Exercise is linked to brain function throughout all stages of life, beginning in infancy and lasting through old age.

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Exercise tones legs, builds bigger biceps and strengthens the heart. But of all the body parts that benefit from a good workout, the brain may be the big winner.

Physical fitness directly affects our mind and plays a crucial role in the way the brain develops and functions. Also, exercise is linked to brain changes throughout all stages of life, beginning in infancy and lasting through old age.

Babies, for example, need regular movement to carve out critical pathways and form connections in the brain. In children, research suggests exercise improves attention, focus and academic performance. And in the elderly, exercise has been shown to help stave off memory loss associated with some forms of dementia, including Alzheimer’s disease.

“Physical activity is crucial to mind and body alike,” said neuroscientist Lise Eliot, who writes about the benefits of movement on the brain in her book “Pink Brain, Blue Brain.”

Scientists used to believe the mind-body connection was a one-way street: The brain helped build a better physique — or else it sabotaged attempts to get to the gym. But scores of studies suggest that what’s good for the body also is nurturing the old noodle.

“We’ve found exercise has broad benefits on cognition, particularly executive functioning, including improvements in attention, working memory and the ability to multitask,” said researcher Charles Hillman, a professor of kinesiology and community health at the University of Illinois at Urbana-Champaign. In fact, an active lifestyle during childhood may confer protective effects on brain health across the life span, Hillman said.

In the mid-1990s, Carl Cotman’s team at the University of California-Irvine showed that exercise triggers the production of a protein called brain-derived neurotropic factor, or BDNF, which helps support the growth of existing brain cells and the development of new ones.

With age, BDNF levels fall. This decline is one reason brain function deteriorates in the elderly, according to Cotman. Certain types of exercise, namely aerobic, are thought to counteract these drops in BDNF and can restore young levels of BDNF in the aging brain.

Over the past two decades, researchers have learned that exercise acts on multiple levels in the brain. The brain’s wiring depends on the integrity of the brain cells or neurons and the connections between the neurons, or the synapses.

As we age, the synapses are lost or break down. Cotman’s work shows that in older rodents, exercise increases the number of synapses and also stimulates the brain to develop more neurons in the hippocampus, which he called “a critical region in learning and memory formation and a target of massive decline in Alzheimer’s disease.”

Still, for those newly created brain cells, or neurons, to work, they need to be plugged into the existing neural network, said Romain Meeusen, chair of the department of human physiology at the University of Brussels.

Exercise helps integrate the new neurons into the brain’s circuitry to help improve learning, Meeusen said.

In general, exercise increases the release of neurotransmitters, or brain chemicals, that relay signals between nerve cells, called neurons, Meeusen said. “This could be one of the mechanisms of the anti-depressive effect of exercise,” he said. “It also helps to ‘train’ cognition and attention at all ages.”

Research also suggests that exercise improves blood flow to the brain and, as a result, enhances cognitive abilities.

The brain loves it when we move and will reward us handsomely if we do, researchers say. Here’s a look at how physical activity can be beneficial during three key stages of life.

Infancy: Mobile children hit their cognitive milestones faster, said Eliot, an associate professor of neuroscience at Rosalind Franklin University’s Chicago Medical School.

When infants are awake, they’re in near-constant motion, which is critical for development, Eliot said. This movement “strengthens their muscles and hones their neural circuits for smooth, purposeful motor skills.”

She worries that babies in the U.S. are spending too much time strapped in devices.

Pre-adolescence: Researchers are asking an intriguing question: What if exercise improves academic success?

Some research suggests it can. Hillman’s team at the University of Illinois’ Neurocognitive Kinesiology Laboratory found that children aged 7 through 9 who participated in a 60-minute after-school exercise program had better focus, processed information more quickly and performed better on cognitive tests than children who didn’t exercise.

Late adulthood: The hippocampus naturally shrinks in late adulthood, leading to impaired memory and increased risk for dementia.

But research suggests aerobic exercise can increase the size of the hippocampus and increase levels of a protein that aids the growth of new brain cells, potentially holding off changes in the brain and improving memory function.