Fasting and your body temperature

Fasting slows down your metabolism in order to reduce energy expenditure (Wang et al., 2004), which may be one of the reasons that animals live longer on dietary regimens such as caloric restriction or intermittent fasting (Liu & Walford, 1972; Rikke & Johnson, 2004; Salerian & Saleri, 2006). Under these regimens metabolic slowing manifests itself in a number of ways, usually a reduction in daily activity and a lower body temperature (Rikke et al., 2003; Severinsen & Munch, 1999). Lane and colleagues (1996) showed that caloric restriction in rhesus monkeys results in a rapid reduction in body temperature, and that the reduction endures for the duration of caloric restriction (in this case years).Humans under caloric restriction also exhibit reductions, albeit smaller, in body temperature. Heilbronn and colleagues (2006) randomized subjects into 1 of 4 groups for 6 months: (1) control (weight maintenance diet); (2) calorie restriction (25% calorie restriction of baseline energy requirements); (3) calorie restriction with exercise (12.5% calorie restriction plus 12.5% increase in energy expenditure by structured exercise); and (4) very low-calorie diet (very low-calorie diet [890 kcal/d] until 15% reduction in body weight, followed by a weight maintenance diet).
Group (3) is particularly interesting, showing that even relatively mild caloric restriction (a degree that can easily occur while intermittently fasting) decreases body temperature. So reducing body temperature is a common response to dietary restrictions, and together with decreases in physical activity are the largest contributors to reducing energy expenditures (reductions in basal metabolism occur, but apparently contribute relatively little).

Evidence suggests that the severity of the dietary restriction dictates the degree of temperature reduction. Rats subjected to total starvation versus caloric restriction (75%, which is quite severe) experienced a significantly greater reduction in body temperature for the duration of restriction, with temperatures in both groups normalizing once ad libitum feeding was reinstated (Severinsen & Munch, 1999).
I haven't found any primate data on body temperature under intermittent fasting, but mice exit torpor, which is a reduction in body temperature during inactive parts of the diurnal cycle, when intermittently fed double and triple rations (Rikke et al., 2003; see also Wan et al., 2003 for body temperature reductions under a more conventional alternate day fasting regimen). Presumably, part of this is due to the caloric expenditure associated with digesting a large meal.

The data also suggest, at least in this mouse strain, that when torpor is re-entered during the fast between meals, that the decrease in body temperature is particularly dramatic.

It's tempting to infer that caloric restriction and/or intermittent fasting cause reductions in body temperature which in turn causes the prolonged lifespans associated with both these dietary regimens. However, a recent experiment using genetically-engineered mice clearly shows that this cannot be the whole story. Conti and colleagues (2006) created transgenic mice where the preoptic area of the hypothalamus, which regulates body temperature, was warmer than normal. This tricked the brain into reducing body temperature by roughly half a degree Celsius, within the range attainable by dietary restrictions or meal patterning. These mice lived 10-20% longer than wild-type mice (in the figure black=wild-type, red=mutant).
The increase in lifespan occurred despite the fact that the transgenic mice ate as much as the normal mice, and actually weighed slightly more. These results indicate that a reduction in body temperature can lengthen lifespan independently of caloric restriction or intermittent fasting. It will be interesting to see whether calorically restricting or intermittently fasting these transgenic mice will lead to even longer lifespans.

I suspect that in practice the wide variation in fasting regimens results in wide variation in the magnitude of body temperature reductions. These factors probably interact with variations in the degree to which individual metabolisms react to dietary restrictions and meal patterning. I would venture to guess based on the evidence that people subjecting themselves to more severe caloric restriction or longer fasts (more severe caloric restriction on a local time scale) will experience the greatest reductions in body temperature. It's unclear to me whether this translates to impaired cold tolerance, although some first-hand reports suggest that it might.