It is very possible to get too much of a good thing. This is certainly the case when it comes to health and fitness. Exercise, for example, is one of the best things we can do for our bodies. It can increase muscle and bone strength, improve blood flow, strengthen the immune system, and benefit us in a host of other ways. But, exercising too much -- or "over-training" -- can have opposite effects on your body; weakened immune system, raised cortisol levels, loss of muscle, etc. This is why exercising in moderation is the best approach. The same applies for the food that we eat. Sure, bananas are healthy, but you don't want to eat a whole bunch of them in one sitting. More specifically, we need vitamins and minerals, but consuming too many can actually be toxic.
Here's a quick breakdown of a few healthy nutrients that you shouldn't overdo.
Protein. It is vital for muscle growth and should be a staple of your diet, but too eating too much can result in gastrointestinal distress. As a general rule, men who work out regularly should aim to eat about 1g of protein per pound of body weight (women should try for around half that amount). If you don't work out, then you can get away with about .5 to .8g per pound of body weight.
Calcium. This mineral offers myriad health benefits when consumed in the appropriate amounts. Too much calcium, however, can actually cause you to also inadvertently absorb too much iron, magnesium and zinc. Clearly this can present a score of potential health issues.
Vitamin E. Despite its many health benefits, consuming too much vitamin E can increase the risk of free-radical development.
Caffeine. There are several studies that demonstrate a variety of benefits to be gained by consuming caffeine. However, when consumed in inordinately high doses, it can lead to a reduction in the body's ability to deliver oxygen to muscles.