The 5 supplements scientists recommend for longevity

• As we age, it becomes harder for our bodies to absorb nutrients.
• Supplements like vitamin D or magnesium may be in order.
• But experts say "do not just take things" without knowing what your body needs.

Dr. Nir Barzilai says supplements are definitely, unequivocally improving the health of one thing.

"Supplements are really good for the economy," the director of the Institute for Aging Research at the Albert Einstein College of Medicine told Business Insider.

"They're supporting a lot of companies, a lot of workers," he said of the roughly $39 billion global supplement market. But whether they're supporting our health is a murkier, more challenging question to answer.

When it comes to aging, Barzilai, and other leading longevity experts say the supplement regimen you should choose to live a long, healthy life won't be one single, straightforward routine. It should be unique to each person, and each moment in time.

"It should be driven by the biology of your body," Dr. Andrea Maier, who co-directs the Centre for Healthy Longevity at the National University of Singapore, told BI. "Do not just take things."

Maier's own research has shown her that many supplements are not living up to their label claims, and that everyone reacts a little differently to each pill they pop. Though she does recommend supplements to some of her patients at private and public longevity clinics in Singapore, she also takes a very personal and scientific approach to the task of prescribing them.

Because our body's ability to absorb nutrients diminishes as we age, it is true that supplements may help some people improve their longevity in certain ways, if taken judiciously.

Here are five of the buzziest science-backed supplements around, and what they may do for aging bodies:

Vitamin D

The supplement is taken by a long list of aging and longevity experts, from immunologist Anthony Fauci, to the biohacker who wants to live forever, Bryan Johnson. Vitamin D is essential for our bone density, helping us convert calcium into strength.

Generally, we absorb most of our vitamin D from the sun, so many people choose to supplement their vitamin D intake in the wintertime, when we aren't getting as many rays outside. But you can also get vitamin D from a diet rich in fatty fish like salmon, as well as fortified milk, and egg yolks.

Barzilai says that even though his vitamin D levels are low, he doesn't supplement this nutrient.

"The only thing that vitamin D helps is for osteoporosis," he said.

And, he says bone scans suggest "my bones are really thick," so he doesn't think he needs to supplement.

Fish oil

Fish oil is a great heart health protector, providing plenty of omega-3 fatty acids to help improve cholesterol. There's some evidence it's also good for preventing and improving the swelling and pain of arthritis, through some anti-inflammatory properties.

But you can also get plenty of omega-3s from eating walnuts, salmon, or tuna. Eating your omega-3s in fish and nuts makes it easier to pack in a whole host of other nutrients too. Nuts, for example, are a great source of fiber, magnesium, and vitamin B6, while salmon is great for improving vitamin D stores and vitamin B12.

NAD+ boosters, like NR

Nicotinamide adenine dinucleotide (NAD+) is a key enzyme that our body uses to create energy. It can help maintain tissue health, keep DNA repaired, and improve immune function and metabolism.

Both nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are precursors that may help us produce more NAD+, and they have become popular anti-aging supplements. But last year, the US Food and Drug Administration said that because NMN is being investigated as a drug, it "may not be marketed as or in a dietary supplement." NR is still allowed to be sold as a supplement, and some studies have suggested that it has decent anti-inflammatory properties that can help support healthy aging. More NR studies are needed targeting populations with NAD+ deficiencies to know for sure what benefits the supplement might provide to those who could potentially use it the most.

Fisetin

This chemical compound, naturally occurring in brightly-hued strawberries, persimmons, onions, and apples, is a great inflammation fighter that may help clear out old, toxic cells that build up as we age.

Longevity researcher Paul Robbins says he takes it every two weeks, and his "cranky" knee always feels better afterwards. Other aging researchers are excited about the potential of fisetin to improve conditions as varied as arthritis and Alzheimer's. But many experts caution people to wait a few years, until more is known about the benefits and risks of popping it at home.

Magnesium

Magnesium is a kind of wonder mineral for the body — great for maintaining healthy muscles, bones, nerves, blood pressure, and cellular health.

Longevity doctor Peter Attia pops several different kinds of magnesium every day, to promote healthy aging. But you can already get a lot of magnesium just from eating a healthful plant-based diet full of dark leafy greens like spinach, bananas, avocados, nuts, and beans.

Fine-tune your diet and exercise routine before you try supplements, experts say

There is no single supplement protocol all aging experts endorse. Instead, they universally agree that a healthy, plant-heavy diet, rich in nuts, legumes, fruits and vegetables, will serve aging bodies very well.

And yet, in the US, more than half of American adults still take some supplements. Maier believes that's, in large part, to help ease anxiety. By buying a pill, we can feel like we're taking charge of our health In a country where there is no drug that's ever been FDA approved for anti-aging, and no supplement recommended to prevent some of the most common age-related diseases and causes of premature death, like cancer and heart disease.

Maier doesn't take any supplements, saying that her healthy vegan diet seems to get her what she needs. Barzilai takes only a homocysteine supplement "that has folic acid and vitamin B, or something like that." He said that's due to a specific genetic imbalance he has.

"That's different than 'let's just take any vitamin that somebody said is nice,'" he said.

Kate Hull contributed reporting to this story.