Found in the sci.med.nutrition newsgroup:

The life-shortening effect of reduced physical activity is abolished by a
fat rich diet.

Mlekusch W, Tillian M, Lamprecht M, Oettl K, Krainz H, Reibnegger G

Institute of Medical Chemistry and Pregl Laboratory,
Karl-Franzens-University Graz, Austria. [log in to unmask]

In female mice on a control diet (3.6% fat) reduced physical activity leads
to a reduction of the average life span. So the average age at death of an
inactive group is 500 +/- 166 compared to 565 +/- 175 days in an active
control group. If the animals are kept on a fat rich diet (12.4% fat) this
effect of physical activity restriction is no longer observable and the
average age at death is 570 +/- 142 days, within the range of the control
animals. The increased fat intake seems to reduce the stress or to increase
the resistance to stress in the activity restricted animals. So stress is a
crucial determinant of life span.