Fortunately, improving mental functioning can be as simple as the foods you eat. In fact, some of the best sources could be considered treats. Though your brain only makes up about two percent of your body weight, it consumes about 20 percent of the calories you eat and the oxygen circulating through your system. Making sure to put the proper fuel in your diet is a smart start to better mental performance.

1. Choose foods that are low on the glycemic index.
You've probably experienced the drop in energy and alertness that follows a lunch containing high glycemic foods, like a sandwich with fries and cola. These foods will spike your blood sugar and then cause you to crash. Instead, opt for foods like non-starchy vegetables, strawberries or other berries, and whole grains. These foods break down more slowly and have less of an effect on blood sugar levels. Mixing carbohydrates with some protein and a little fat in every meal creates a gently rising curve rather than a spike. To maintain a steady blood sugar level, don't skip meals or increase exercise without increasing your caloric intake.

2. Enjoy a cup of java.
If you're a coffee drinker, you'll be happy to know that coffee beans contain a number of brain-friendly substances. Coffee beans have been called the number one source of antioxidants in the typical American diet. In addition to caffeine, coffee provides amino acids, vitamins, and minerals. Studies have shown that drinking coffee reduces the risk of mental decline: coffee drinkers have a lower incidence of dementia and Alzheimer's than non-coffee drinkers. Keep in mind that drinking your coffee with shots of high-sugar caramel or mounds of sugar will spike your blood sugar. The best choice is probably black coffee or a flavorless espresso drink made from freshly ground beans.

3. Include some berries in your daily diet.
Berries are good for you and the fresher, the better. These tasty treats have been linked to improving brain function because of their high levels of antioxidants--especially blueberries. The acai berry is another remarkable example. It has one of the highest levels of antioxidants ever measured. The difficulty with acai may be in getting fresh berries instead of the juices made from concentrate. Since the berry is very tart, many of the available sources on the supermarket shelf also have high levels of sugar.

4. Regenerate with green tea.
This beverage not only provides high levels of antioxidants, but also vitamins, minerals, and catechins. A Japanese green tea called matcha was traditionally used by Japanese monks preparing for a full day of meditation. Touted for its ability to both calm and focus while simultaneously stimulating, it's a great source of all the goodness green tea has to offer. The prime tea leaves, or "gyokuru," are selected for their color, dried, and then ground into a powder. When you drink the opaque tea made from this powder, you're actually consuming the green tea leaf--not just the water that the leaves steeped in. As a result, you get a much higher level of all the substances in the tea, including L-theanine, an amino acid associated with increased focus and concentration.

5. Eat more (cacao) chocolate.
One of the best foods for maintaining a healthy brain is the cacao bean, better known as the chocolate bean. Not only is it a high source of antioxidants, but it contains an alkaloid called theobromine, known for its calming, bliss-enhancing effects while increasing mental clarity and focus. The cacao bean isn't always the same thing as chocolate. Many chocolate-flavored foods have low levels of cacao and high levels of sugar and other flavorings. In choosing your chocolate, look for higher levels of cacao--at least 75 percent--which you'll probably find more often in dark chocolate varieties.