butable to coffee; but no difference
between those of coffee and of non-coffee drinkers (ascertained by
careful investigation of their life history) could be discerned.[216] In
the long run, it is safe to say that the effect of coffee drinking upon
the prolongation or shortening of life is neutral.

_Coffee in the Alimentary Tract_

When coffee is taken _per os_ it passes directly to the stomach, where
its sole immediate action is to dilute the previous contents, just as
other ingested liquids do. Eventually the caffein content is absorbed by
the system, and from thence on a stimulation is apparent. Considerable
conjecture has occurred over the difference in the effects of tea and
coffee, the most feasible explanation advanced being one appearing in
the London _Lancet_.[217]

The caffein tannate of tea is precipitated by weak acids, and the
presumption is that it is precipitated by the gastric juice and,
therefore, the caffein is probably not absorbed until it reaches
the alkaline alimentary tract. In the case of coffee, however, in
whatever form the caffein may be present, it is soluble in both
alkaline and acid fluids, and, therefore, the absorption of the
alkaloid probably takes place in the stomach.

This theory, if true, goes far toward explaining the more rapid
stimulation of coffee.

The statement has sometimes been made that milk or cream causes the
coffee liquid to become coagulated when it comes into contact with the
acids of the stomach. This is true, but does not carry with it the
inference that indigestibility accompanies this coagulation. Milk and
cream, upon reaching the stomach, are coagulated by the gastric juice;
but the casein product formed is not indigestible. These liquids, when
added to coffee, are partially acted upon by the small acid content of
the brew, so that the gastric juice action is not so pronounced, for the
coagulation was started before ingestion, and the coagulable
constituent, casein, is more dilute in the cup as consumed than it is in
milk. Accordingly, the particles formed by it in the stomach will be
relatively smaller and more quickly and easily digested than milk _per
se_. It has been observed that coffee containing milk or cream is not as
stimulating as black coffee. The writer believes that this is probably
due to mechanical inclusion of caffein in the casein and fat particles,
and also to some adsorption of the alkaloid by them. This