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Alcohol Absorption and Elimination

The Science Behind BAC Evidence in Texas DWI Cases

When a prosecutor presents a blood alcohol concentration (BAC) result in a Texas DWI case, they are presenting a number produced by a chemical test administered at a specific point in time and not a measurement of the defendant’s BAC at the time they were driving. Understanding the difference between those two things, and why it matters, requires understanding how alcohol moves through the body: how it is absorbed, how it is distributed, and how it is eliminated.

Managing Partner Deandra Grant holds a Master’s Degree in Pharmaceutical Science which is the discipline that encompasses pharmacokinetics, the study of how substances are absorbed, distributed, metabolized, and eliminated by the body. She holds the ACS-CHAL Forensic Lawyer-Scientist designation, has been a Texas Super Lawyer since 2011, and holds the AV® Preeminent rating from Martindale-Hubbell®. The alcohol pharmacokinetics that most defense attorneys address through general argument, Deandra addresses through scientific analysis.

Phase 1: Absorption

After alcohol is consumed, it passes from the mouth and esophagus into the stomach and small intestine, where it is absorbed into the bloodstream. Absorption is not instantaneous. The rate at which alcohol enters the bloodstream depends on a number of variables:

Food in the stomach.  Food (particularly food with fat and protein) slows gastric emptying and delays the absorption of alcohol into the small intestine. A person who consumed alcohol after a substantial meal will absorb that alcohol more slowly than a person who drank on an empty stomach. The peak BAC achieved will be lower, and the time to reach peak BAC will be longer.

Type of beverage.  The concentration of alcohol in the beverage affects absorption rate. Carbonated beverages tend to accelerate gastric emptying and therefore speed absorption. Beverages with very high alcohol concentrations may temporarily slow gastric emptying due to pyloric valve constriction.

Individual physiology.  Body weight, body composition (the ratio of muscle to fat tissue), metabolic rate, and gastric motility all affect the rate and extent of alcohol absorption. Two people who drink the same amount in the same time period under the same conditions can have meaningfully different BAC profiles.

The absorption phase and the rising BAC defense.  During the absorption phase, BAC is rising. If a person was still in the absorption phase at the time they were driving (still absorbing alcohol consumed shortly before or while driving) their BAC may have been lower at the time of driving than at the time the chemical test was administered. This is the pharmacokinetic basis of the rising BAC defense.

Phase 2: Distribution

Once absorbed into the bloodstream, alcohol is distributed throughout the body’s water compartments. Alcohol is hydrophilic meaning it dissolves in water and distributes primarily through body water rather than fat tissue. This is why body composition matters: a person with a higher ratio of lean muscle to fat has a greater volume of distribution for alcohol, which produces a lower peak BAC for the same amount of alcohol consumed compared to a person with more body fat and less lean tissue.

The Widmark formula (the foundational equation in forensic alcohol pharmacokinetics) estimates BAC based on the amount of alcohol consumed, body weight, and a factor (r) that accounts for body composition. The r factor is typically estimated using population averages (approximately 0.68 for men, 0.55 for women), but individual variation is real and meaningful. A retrograde extrapolation opinion that uses a population-average r factor without accounting for the specific defendant’s body composition is subject to challenge.

Phase 3: Elimination

The body eliminates alcohol primarily through hepatic oxidation which is enzymatic breakdown in the liver by alcohol dehydrogenase (ADH). A small percentage is eliminated through breath, urine, and sweat, which is the basis for breath testing. The liver metabolizes alcohol at a rate that is relatively constant for a given individual (sometimes described as zero-order kinetics) typically in the range of 0.010 to 0.025 grams per deciliter per hour, with a commonly cited average of approximately 0.015 to 0.018 g/dL/hr.

Individual variation in elimination rate.  Chronic, heavy alcohol users typically have elevated liver enzyme activity and eliminate alcohol faster than the population average. Individuals who rarely drink may eliminate more slowly. Certain medications affect ADH activity and alter elimination rate. The assumption that any particular defendant eliminates alcohol at the population average rate is a starting point, not a conclusion.

Retrograde extrapolation and its limits.  Retrograde extrapolation is the process of estimating a defendant’s BAC at an earlier point in time (typically the time of driving) by working backward from the BAC measured at the time of the chemical test. This calculation requires knowing or accurately estimating the elimination rate, the time elapsed between driving and testing, and whether the defendant was in the absorption phase or the elimination phase at the relevant times. An expert who offers a retrograde extrapolation opinion without accounting for these variables, or who presents a single-point estimate without acknowledging the range of uncertainty, is presenting a misleadingly conclusion.

Deandra Grant Law – Criminal & DWI Defense Helps Residents Across Texas with DWI Defense – Including: Allen , Arlington, Belton, Cleburne, Collin County, Dallas, Denton, Fairview, Fort Worth, Frisco, Gainesville, Granbury, Hood County, Johnson County, Lewisville, Little Elm, McKinney, Park Cities, Parker County, Plano, Princeton, Richardson, Rockwall, Rowlett, Royse City, Southlake, Sunnyvale, The Colony, Waco, and Weatherford.

Why This Science Matters in Your Case

The gap between the time of driving and the time of the chemical test is where alcohol pharmacokinetics becomes legally decisive. In a typical DWI stop, 30 minutes to 2 hours may elapse between the time the defendant was driving and the time the breath or blood test is administered. If the defendant was still absorbing alcohol during that period, the BAC at the time of the test may be higher than the BAC at the time of driving which means the number the prosecution presents to the jury may overstate the defendant’s actual BAC while they were behind the wheel.

This is not a theoretical argument. It is a documented pharmacokinetic phenomenon that is addressed in peer-reviewed toxicology literature and that courts have recognized as a legitimate basis for challenging BAC evidence. Evaluating whether it applies to a specific defendant’s case requires analysis of the consumption timeline, the type and quantity of alcohol consumed, the presence or absence of food, the defendant’s body weight and composition, and the time between driving and testing.

That analysis (pharmacokinetic evaluation of BAC evidence in the context of a specific defendant’s case) is precisely what a Master’s Degree in Pharmaceutical Science is designed to perform. It is what distinguishes a scientific challenge to BAC evidence from a legal argument about admissibility. For more on how this applies to breath and blood test evidence specifically, see our pages on DWI blood tests and DWI breath tests.

Speak With Deandra Grant Law

Understanding the science behind BAC evidence is not optional in a DWI defense. It is the foundation. Deandra Grant Law has offices in Dallas, Fort Worth, Allen, Denton, Waco, and Rockwall. We have appeared in North and Central Texas courts for more than 30 years, across more than 500 trials to verdict.

Call (214) 225-7117 for a confidential consultation.