We've all had those days when we just can't seem to concentrate. And while there's no magic pill to bring us back to the height of our cognitive powers, there are some foods that have been shown to improve brain function, protect against age-associated cognitive decline and encourage focus and clarity.

But before you dismiss the diet-brain connection as mere conjecture, keep in mind that study after study has found a relationship between what we put in our mouths and how well we can perform important thinking and memory tasks. While certain nutrients may specifically assist brain function, there is also the totality of our diets to consider. One recent U.K. study found that a diet high in saturated fat actually caused damage to neurons that control energy and appetite in mice. And several well-regarded studies have shown that meal timing is an important predictor of performance. For example, research shows that eating breakfast can improve the memory and acquisition skills of schoolchildren.

We put together the top brain foods -- tell us in the comments if any have made a difference for you. After all, who couldn't use a little extra oomph up there?

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  • Walnuts

    Walnuts are chock-full of heart-healthy and anti-inflammatory nutrients, and are the only good nut source of alpha linolenic acid (ALA), <a href="http://www.huffingtonpost.com/2012/06/29/healthy-food-healthiest-list_n_1636409.html#slide=1162293">HuffPost Healthy Living earlier reported</a>. That means they help promote blood flow, which in turn allows for efficient delivery of oxygen to the brain. And <a href="Diseasehttp://alzheimers.org.uk/site/scripts/news_article.php?newsID=730">research presented at the 2010 International Conference on Alzheimer's</a> found that mice with the disease who were regularly fed walnuts had improved memory, learning and motor skill coordination, <a href="http://www.myhealthnewsdaily.com/758-best-foods-brain-health.html">according to MyHealthNewsDaily</a>.

  • Olive Oil

    Olive oil is a great source of monounsaturated fats, which have been shown to <a href="http://www.boston.com/dailydose/2012/05/18/tweaking-dietary-fat-intake-could-help-slow-brain-aging-study-suggests/OO7tmvxhB2E8V0algT7DlL/story.html">actually <em>slow</em> brain aging</a>.

  • Berries

    Animal studies have long indicated a link between berry consumption and brain health. But a recent study published in the <em>Annals of Neurology</em> found that a diet high in blueberries, strawberries and others were linked to a <a href="http://healthland.time.com/2012/04/26/brain-food-berries-can-slow-cognitive-decline/">slower mental decline in areas like memory and focus</a> in a large sample of middle-aged women, reported <em>TIME</em>'s Alice Park.

  • Sardines

    Fatty fish like sardines (<a href="http://www.webmd.com/add-adhd/slideshow-brain-foods-that-help-you-concentrate">and salmon!</a>) are a well-known brain booster, thanks to the omega-3 fatty acids, EPA and DHA, which <a href="http://www.hsph.harvard.edu/nutritionsource/questions/omega-3/index.html">have been linked to lower risk of dementia, improved focus and memory</a>.

  • Coffee

    Caffeine, the mild stimulant found in coffee, improves mental acuity, though the drink's enthusiasts -- <a href="http://www.bbc.co.uk/sn/humanbody/truthaboutfood/best/caffeinebrain.shtml">who guzzle 120,000 tons of the stuff each year</a> -- likely already know that. Aside from caffeine's brain boosting effects, coffee's antioxidant richness helps maintain brain health. And some research suggests that drinking coffee can actually <a href="http://www.huffingtonpost.com/2011/09/27/coffee-cuts-depression-women_n_982122.html">stave off depression in women</a>.

  • Spinach

    Spinach is rich in the antioxidant lutein, which is thought to help protect against cognitive decline, <a href="http://www.nutraconference.com/networknow/public/SessionDetails.aspx?SessionID=1004689&FromPage=nz_ALSessionSearch.aspx">according to researchers from Tufts University</a>. And <a href="http://www.shape.com/healthy-eating/diet-tips/11-best-foods-your-brain?page=11">a longitudinal study at Harvard Medical School</a> found that women who reported eating the most leafy green and cruciferous vegetables had a markedly lower rate of cognitive decline, compared to those who ate the least.

  • Dark Chocolate

    Antioxidant-rich dark chocolate is healthy for your whole body, but its <a href="http://www.huffingtonpost.com/2012/06/18/brain-health-foods_n_1593650.html#slide=1087860">caffeine content is thought to play a role</a> in maintaining mental acuity. What's more, chocolate is rich in flavonoids, a class of antioxidant that helps to improve blood flow (and thus brain health) by regulating cholesterol and lowering blood pressure.

  • Avocados

    Avocados are full of monounsaturated fats that <a href="http://www.webmd.com/add-adhd/slideshow-brain-foods-that-help-you-concentrate">improve vascular health and blood flow</a>, making them another brain food.

  • Water

    When a person becomes dehydrated, their brain tissue actually shrinks. And several studies have shown that <a href="http://www.ncbi.nlm.nih.gov/pubmed/20336685">dehydration can affect cognitive function</a>. Dehydration can impair short-term memory, focus and decision making, <a href="http://www.psychologytoday.com/blog/you-illuminated/201010/why-your-brain-needs-water">according to <em>Psychology Today</em></a>.

  • Wheat Germ

    Wheat germ is a rich vegetarian source of choline -- a nutrient that is involved in the body's production of acetylcholine, a neurotransmitter that boosts memory, <a href="http://www.shape.com/healthy-eating/diet-tips/11-best-foods-your-brain" target="_hplink">according to <em>Shape</em></a>. Eggs are another good choline source.

  • Beets

    Beets are a good source of naturally-occurring nitrates, which help improve blood flow to the brain, <a href="http://www.shape.com/healthy-eating/diet-tips/11-best-foods-your-brain">according to <em>Shape</em></a>.

  • Garlic

    Garlic may help stave off some forms of brain cancer, according to research published in <em>Cancer</em>, the medical journal of the American Cancer Society. Investigators found that <a href="organo-sulfur compounds found in garlic have been identified as effective against glioblastoma">the organo-sulfur compounds in garlic actually worked to kill glioblastoma cells</a> -- a type of malignant tumor cell.