The average human brain shrinks by approximately 5% per decade after the age of 40. This can have a major impact on memory and focus.

What’s more, brain disorders are on the rise. In 2020, 54 million people worldwide had Alzheimer’s disease or other dementias, and that number is expected to grow.

But serious mental decline doesn’t have to be an inevitable part of aging. In fact, certain lifestyle factors have a greater impact than your genes do on whether you’ll develop memory-related diseases.

As a neuroscience researcher, here are seven hard rules I live by to keep my brain sharp and fight off dementia.

Your heart beats roughly 115,000 times a day, and with every beat, it sends about 20% of the oxygen in your body to your brain.

High blood pressure can weaken your heart muscle, and is one of the leading causes of strokes. Ideally, your blood pressure should be no higher than 120/80.

Cholesterol is critical to your brain and nervous system health, too. The American Heart Association recommends getting your cholesterol levels measured every four to six years.

Blood sugar is the primary fuel of the brain. Not enough of it, and you have no energy; too much, and you can destroy blood vessels and tissue, leading to premature aging and cardiovascular disease.

Keep in mind that sugar isn’t enemy, excess sugar is. It’s easy for grams of sugar to add up, even if you think you’re being careful — and usually, sugar will sneak in through packaged foods.

Where is the sugar hidden? Look for these in the ingredients list:

• Dextrose
• Fructose
• Galactose
• Glucose
• Lactose
• Maltose
• Sucrose

And be wary of any product that includes syrup, such as agave nectar syrup or high-fructose corn syrup.

Studies show that people with untreated sleep apnea raise their risk of memory loss by an average of 10 years before the general population.

For most people, a healthy brain needs somewhere between seven and nine hours of sleep a night.

My tips for memory-boosting, immune-enhancing sleep:

• Keep a consistent bedtime and wake-up schedule.
• Turn off devices one hour before bedtime.
• Do something relaxing before bedtime, like listening to soft music or doing mindful breathing exercises.
• Go outside and get in natural sunlight as soon as you can after waking up.

One way I keep things simple is to have most, if not all, of these items in my grocery cart:

• Fatty fish like salmon
• Avocados
• Nuts
• Blueberries
• Cruciferous veggies like arugula, broccoli, Brussels sprouts and collard greens

When food shopping, I ask myself three questions to help determine whether something is good for my brain:

1. Will it spoil? In many cases, perishable is a good thing. The additives and preservatives that keep food from spoiling wreak havoc on your gut bacteria.

2. Are there tons of ingredients in that packaged food? And for that matter, can you pronounce the ingredients? Or does it look like the makings of a chemical experiment? Also avoid anything where sugar is one of the first few ingredients.

3. Do you see a rainbow on your plate? The chemicals that give fruits and vegetables their vibrant colors help boost brain health.

Smokers have a 30% higher risk of developing dementia than non-smokers. They also put those around them at risk: Secondhand smoke contains 7,000 chemicals — and at least 70 of them can cause cancer.

Then there’s thirdhand smoke, which is not actually smoke. It’s the residue of cigarette smoke that creates the telltale smell on clothing or in a room. That residue alone can emit chemicals that are toxic to the brain.

In a recent study, people over the age of 55 who regularly participated in dinner parties or other social events had a lower risk of losing their memory. But it wasn’t because of what they ate, it was the effect of the repeated social connection.

To lessen isolation and loneliness, you can also boost brain chemicals like serotonin and endorphins by performing small acts of kindness:

• Wish others well or check in with somebody.
• Give a compliment without expecting anything in return.
• Make a phone call to somebody you don’t usually reach out to.

Maintaining a strong memory is not all about brain games like Sudoku, Wordle and crossword puzzles.

Learning skills and acquiring information are much more effective ways to make new connections in the brain. The more connections you make, the more likely you are to retain and even enhance your memory.

When you think about learning something new, approach it the way you would with fitness training. You want to work out different muscles on different days. The same goes for the brain.

Over the course of this week, try cross-training your brain by mixing mental activities (learning a new language or reading a book) and physical learning activities (playing tennis or soccer) .

Marc Milstein, PhD, is a brain health expert and author of “The Age-Proof Brain: New Strategies to Improve Memory, Protect Immunity, and Fight Off Dementia.” He earned both his PhD in Biological Chemistry and his Bachelor of Science in Molecular, Cellular, and Developmental Biology from UCLA, and has conducted research on genetics, cancer biology and neuroscience. Follow him on Twitter and Instagram.

Don’t miss:

Want to earn more and work less? Register for the free CNBC Make It: Your Money virtual event on Dec. 13 at 12 p.m. ET to learn from money masters like Kevin O’Leary how you can increase your earning power.