"I Am Thankful For Laughter, Except When Milk Comes Out Of My Nose." - Woody Allen

New research from Psychosomatic Medicine:

Divergent Effects of Laughter and Mental Stress on Arterial Stiffness and Central Hemodynamics

• Laughter decreased arterial stiffness. (Participants watched a 30-minute laughter-inducing film. Wish I could get into this study to see the film.)
  
  
• Stress increased arterial stiffness. (Participants watched a 30-minute stress-inducing film.)

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And an older one (which shows that acute stress, such as seen above, can have a prolonged effect):

Acute Mental Stress Has a Prolonged Unfavorable Effect on Arterial Stiffness and Wave Reflections, Psychosomatic Medicine, 2006

• Stress* increased arterial stiffness.
  
  * "Participants were instructed to subtract the number 7 from a 4-digit number, continuously and as quickly and as accurately as possible, for 3 minutes. During the test, participants were intentionally frustrated by being asked to perform faster and by being immediately corrected in case of wrong answers. A metronome was played loudly as an additional distracter."

Arterial stiffness is linked to blood pressure - the stiffer the arteries the higher the pressure. Stiff arteries, as well as hypertension, are also related to atherosclerosis and cardiovascular disease.

This is one reason nutritional studies are difficult to interpret. Non-dietary factors can confound results. It doesn't detract from the worth of studies; it just reveals the challenges in controlling and analyzing them.

Where health (or disease) lies on the right side of the equation, the number of variables on the left is probably innumerable.
________Photo of orangutans from the Guardian. Their gallery here.