Exercise has been shown to stave off memory loss associated with some forms of dementia among the elderly.

Exercise tones the 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. Moreover, exercise is linked to brain changes throughout all stages of life, from infancy 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, author of the book “Pink Brain, Blue Brain.” “The brain benefits as much as the heart and other muscles from physical activity.”

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. Exercise, it turns out, can help improve cognition in ways that differ from mental brain-training games.

“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 first showed that exercise triggers the production of a protein called brain-derived neurotrophic 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 age-related drops in BDNF and can restore young levels of BDNF in the aging brain.

“In a sense, BDNF is like a brain fertilizer,” said Cotman, a professor of neurology and neurobiology and behavior and founding director of the Institute for Memory Impairments and Neurological Disorders (UCI Mind). “BDNF protects neurons from injury and facilitates learning and synaptic plasticity.”

Over the last 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, as well as the connections between the neurons, or the synapses.

As we age, the synapses are lost or break down. Cotman’s work has shown 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.”

Exercise helps integrate those newly created brain cells, or neurons, into the brain’s circuitry to help improve learning, said Romain Meeusen, chairman of the department of human physiology at the University of Brussels.

In general, exercise increases the release of neurotransmitters, or brain chemicals that relay signals between nerve cells, called neurons, Meeusen said.