Do I burn more calories when it is hot outside or cold?

Do I burn more calories when it is hot outside or cold?
 
It seems as if the extra energy that is needed to keep you warm when exercising in a cold environment would translate into extra calories burned. However, exercise raises the body’s temperature on its own without needing to expend more energy to do this. In hot weather, it takes more cardiovascular effort to COOL the body, which occurs by pumping blood to the skin to promote sweating. Therefore, exercising in warm weather would actually use more energy than exercising at a temperature near freezing.

In one study, nine male subjects cycled for 90 minutes in different temperatures: -10 degrees Celsius, zero degrees Celsius, 10 degrees Celsius and 20 degrees Celsius. During the two colder temperatures, there was a higher respiratory exchange ratio indicating that more carbohydrate was used for fuel and less fat was oxidized than during the warmer trials (1).

In another study of eight male cyclists, subjects rode to exhaustion in four different temperatures (approximately 4, 10, 20 and 30 degrees Celsius). The longest time to exhaustion was found at 10 degrees Celsius and the shortest was at 30 degrees Celsius. This research showed a relationship between temperature and exercise capacity with the best exercise able to be performed in the moderate ranges with the lowest capacity on either end of the temperature extremes. (2)

These two studies suggest that working out in moderate to warm temperature is best for burning fat and exercising longer, so more calories are burned overall.
Cold weather doesn’t increase caloric expenditure unless, however, the body starts to shiver. When a person is shivering, the body needs to work harder to maintain thermoregulation (body temperature). According to Nancy Clark, M.S., R.D., shivering can burn about 400 calories per hour and it depletes glycogen stores and leaves you feeling fatigued (3).

In this situation the energy expenditure in cold weather is greater than in warm weather, but the actual amount of additional calories burned due to shivering depends on the temperature, how long the person was exposed, and the type of clothing.

Other factors can cause weight gain during winter months. Inclement weather and less daylight can decrease activity. Winter blues, also known as seasonal affective disorder, is related to low levels of serotonin production (the feel-good hormone in the brain). It has been shown that eating carbohydrates increases the amount of serotonin in the brain, which may also be why we reach for a cookie after a tough day (4). Also, many people find it hard to resist the many high-calorie treats that are available in abundance during the holidays.
References:

1. Medicine and Science in Sports and Exercise, 34 (5), 774-779.
2. Medicine and Science in Sports and Exercise, 29 (9), 1240-1249.
3. Clark, Nancy (2004).  Winter and Nutrition: Fueling for Cold Weather Exercise
4. Wurtman, J.J. (1988).   Carbohydrate craving, mood changes, and obesity.  Journal of Clinical Psychiatry, Aug (49) Suppl 37-9.