life stage and gender group is the Estimated Average Requirement (EAR).

The RDA is the average daily nutrient intake level sufficient to meet the requirements of nearly all (97% to 98%) individuals in a particular life stage and gender group.

An Adequate Intake (AI) is set instead of an RDA if sufficient scientific evidence is not available to calculate an EAR or RDA. The AI is based on estimates of nutrient intake by a group (or groups) of apparently healthy people.

The highest average daily nutrient intake level likely to pose no risk of adverse health effects to almost all individuals in the general population is the Tolerable Upper Intake Level (UL).

Describe the relationship between certain dietary fats and heart disease

Dietary fats most strongly influence the incidence of coronary heart disease (CHD), but evidence linking dietary fat and the risk for cancer or obesity is much weaker. 

Dietary fats most strongly influence the incidence of coronary heart disease (CHD), but evidence linking dietary fat and the risk for cancer or obesity is much weaker. Figure 27.9 from Lippincott Illustrated Reviews: Biochemistry, 8e.
Correlation of the death rate from coronary heart disease with the concentration of plasma cholesterol. (Note: The data were obtained from a multiyear study of men with the death rate adjusted for age.) Figure 27.10 from Lippincott Illustrated Reviews: Biochemistry, 8e.
Effects of dietary fats. LDL = low-density lipoprotein
HDL = high-density lipoprotein
DHA = docosahexaenoic acid
Figure 27.13 from Lippincott Illustrated Reviews: Biochemistry, 8e.

Describe the biochemical basis of Kwashiorkor and Marasmus

23i

Kwashiorkor

Sufficient calorie intake, severely inadequate protein intake

  • Inadequate adaptation; insufficient dietary nutrient intake
  • Extreme protein deficiency ↓ liver protein synthesis osmotic imbalance edema, abdominal distension
  • ↓ Lymphatic function ↓ fluid recovery, low lipid absorption further abdominal distension
  • Hypoglycemia
  • ↓ blood lipids
  • Hypoalbuminemia, hypoproteinemia (transferrin, essential amino acids, lipoprotein)
  • Anemia (normochromic-normocytic/ hypochromic microcytic/macrocytic)
  • Electrolyte depletion hypocalcemia, hypophosphatemia, hypomagnesemia, hypokalemia

Marasmus

Severe malnutrition: inadequate calorie/protein intake

  • Insufficient energy balance evolving adaptation
    1. ↓ intake, ↑ loss (e.g. emesis, diarrhea, burns), ↑ energy expenditure negative energy balance
    2. Negative energy balance adaptations:
    3. ↓ physical activity, lethargy, ↓ basal metabolic rate, growth retardation, weight loss
  • Hypotension
  • Hypothermia
  • Hypoglycemia
  • Anemia (normochromic-normocytic/hypochromic microcytic/macrocytic)
  • Hyponatremia +/– non-specific electrolyte imbalances

 

Describe how nutrition needs change during life stages

(Adapted from Lippincott Illustrated Reviews: Biochemistry, 8e.)

Macronutrient energy sources, micronutrients, essential fatty acids, and essential amino acids are required at every life stage. Additionally, each stage has specific nutrition needs. 

Infancy, childhood, and adolescence

The rapid growth and development in infancy (birth to one year) and childhood (one year to adolescence) necessitate higher energy and protein needs relative to body size than are required in subsequent life stages. In adolescence, the marked increases in height and weight increase nutritional needs. 

Infants

Ideal infant nutrition is based on human breast milk because it provides calories and most micronutrients in amounts appropriate for the human infant. Carbohydrates, protein, and fat are present in a 7:3:1 ratio. (Note: Human milk contains nearly 200 unique oligosaccharides besides the disaccharide lactose. About 90% of the microbiota [the population of microbes] in the breast-fed infant's intestine is represented by one type, Bifidobacterium infantis, which expresses all the enzymes needed to degrade these complex sugars. The sugars, in turn, act as prebiotics that support the growth of B. infantis, a probiotic [helpful bacteria].)

Breast milk is low in vitamin D; however, exclusively breast-fed babies require vitamin D supplementation. (Note: Human milk provides antibodies and other proteins that reduce the risk of infection.) 

Children

As with infants, children have an increased need for calories and nutrients. However, the primary concerns in this stage are iron and calcium deficiencies. 

Adolescents

In the teen years, the increases in height and weight increase the need for calories, protein, calcium, iron, and phosphorus. Eating patterns in this stage can result in overconsumption of fat, sodium, and sugar and underconsumption of vitamin A, thiamine, and folic acid. 

Adulthood

Overnutrition is a concern in young adults, whereas malnutrition is a concern in older adults. 

Young adults

Nutrition in young adults focuses on the maintenance of good health and the prevention of disease. The goal is a diet rich in plant-based foods (with a focus on fiber and whole grains), limited intake of saturated fat and trans fatty acids, and balanced intake of ω-3 and ω-6 PUFA. 

Pregnant or lactating women

The requirements for calories, protein, and virtually all micronutrients increase in pregnancy and lactation. Supplementation with folic acid (to prevent neural tube defects [see Chapter 28, Section II 2]), vitamin D, calcium, iron, iodine, and DHA is typically recommended. 

Older adults

Aging increases the risk of malnutrition. Decreased appetite resulting from a reduced sense of taste (dysgeusia) and smell (hyposmia) decreases nutrient intake. Inadequate protein, calcium, and vitamins D and B12 intake is cal. B12 deficiency can result from decreased absorption caused by achlorhydria (reduced stomach acid). In aging, lean muscle mass decreases and fat increases, decreasing RMR.

Describe how ethanol consumption can lead to hypoglycemia

A major route of ethanol metabolism in the liver is liver alcohol dehydrogenase (ADH), a cytosolic enzyme that oxidizes ethanol to acetaldehyde with a reduction of NAD+ to NADH. 

Approximately 90% of the generated acetaldehyde from the ADH reaction is metabolized to acetate in the liver. The primary enzyme involved is the mitochondrial acetaldehyde dehydrogenase (ALDH), which oxidizes acetaldehyde to acetate with a generation of NADH.

An increase in NADH/NAD+ ratio can also cause hypoglycemia in a fasting individual who has been drinking and is dependent on gluconeogenesis to maintain blood glucose levels. Alanine and lactate are major gluconeogenic precursors that enter gluconeogenesis as pyruvate. The high NADH/NAD+ ratio shifts the lactate dehydrogenase equilibrium to lactate so that pyruvate formed from alanine is converted to lactate and cannot enter gluconeogenesis. The increased NADH/NAD+ ratio prevents other major gluconeogenic precursors, such as oxaloacetate and glycerol, from entering the gluconeogenic pathway. 

Chapter 23, Lippincott Illustrated Reviews: Biochemistry, 8e.

Image credits

Unless otherwise noted, images are from Adobe Stock.