Think of vitamins and nutrients as an army that will fight off age-related ailments. And the best way to build this army is by eating a healthy, well-rounded diet, says Kristin Kirkpatrick, MS, RD, the manager of wellness nutrition programs at the Cleveland Clinic Wellness Institute. While it's always important to eat well, it becomes especially essential around age 40 because that's when the rules start to change, she says. 

"Your body probably isn't working the same way at 40-plus as it was at 20," she says. Muscle mass starts to deteriorate, we're much more likely to put on weight, menopause may (or may soon) start, and risk of chronic diseases like cancer, heart disease, and diabetes begins to increase—which means your battle plan needs to start looking a little different.

One of the best ways to stay healthy is by getting enough of the right vitamins and nutrients. Whole food sources are typically a better bet than supplements because they're easier for the body to absorb, Kirkpatrick says. However, if you follow a special diet or have certain medical concerns you may benefit from taking a supplement, too. Ask your healthcare provider what's right for you.

Here, the key nutrients to look out for and the best ways to get them.

Vitamin B12

Once you turn 40 (and definitely after turning 50), vitamin B12 should be on your radar. It's essential for normal blood and brain function, Kirkpatrick says. And while children and younger adults are likely to get the B12 they need from food—it's in meat and animal products including chicken, fish, dairy, and eggs—B12 is more poorly absorbed as the body ages, typically starting around 50 because that's when stomach acid levels deplete.

Any time after 40 and before turning 50 is a good time to start getting B12 from a supplement or multivitamin. Aim for 2.4 mg per day (the current recommended dietary allowance), though there's no need to worry about taking too much, Kirkpatrick adds. Because it's a water-soluble vitamin, you pee out what you don't need. 

Calcium

It's hard to know what to think about calcium: A recent analysis of 59 studies designed to measure the role it plays in preventing fractures for men and women older than 50 found that increasing calcium intake—either from foods or supplements—was not likely to significantly reduce fracture risk. And other research has linked calcium supplements to increased risk of heart attack, stroke, and cardiac death for postmenopausal women.

But even though our bones absorb most of the calcium they need earlier in life (typically before age 30), the nutrient does play a role in maintaining bone health later in life, too, according to Kirkpatrick. The nutrient is needed for other basic body functions like muscle contraction, nerve and heart functioning, and other biochemical reactions—and if you're not getting enough calcium from your diet, the body steals calcium from your bones (and weakens them). 

The bottom line is that you do need calcium at 40 and beyond, but these latest findings tell us you don't need to go overboard because more calcium does not necessarily mean more benefit and may even be harmful to heart health, she says. Most women can get the calcium they need—1,000 mg a day for women 40 to 50, and 1,200 mg for women older than 50—if they eat a well-rounded diet with calcium-rich foods like dairy, tofu, sardines, broccoli, almonds, and spinach. Women who are vegan and lactose intolerant should ask their physician if taking a supplement may be beneficial. 

Vitamin D

D is a biggie, Kirkpatrick says, especially after 40, because it helps protect against the age-related changes that start to kick in.  Vitamin D deficiencies have been linked to diabetes, heart disease, multiple sclerosis, and breast and colorectal cancers—all of which are more likely to crop up the older you get. Plus, D is essential for absorption of calcium in the body, she says. 

Dietary sources include fish and fortified dairy, grains, and cereals, but generally, the D you get from food is poorly absorbed. The sun is the best source of the vitamin, but not everyone lives close enough to the equator to be exposed to the strong rays that will deliver the D you need, Kirkpatrick explains. 

"If you're living anywhere above Georgia, you're probably not getting enough vitamin D from the sun," she says. Plus, you don't absorb it with sunscreen on—and you definitely don't want to be hanging out in the sun without sunscreen (despite any vitamin D benefits). She recommends a D3 supplement (D3 is the type of vitamin D closest to what you would get from the sun). You should be getting at least 600 IU per day (and 800 IU per day after 50), according to current National Institutes of Health recommendations. The tolerable upper limit (i.e., the amount that will not cause harm) is as much as 4,000 IU per day. 

Magnesium

A key function of magnesium is to help regulate blood pressure, which is especially important for women 40-plus, who are already at risk of high blood pressure due to normal aging. Deficiencies in magnesium have been linked to heart disease, diabetes, and inflammation, Kirkpatrick adds. Plus, it helps the body absorb calcium and plays a role in muscle, nerve, and heart function, as well as blood glucose control. 

Your doc can test your magnesium levels if you think you might be deficient (and would need a supplement). But if you're eating a healthy, balanced diet, you're likely to get all the magnesium you need (320 mg a day for women 40 and up) from food, Kirkpatrick says—it's found in dark leafy greens, beans, soy, nuts, seeds, and avocados. Too much magnesium does not necessarily pose health risks but may cause diarrhea, nausea, or cramping.

Potassium

Potassium plays a key role in keeping blood pressure in check, no matter your age, Kirkpatrick says. In postmenopausal women, research has linked higher intake of potassium from food to decreased risk of stroke—though "high" intake was considered approximately 3.1 g, which is still lower than the recommended 4.7 g per day. And the benefits were seen in those getting as little as 2 g per day, says study author Sylvia Wassertheil-Smoller, PhD, a professor in the department of epidemiology and population health at Albert Einstein College of Medicine.

Potassium is definitely a nutrient you want to be getting enough of, but unless your MD prescribes it for another medical condition, Kirkpatrick cautions against taking potassium supplements. Too much potassium can damage the gastrointestinal tract and the heart, and can cause potentially life-threatening cardiac arrhythmias. Most people can get the potassium they need by eating a varied, healthy diet that includes bananas, sweet potatoes, chard, beans, and lentils. You're highly unlikely to get enough potassium in your diet to be dangerous, Kirkpatrick says. If your doctor does prescribe supplements, she should carefully monitor how they affect you, she says.

Omega-3s

Technically not a vitamin, omega-3 fatty acids still deserve a place on this list because of their myriad health benefits, Kirkpatrick says—and especially because they help counteract some of the negative changes that come with aging, like increased heart disease risk and cognitive decline. Research has shown that omega-3s help lower blood pressure (check out these other ways to lower your blood pressure naturally) and LDL ("bad") cholesterol levels, reduce the risk of heart disease, and play a role in keeping memory and thinking sharp.

In fact, a recent study found that people with higher levels of omega-3 fatty acids in their blood had larger brains and performed better on memory tests, planning activities, and abstract thinking, compared with individuals with lower levels—which suggests that omega-3 fatty acids play a role in maintaining brain health in addition to the other known benefits, says the study's lead author, Zaldy S. Tan, MD, MPH, medical director of the Alzheimer's and Dementia Care Program at UCLA.

Though you can get omega-3s from foods like fish, walnuts, flaxseeds, and leafy vegetables, taking a supplement is a good way to make sure you're getting enough, Kirkpatrick says. Either way, aim for 500 mg if you're healthy, 800 to 1,000 mg if you have heart disease, and 2,000 to 4,000 mg if you have high triglyceride levels. And be sure to ask your doctor about the right dose if you're taking anticoagulant drugs, which can have serious side effects.

Probiotics

Probiotics are not technically vitamins or minerals either, but they're important essentials for women 40 and up, Kirkpatrick says. Mounting evidence suggests probiotics play a role in keeping the gut healthy and weight down, and even in lowering risk of heart disease, diabetes, and stroke—all of which is especially important around 40 when muscle mass starts to decrease, making it easier to put on weight and develop insulin resistance.

And though you can get probiotics in some dairy and fermented soy products like seitan, foods typically will not contain as many strains as a supplement—and each strain comes with its own benefit, some for helping to control weight, others for helping prevent diarrhea. Plus, because probiotics are actually live and active cultures, you won't be able to get them from foods that are cooked or heated.