It's great to hear about a 12 year old out there who is concerned enough about cycling to be thinking about their nutrition - kudos on that
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I'd say that at your age, because you will likely be growing so much over the next few years of your life, be careful to not make too many drastic changes in your eating patterns. Focus on whole grains, protein from nuts, seeds, fish and poultry, and (to hit this home again) LOTS of fresh fruit and vegetables. I wouldn't worry at all about any supplements. I don't know much about your build/stature, but considering the caloric strain that cycling can put on peoples diets, keep a keen eye on how energized you feel before, during, and after rides. If you feel tired, or overly fatigued, make sure you are drinking plenty of water, and try eating a bit more during your day. Just listen to your body - generally you'll know if you aren't eating right.
Work to avoid processed foods if you can. This can be difficult because there are so many out there. In general, if it looks like it came from a machine, and not out of the ground or the sea... Avoid it. Now I'm not saying that the ocasional indulgence is going to kill your riding ability, but out of long-term health interests, don't be scarfing down hohos or the like
.
And I concur fully with every person who says CUT OUT THE SODA. It isn't that hard to do, and honestly, the on-bike benefits are tremendous once you're "off the juice"
. Soft-drinks contain lots of high-fructose corn syrup which will do nothing except elevate your blood sugar in a heartbeat, and then leave you crashing a short while later. Tea, skim milk,
some diet sodas, and
yes even crystal light work well as substitutes.
While on the bike, bring some sort of easily digestable, carbohydrate-filled snack. Granola bars, dried fruit, half a pbj sandwich on wheat are some of my favorites. Find out what works for you.
If you really become interested, or run into problems that basic guidelines such as those posted in this forum can't fix, visit a sports nutritionist to see what they can do to help. Again, it's awesome to see somebody this dedicated at 12 years of age.