What is the major difference between training indoors and outdoors?

The environment is yet another major difference between riding on a trainer versus riding on the road. Heart rate is usually higher on the trainer because of those warmer conditions and lack of cooling. On the open road, wind blows by us constantly providing a very good cooling effect. Indoors it is normally warmer, and even in the best conditions using a fan, there are not the same cooling effects on the trainer. Most of us just get too warm riding on the trainer. The cooler you can get the environment and the more airflow you can get around the body (with more fans) the cooler you will be during the workout. You’ll be amazed by how adding a couple of fans to a room can increase your capacity to produce power on the trainer. Keeping cool will have a huge effect on the quality of your workouts. You’ll also notice that with better cooling your heart rate will be lower and more in line with what is expected from outdoor riding.

Exercise outside whenever possible, so you can get some sunshine, fresh air, and vitamin D in the process. We have been told to stay out of the sun to prevent cancer. However, this is awful advice. More than 90 percent of our vitamin D comes from sunshine. A number of studies have shown an association between vitamin D deficiency and diabetes, including a UCLA study published in the American Journal of Clinical Nutrition in 2004 that showed that people with vitamin D deficiency are at higher risk of both insulin resistance and metabolic syndrome-two conditions that drive sugar cravings. What you want to do is avoid sunburn, not sunshine.