Myth: If You Eat or Exercise Consistently, You Will Never Gain Weight

A common myth is that people who exercise should not gain weight.  The common belief is that fat people are lazy and if they exercise it would fix their obesity.  The fact is that weight gain is mostly an input and not an output problem.  You need to be willing to make lifestyle changes and dietary adjustments as you age, change, and grow. 

I am going to attack this in two parts:  

1. Diet.  Eating regularly will not in itself help maintain or lose weight.  It will help prevent you from having cravings or snacking, but the problem is that if you eat more calories than you burn, you will gain weight.  You must control your intake in addition to eating regularly to prevent weight gain.  Also, there is research to show that skipping meals will induce weight loss.  I do not recommend it because it will cause cravings and hunger.  There are many studies that show that diet is superior to exercise alone​[1]​,.

2. Exercise.  Exercise can produce weight loss, but the amount of time you would have to set aside to induce significant weight loss by itself is almost not realistic.  For example, to lose two pounds a week, you would have to burn 1,000 extra calories which would be 1 hour of high-intensity exercise per day and 2 hours per day of moderate-intensity exercise.  One study looked at exercise for weight control in postmenopausal females and found that although exercise works it can take up to 77 hours of exercise to lose 2.2 pounds of body weight[2].  Another study looked at research over the past 25 years.  The meta-analysis found that exercise was the least successful means to lose weight and improve body composition[3].  

The Bottom Line:  In theory, both of these could be true, but the fact is they are unrealistic to maintain.  You must pay attention to what you eat and combine exercise and diet to prevent weight gain.  It is clear based on most research that diet combined with exercise is superior to promote long-term weight loss​[4]​, but no plan is guaranteed to keep the weight off without keeping the energy usage either balanced or negative.   

Footnotes
[1]Miller, Koceja, and Hamilton, “A Meta-Analysis of the Past 25 Years of Weight Loss Research Using Diet, Exercise or Diet plus Exercise Intervention.”
[2]Friedenreich et al., “Adiposity Changes after a 1-Year Aerobic Exercise Intervention among Postmenopausal Women: A Randomized Controlled Trial.”
[3]Miller, Koceja, and Hamilton, “A Meta-Analysis of the Past 25 Years of Weight Loss Research Using Diet, Exercise or Diet plus Exercise Intervention.”
[4]Wu et al., “Long-Term Effectiveness of Diet-plus-Exercise Interventions vs. Diet-Only Interventions for Weight Loss: A Meta-Analysis.”

References:

1. Friedenreich, CM, CG Woolcott, A McTiernan, T Terry, R Brant, R Ballard-Barbash, ML Irwin, et al. “Adiposity Changes after a 1-Year Aerobic Exercise Intervention among Postmenopausal Women: A Randomized Controlled Trial.” International Journal of Obesity (2005) 35, no. 3 (September 7, 2010): 427–35. [PMC]
2. Miller, WC, DM Koceja, and EJ Hamilton. “A Meta-Analysis of the Past 25 Years of Weight Loss Research Using Diet, Exercise or Diet plus Exercise Intervention.” International Journal of Obesity and Related Metabolic Disorders : Journal of the International Association for the Study of Obesity 21, no. 10 (October 1, 1997): 941–47. [PubMed]
3. Wu, T., X. Gao, M. Chen, and R. M. van Dam. “Long-Term Effectiveness of Diet-plus-Exercise Interventions vs. Diet-Only Interventions for Weight Loss: A Meta-Analysis.” Obesity Reviews. Wiley-Blackwell, May 2009. doi: 10.1111/j.1467-789x.2008.00547.x