Alcohol consumption is a global health concern. Ethanol is broken down in the liver to acetaldehyde, not only using up NAD+, but also creating a compound (acetaldehyde) that is more toxic than its ethanol precursor. Ethanol causes damage to many organs including the liver and the central nervous system.

Alcohol's Travel in Your Body

Alcohol (See Figure 2) bypasses the typical digestion in the small intestine and stomach that many foods undergo. Rather than being broken down, it travels intact into the blood stream and is absorbed by many tissues. When you consume alcohol, it is absorbed into your bloodstream through the stomach walls, with about 20% traveling through the bloodstream to your organs. The remaining 80% passes through the pylorus valve to the small intestine, where it is quickly absorbed, resulting in rapid intoxication.

Alcohol is diluted by the body's water. Since muscle has more water than fat, a muscular person will have a lower concentration of alcohol in their system and consequently be less intoxicated than a person of the same weight and gender with a higher percentage of body fat.

Women typically have higher body fat and less muscle relative to men. Thus, a woman will have a higher systemic concentration of alcohol and have more severe effects than a man of equal weight who consumed the same quantity of alcohol. If the alcohol concentration is high enough (due to rapid consumption or high percentage of ethanol), the pylorus valve may spasm, enabling a higher percentage of alcohol to enter the blood via the stomach. Since the alcohol can not leave the stomach to be absorbed into the blood more quickly in the small intestine, fast drinking can actually postpone intoxication. (Dunlap)

Ethanol and the Brain

The liver is the body’s main device to screen out harmful substances. A major portion of systemic ethanol is metabolized by the liver. The alcohol that is not metabolized in the liver goes on to other organs and to the nerves depressing the Central Nervous System. Alcohol is one of a few compounds that can cross the blood-brain barrier, which is what causes intoxication. (Chudler 2006) Acetaldehyde, a byproduct of alcohol breakdown, is one of the most common neurotoxins in humans today, promoting brain damage in tens of millions of people. (South)

Metabolism of Alcohol

Because alcohol itself can cause serious acute and chronic problems, your body breaks it down to excrete it. First, alcohol is oxidized by alcohol dehydrogenase while reducing NAD+ and H+ to form NADH. (See Figure 1 for a depiction of the reaction and see Figure 3 for a picture of the enzyme catalyzing the reaction.) This reaction forms acetaldehyde, which can potentially cause many problems, including liver disease and possibly cancer. (See Figure 4 for a picture of acetaldehyde.) This metabolite is thought to have more severe consequences then the original alcohol molecule according to a new research experiment by Pivetta et al. Acetaldehyde is then broken down by acetaldehyde dehydrogenase to form acetic acid, a nontoxic product.

Sufficiently high levels of ethanol will lead to oxidation by the endoplasmic reticulum’s microsomal ethanol oxidizing system. This uncouples a pathway that usually produces energy from NADPH and O2, draining the cells of valuable energy. (Leeds Dental Institute)

Other Sources of Alcohol

Other sources of acetaldehyde include cigarette smoke, car exhaust, and industrial waste. In cigarette smoke, acetaldehyde dissolves in the salvia, causing upper intestinal track damage and cancer. (Salaspuro 2006) It is also found as a byproduct of fireplaces, coffee roasting, coal refining, and higher plant respiration. (US Environmental Protection Agency)

Other Fates for Alcohol and its Metabolite Acetaldehyde

Acetaldehyde is used as a commercial intermediate for the production of acetic acid, acetic anhydride, pyridines, peracetic acid, penteerthritol, ethyl acetate, alkylamines, lactic acid and crotonaldehyde. (California Environmental Protection Agency)

As you can see, alcohol can have many negative biological effects, read on to find out some of these problems.

Figure 2: An Ethanol Molecule

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Figure 3: The Enzyme Alcohol Dehydrogenase

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Figure 4: Acetaldehyde

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