Why summer is the perfect time of year to get in shape...

Your body will respond very differently to the same program of diet and exercise in the summer than it does in the winter. In fact, I think the summer is the easiest time of year to get in shape.

This does depend on where you live. What you're about to read assumes that it gets colder and darker in the winter, and lighter and hotter in the summer.

Firstly, the temperature in which you exercise affects the number of fat calories your body burns for energy. Some evidence for this comes from a trial published in the journal Medicine and Science in Sports and Exercise [5]. Nine male subjects cycled for 90 minutes in several different temperatures.

• Minus 10 degrees Celsius

• 0 degrees Celsius

• 10 degrees Celsius

• 20 degrees Celsius

The number of fat calories burned for energy was reduced at both minus 10 degrees Celsius (0.15 grams of fat per minute) compared with 10 degrees Celsius (0.35 grams of fat per minute) and 20 degrees Celsius (0.40 grams of fat per minute). Previous research at Kent State University also shows an increase in protein breakdown when you exercise in the cold [1].

During colder temperatures, the flow of blood to subcutaneous fat (that's the fat under your skin) is reduced, limiting the amount of it burned for energy.

But this doesn't mean that you can't exercise in the cold weather. Subjects in this study wore very little clothing. Unless there's a nude gym opening in your area, these findings don't have a great deal of practical value.

If you spend a lot of time swimming, however, the amount of subcutaneous fat burned for energy could be reduced (compared with other forms of exercise). Indeed, swimmers seem to have a harder time losing weight compared to cyclists or walkers.

This is not the only study to look at the effect of cold air on fat metabolism. And the results are far from conclusive. In fact, during submaximal exercise in the cold, fat metabolism has been reported to be elevated, unchanged or reduced.

Other studies have combined whole body precooling before the exercise. This can reduce core body temperature, leading to a "shivering" response. It might explain why some trials show that the cold actually increases the amount of fat burned for energy.

In this study, the normal rise in core temperature associated with exercise stopped the subjects from shivering. That's why I think it's more relevant to people like you and me who exercise regularly.

Skin temperature also affects growth hormone levels. In fact, simply taking a hot (38-39 degrees Celsius) bath for 25 minutes will raise growth hormone levels more than ten-fold [4].

A single surge in growth hormone increases both the number of fat calories your body burns for energy and your metabolic rate [3]. That's why growth hormone therapy often leads to a reduction in fat mass (but it doesn't mean you can lose fat sitting in the bath — sorry). You can read more about supplements designed to raise growth hormone levels elsewhere on this site.

Hibernation

Hibernating animals (those that sleep during the winter) tend to store fat before they hibernate. One of the ways they accomplish this is via an increase in the activity of enzymes (such as lipoprotein lipase, known also as LPL) that promote the storage of fat.

More interesting still, LPL levels in humans also rise and fall in tandem with the seasons [2].

Researchers from the University of Colorado studied a group of 12 women and 6 men in both the summer and winter.

Summer was classed as May through August. Winter was classed as November through February. LPL activity in both muscle and fat increased during the winter, and dropped during the summer.

The summer also sees a change in the activity of several fat-burning and muscle-building hormones.

Cortisol levels, for example, reach a low point in the summer [7]. Not only is cortisol associated with the storage of abdominal fat, it's also been linked to all kinds of health problems, including diabetes, heart disease and depression. Cortisol may also weaken your immune system, leaving you more susceptible to colds and flu.

It appears that living by the earth's natural cycle of light and darkness keeps cortisol at its proper level. Make sure to exercise outside whenever you get the chance, or join a gym that lets in plenty of natural sunlight. Treatment with natural light has been shown to lower cortisol in subjects with seasonal affective disorder - a type of depression many people suffer from during the winter [9].

Testosterone, a powerful hormone which helps you build muscle and lose fat, tends to peak in the summer and early autumn. It also reaches a low point in the winter and early spring [6].

Serotonin

There's also evidence to show that you'll find it easier to control your appetite in the summer rather than the winter. Some studies, for instance, show a link between the "winter blues" and a drop in serotonin levels [7].

Serotonin is a chemical that helps messages pass from one nerve cell to another. It helps different parts of your brain "talk" to each other.

When serotonin drops below a certain level, your brain "thinks" that your body is starving and "tells" you to start eating. In fact, some researchers believe that there's direct link between obesity (due to overeating) and decreased brain serotonin levels.

Overweight people with low levels of serotonin feel almost compelled to eat more. Once they get their carbohydrate "fix", serotonin levels rise, and they feel better again — albeit temporarily.

Dr. Albert Stunkard, a professor of psychiatry at the University of Pennsylvania, thinks that people with an almost uncontrollable urge to raid the fridge late at night are doing it to help themselves sleep by boosting serotonin levels.

In other words, some people who suffer from the "winter blues" may use foods high in carbohydrate to make themselves feel better.

The bottom line is that your body will respond very differently to the same program of diet and exercise in the summer than it does in the winter.

Summer is the perfect time to to renew your commitment to building a leaner, stronger body. If the program you're following now isn't delivering the results you want, consider using the fat-burning or muscle-building programs available on this site. The programs will require both time and effort from you, but they work well for almost everyone who tries them.

References
1. Dolny, D.G., & Lemon, P.W. (1988). Effect of ambient temperature on protein breakdown during prolonged exercise. Journal of Applied Physiology, 64, 550-555
2. Donahoo, W.T., Jensen, D.R., Shepard, T.Y., & Eckel, R.H. (2000). Seasonal variation in lipoprotein lipase and plasma lipids in physically active, normal weight humans. Journal of Clinical Endocrinology and Metabolism, 85, 3065-3068
3. Gravholt, C.H., Schmitz, O., Simonsen, L., Bulow, J., Christiansen, J.S., & Moller, N. (1999). Effects of a physiological GH pulse on interstitial glycerol in abdominal and femoral adipose tissue. American Journal of Physiology, E277, 848-854
4. Koska, J., Rovensky, J., Zimanova, T., & Vigas, M. (2003). Growth hormone and prolactin responses during partial and whole body warm-water immersions. Acta Physiologica Scandinavica, 178, 19-23
5. Layden, J.D., Patterson, M.J., & Nimmo, M.A. (2002). Effects of reduced ambient temperature on fat utilization during submaximal exercise. Medicine and Science in Sports and Exercise, 34, 774-779
6. Smals, A.G., Kloppenborg, P.W., & Benraad, T.J. (1976). Circannual cycle in plasma testosterone levels in man. Journal of Clinical Endocrinology and Metabolism, 42, 979-982
7. Walker, B.R., Best, R., Noon, J.P., Watt, G.C., & Webb, D.J. (1997). Seasonal variation in glucocorticoid activity in healthy men. Journal of Clinical Endocrinology and Metabolism, 82, 4015-4019
8. Wurtman, R.J., & Wurtman, J.J. (1995). Brain serotonin, carbohydrate-craving, obesity and depression. Obesity Research, 3, 477S-480S
9. Wirz-Justice, A., Graw, P., Krauchi, K., Sarrafzadeh, A., English, J., Arendt, J., & Sand, L. (1996). 'Natural' light treatment of seasonal affective disorder. Journal of Affective Disorders, 37, 109-120