The other energy source is alcohol. The alcohol we consume contains two carbons and is known as ethanol.
Figure 6.51 Structure of ethanol1
Ethanol is passively absorbed by simple diffusion into the enterocyte. Ethanol metabolism occurs primarily in the liver, but 10-30% is estimated to occur in the stomach2. For the average person, the liver can metabolize the amount of ethanol in one drink (1/2 ounce) per hour3. There are three ways that alcohol is metabolized in the body.
1. Catalase - an enzyme that we will cover again in the antioxidants section. Catalase is estimated to metabolize less than 2% of ethanol, so it is not shown below or discussed further4.
2. Alcohol dehydrogenase (ADH) - This is the major ethanol-metabolizing enzyme that converts ethanol and NAD to acetaldehyde and NADH, respectively. Aldehyde dehydrogenase (ALDH) uses NAD, CoA, and acetaldehyde to create acetyl-CoA and to produce another NADH. The action of ADH is shown in the figure below.
Figure 6.52 Ethanol Metabolism1,5
3. Microsomal ethanol oxidizing system (MEOS) - When a person consumes a large amount of alcohol the MEOS, is the overflow pathway, that also metabolizes ethanol to acetaldehyde. It is estimated that the MEOS metabolizes 20% of ethanol3, and it differs from ADH in that it uses ATP to convert reduced nicotinamide adenine dinucleotide phosphate (NADPH + H+) to NADP+. The action of the MEOS is shown in the figure above.
At high intakes or with repeated exposure, there is increased synthesis of MEOS enzymes resulting in more efficient metabolism, also known as increased tolerance. ADH levels do not increase based on alcohol exposure. MEOS also metabolizes a variety of other compounds (drugs, fatty acids, steroids) and alcohol competes for the enzyme's action. This can cause the metabolism of drugs to slow and potentially reach harmful levels in the body3.
Females have lower stomach ADH activity and body H2O concentrations. As a result, a larger proportion of ethanol reaches circulation, thus, in general, females have a lower tolerance for alcohol. About 50% of Taiwanese, Han Chinese, and Japanese populations have polymorphisms in ALDH which cause the enzyme to have low activity6. This leads to buildup of acetaldehyde and undesirable symptoms such as: flushing, dizziness, nausea, and headaches2. The following short video explains what happens when the MEOS system gets involved in alcohol metabolism.
References & Links
2. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
3. Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.
4. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
6. Zakhari, S. (2006) Overview: How Is Alcohol Metabolized by the Body? (2006) Alcohol Research and Health. 29 (4) 245-254.
MEOS Overflow Pathway - http://nutrition.jbpub.com/resources...?id=20&debug=0