Pro and amateur cyclists usually gain weight in the winter. 

This is an inevitable consequence of interrupted or diminished training regimens, but it is also, partially at least, a deliberate choice. 
Renown champions of the more or less recent past talk about incremental weight gain of between 3 and 6 kg or more in November - December of every year. 
Many of them are convinced that this practice of “Overfeeding” (a kind of super-nutrition schedule) pays off during the following season. 

It’s important to note that a 3-6 week period of Overfeeding can produce an increase in weight of which 50-60% is lean body mass (source: Ann. N.Y. Acad. Sci. 2000; 904: 359-365). This percentage can in turn be increased to 75-80% with a program of power training at a gym. 

On the other hand, a period of relative “under nourishment” (called “Underfeeding”) can produce weight reduction at the expense of both the lean body mass and the lean muscle mass. But this muscular catabolism is less pronounced in individuals who had earlier gone through a period of Overfeeding. 

The same phenomenon can be observed in migrating birds and in hibernating animals. 

It has been documented (source: Am. J. Clin. Nutr. 1989; 49: 608-611) that Overfeeding produces important hormonal responses, with an increase in insulin, IGF-1, and testosterone in both men and women. 

Personally, I find that a controlled weight increase (3-6 kg) in winter is useful, especially in athletes who are slightly lacking in muscle force. Such athletes can profit even more, in terms of lean body mass, by combining Overfeeding with appropriate gym workout regimens.

 

Overfeeding must be balanced. Proteins should correspond to 15-20% of total Kcal; fats 30-35%; carbs 45-55%. 

In a hypothetical diet of 3000 Kcal, this works out as follows: 

- 450-600 Kcal from 110-150 g protein 
- 900-1050 Kcal from 110-115 g fats 
- 1350-1650 Kcal from 340-410 g carbs 

Alcohol should be avoided or at least limited, since it inhibits hormonal adaptation to Overfeeding.