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I've only had 1 year of basic Chemistry at school, so I am waay too uneducated to answer this question myself: What is the true caloric impact of ingested alcohol (ethanol) in the human body?

Here are a few things I presume to be fact:

  • When testing, how many calories a substance contains, it is done by setting that substance on fire, then measuring the missing oxygen, maybe the resulting CO2 and the heat.
  • Alcohol is poisonous to the human body, and cannot be used directly as an energy source
  • the human body has to expend energy to turn alcohol (and starch ect) into some usable form. (by oxygenating or hydrogenation or something alike)
  • every energetic substance we consume will be turned into adenosine triphosphate (ATP), which is the only 'energy carrier' our body uses when powering muscles

Conclusions I draw:

  • When we test things for calories, we get a good estimate of how much 'energy' it would serve us
  • yet the testing is just a guideline, as human digestion and internal biochemical processes are a bit more convoluted than the simple 'set it on fire' combustion
  • When we have two different substances with the same caloric value, consuming them might yield different energetic value to the body

So, how to formulate this mathematically:

  • tcv is the tested caloric value
  • ecv is the effective caloric value for the human body
  • tcv >= ecv
  • for those sugars, that can be used directly (e.g. converted into ATP): tcv = ecv
  • for most things, where internal biochems have to turn it into usable form : tcv > ecv
  • alcohol: tcv > ecv

So how energetically effective is alcohol to the human body, compared to sugars?

  • on a pure chemical level: ecv/tcv=? 50%? 70%?
  • considering neutralizing / storing / discarding unwanted byproducts
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  • $\begingroup$ Ethanol does not bring any energy to our body. it just allows arteries to be a bit enlarged, so that the blood gets quicker to the capillaries, and gives an impression of more heat being produced. $\endgroup$ – Maurice Mar 14 '20 at 17:25
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    $\begingroup$ @Maurice Ethanol does have calories; it is converted to acidic acid and funneled into the citric acid cycle: ncbi.nlm.nih.gov/pmc/articles/PMC4338356 $\endgroup$ – Karsten Theis Mar 14 '20 at 17:32
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    $\begingroup$ The method is a bit more sophisticated, but still a rough estimation: mcgill.ca/oss/article/nutrition/… $\endgroup$ – Karsten Theis Mar 14 '20 at 17:35
  • $\begingroup$ @Maurice Just wrong. But there is an interesting question as to whether conventional ways of measuring food calorific value apply to alcohol as alcohol metabolism doesn't go through exactly the same rout as carbohydrates. It is both an interesting and important question. $\endgroup$ – matt_black Mar 14 '20 at 23:13
  • $\begingroup$ @KarstenTheis hey man, thanx a lot for the links! Quite interesting to read. Yet they basically state that the full impact on our metabolical digestion has by far not been understood or measured, that there are only preliminary thesises and estimations, especially not on the chemical level. Also cleared up my assumption about how caloric testing is achieved, but the article is still a little vague on that. Thanx also to matt_black $\endgroup$ – JayC667 Mar 17 '20 at 20:40
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An interesting study, 'Can calories from ethanol contribute to body weight preservation by malnourished rats?', to quote:

In terms of caloric content, ethanol is a unique drug whose oxidation by alcohol dehydrogenase (ADH) produces 7.1 kcal per gram of ethanol oxidized. However, it is not known to what extent these calories are converted into body weight. There is no controversy about the malnutrition produced by chronic ethanol intake in alcoholics. In addition to the known toxic effects on absorption and synthesis of essential nutrients (1), some important metabolic changes occur. Pirola and Lieber (2) reported that activation of the microsomal ethanol oxidizing system (MEOS) in hepatocytes of alcoholics may consume one third of the caloric content of ethanol and impair ATP synthesis. Lands and Zakhari (3) suggested the existence of a futile cycle in ethanol metabolism which would account for a loss of six ATPs per gram of ethanol oxidized. They proposed that ethanol may be oxidized to acetaldehyde, consuming three ATPs, and acetaldehyde may be again reduced to ethanol, consuming three extra ATPs. However, there is some controversy about what happens to ethanol calories when consumption is moderate and not related to the metabolic changes of the liver that characterize chronic ethanol intake. It has been suggested that calories derived from ethanol may function as do calories derived from carbohydrates, producing a thermogenesis value of about 10% (4,5). This thermogenic value may be higher than that of fatty food (3%) and lower than that of proteins (about 20%) (6).

The contribution of calories provided by ethanol to body weight gain has also been studied but there seems to be no consensus about the data from animal and human studies.

Moderate ethanol consumption seems to favor an increase in fat storage and could result in weight gain, especially in overweight individuals (7). In an epidemiological study involving 89,538 women and 48,493 men, Colditz et al. (8) reported a strong negative association between alcohol intake and body weight index for women and the absence of this association for men.

Animal studies are consistent in reporting a decrease in the body weight of rats receiving ethanol solutions as the only source of liquids. Concentrations of ethanol as low as 5% (v/v), which are similar to the ethanol content of a Brazilian beer, or as high as 40% (v/v), solution similar to spirit drinks, are related to decreased body weight gain (9). Similar results have been reported for 20% (v/v) ethanol solution (10).

Different results have been obtained for malnourished animals. Da-Silva et al. (11), studying rats which had been treated with ethanol for 90 days, reported a significant weight gain by malnourished rats (50% food restriction) drinking a 20% (v/v) ethanol solution when compared to malnourished rats drinking water. A more recent study (12) reported improvement in somatic and motor development and a decrease in the mortality rate of the offspring of malnourished rats drinking low doses of ethanol (5%, v/v). These data suggest that malnourished rats can benefit from ethanol calories.

In summary, in spite of the large number of studies on the effects of ethanol in well-nourished animals and humans, there is still controversy about how well ethanol-derived calories can be utilized. Fewer studies are available about special physiological conditions such as malnutrition. Over the last few years, scientific research has mainly focused on obesity, an increasing problem in developed countries, which led us to the false belief that malnutrition was no longer a problem worth investigating. However, there are still 800 million malnourished people in the world (13). The decreasing interest of the scientific community in problems related to malnutrition has left many questions without an answer. Ethanol consumption and its consequences on the malnourished organism are among them.

In view of the importance of malnutrition in Brazil - 22% of the population or 40 million people are malnourished (14), as well as ethanol consumption and alcoholism - 11% of Brazilian population are alcoholics (15), the aim of the present study was to assess the use of ethanol calories in a dose/effect model by evaluating body weight before and after the installation of malnutrition.

My take, ethanol is apparently somewhat unique and there remains some controversy about how well ethanol-derived calories can be utilized. Study classification factors include well-nourished vs. malnourished, low doses (5%) vs high doses (20%), producing significant variations in effects.

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