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Having difficulty making the data in the below charts to apply to daily life. What is the right interpretation and implementation? How are the “in vitro” results of the below charts relevant to levels that cells would experience "in vivo" after the consumption of caffeinated beverages?

Fig 1

Fig 2

Link to the medical research article which the two above charts were taken from

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  • $\begingroup$ Good - hope you get some useful answers. I could add that in the lab I work in, we often do in vitro experiments with cells or tissue in a dish, and it's a normal part of our daily business to ask questions exactly like this - it's a great question, though can also be difficult to answer sometimes. The science that figures out answers to these types of questions is called "pharmacology", by the way. $\endgroup$
    – Bryan Krause
    Jan 24, 2020 at 3:35
  • $\begingroup$ You're asking how skin growth rate is affected in vivo "confluent fibroblasts" by caffeine and it looks like that without the factors and matrix growth environment, that the caffeine metabolically stimulates the fibroblasts away from confluence in graph 1. $\endgroup$ Jan 28, 2020 at 9:07

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In the mentioned study, they tried to support the evidence from earlier studies in which caffeine consumption was associated with skin aging and slow wound healing:

Our research confirms our earlier observations that caffeine may have an adverse effect on the wound healing process, as well as on the aging process, of the human skin.

...and by "earlier observations" they mean another in vitro study.

In the study, they used 1, 2 and 5 mM caffeine solutions. 1 cup (~250 mL) of brewed tea can contain 95-165 mg caffeine (source), which makes it 2-3.4 mM, so much like the solutions used in the study. When you drink 1 cup of coffee (let's say, 2 mM caffeine solution), the caffeine, which is water- and lipid-soluble (StatPearls), will be dissolved in the body water and fat, which can together make ~50 kg in a 70 kg person. So, you would need to drink ~200 cups of coffee (~20 g caffeine) in ~2 hours (caffeine half-life is ~5 hours - source) to achieve 2 mM caffeine solution in the skin cells.

I haven't found any actual human clinical trial in which caffeine consumption would be associated with adverse effects on the skin. In a recent review: Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes (BMJ, 2017), they don't even mention skin or wound healing and regular drinking of 3-4 cups of coffee per day was not associated with an increased risk of various health conditions (except in pregnancy).

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    $\begingroup$ Since caffeine is water soluble, wouldn't you have to consume (approximately) 194 mg per liter of water to reach that concentration in skin? That is above the LD50 for caffeine. $\endgroup$ Jan 24, 2020 at 14:55
  • $\begingroup$ I edited the bold paragraph about the amounts. The concentrations of caffeine solutions used in the study and in coffee seem to be about the same, but when you drink a cup of coffee, caffeine would be dissolved in the whole body and would not be so concentrated in the skin cells any more. I think you would need unrealistic amounts of coffee to reach such concentrations. $\endgroup$
    – Jan
    Jan 24, 2020 at 14:59
  • $\begingroup$ The LD50 of caffeine is 150-200 mg per kilogram of body weight. So, the lethal dose for a 70 kg man would need to be at least 10 grams of caffeine (~100 cups of coffee) consumed in a short time. $\endgroup$
    – Jan
    Jan 24, 2020 at 15:19
  • $\begingroup$ Ah yes, weight not volume, so the 2 molar solution is about the LD50. $\endgroup$ Jan 24, 2020 at 15:22
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How can one relate in vitro studies of caffeine (dose expressed as concentration) to dietary intake of caffeine (dose in mass)?

Caffeine rapidly diffuses throughout the body's water, both intra and extracellular.

Therefore, since the caffeine will be relatively evenly distributed, you can calculate the amount of caffeine needed to reach a given molar concentration within any particular cell by knowing the total water in a person and the molar mass of caffeine (194.19 g/mol ). A 72 kg person has about 40 liters of water in their body.

Combining those numbers, reaching 1mM requires 7.7676 grams of caffeine. Reaching 5mM requires 38.838 grams.

How are the “in vitro” results of the below charts relevant to levels that cells would experience "in vivo" after the consumption of caffeinated beverages?

Wikipedia puts the LD50 of caffeine at 150-200 mg/kg, or 10.8-14.4g for our hypothetical 72kg man. These numbers are likely very approximate as there isn't going to be a lot of well controlled clinical data on caffeine toxicity in humans, but someone reaching those concentrations stands a good chance of dying from caffeine poisoning.

For that reason I don't think these results are relevant to normal caffeine consumption. I interpreted the high concentrations used in that study as a deliberate attempt to induce cell injury, not as a means to simulate normal caffeine consumption.

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