Nutritionists are continually spouting the benefits of foods like tomatoes, avocados and fish, but overdoing it on these healthy foods actually can be harmful.

"Even nutritious food can be too much of a good thing if you eat it in too large a quantity or too often," said Elisa Zied, New York-based dietitian, nutritionist and author of "Younger Next Week." "For one, anything that has calories -- even if they're quality calories -- can add up if your portion gets too big. Also, if you overdo any one food, you will leave less room for other foods that provide a different mix of nutrients."

Here are some ways to find the right balance so you don't eat too much of a good thing.

 

OLIVE OILWhy it's good for you: A major component of the healthful Mediterranean diet, olive oil helps lower the risk of heart disease, stroke and high blood pressure because it contains monounsaturated fatty acids (as opposed to saturated fats or trans fats). A study published in Neurology found that older people who regularly consume olive oil have a 41 percent lower risk of stroke compared with those who never consume it. Other studies have found that olive oil in the diet helps maintain healthy cholesterol levels, guards against Alzheimer's disease, prevents acute pancreatitis and protects the liver from oxidative stress in addition to other diseases.

Too much of a good thing: "Because olive oil is looked upon as a healthy fat, people think they should not be concerned about calories," said Andrea Giancoli, a Los Angeles-based registered dietitian and nutrition consultant. "But calories count."

Stick to this: Giancoli recommends sticking to one tablespoon of olive oil daily, which is 120 calories. If you want more than one tablespoon, you should cut calories in other areas of your diet that day.

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AVOCADO

Why it's good for you: Avocados are high in monounsaturated fat, which reduces bad cholesterol, lowers your risk of stroke, heart disease and cancer, and may promote a healthy body weight. One medium avocado contains about 4 grams of protein and is high in vitamins K, B, C and E.

Too much of a good thing: "Each one also contains 322 calories and 29 grams of fat," said Allison Parker, registered and licensed dietitian for Mariano's, an Illinois grocery chain.

Stick to this: Parker uses one-quarter to one-third of a medium avocado for a serving of fat in her meals or snacks -- essentially using the avocado as a replacement for another fat, such as butter or mayonnaise.

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TOMATO AND ORANGEWhy they're good for you: Tomatoes are high in vitamins A, B6, E and K, and they're also a good source of copper, potassium, fiber and phosphorus. Oranges are packed with vitamin C, phytochemicals and flavonoids, which have anti-inflammatory properties -- and they are only about 80 calories.

Too much of a good thing: "If you overdose on them, one thing that comes to mind is tooth enamel," Zied said. "Too much acidity can wear it away, so it's good to eat acidic fruits and vegetables for their nutrients and water content but to also choose other options in those categories -- for example, hard, crunchy fruits like apples, carrots and celery that stimulate the flow of saliva and neutralize the acids in foods that can erode enamel." Stick to this:One-half to 1 cup of tomato or an orange or clementine is great per day.

 

NUTS

Why they're good for you: Most nuts boast a good dose of monounsaturated fat. When used to replace saturated fats and trans fats, that can reduce blood cholesterol and lower heart disease and stroke risk, Zied said. "Nuts also provide polyunsaturated fats, which are essential fats our bodies need from the diet, since it can't make them," Zied said.

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Too much of a good thing: Nuts are easy to overdo because they're a concentrated source of calories -- a lot of calories in a small portion, Zied said.

Stick to this: An ounce of nuts per day -- or up to 1 ½ ounces if you can afford the calories. Mix types of nuts for different proportions of nutrients and flavors in your diet. An ounce of almonds is about 24 whole almonds or 4 tablespoons chopped. An ounce of walnuts is about 14 halves or 4 tablespoons chopped. An ounce of pistachios is about 48 pistachios.

LARGE FISH (such as tuna, swordfish or mackerel)Why it's good for you: These fish varieties have lean protein and are high in B12, vitamin D, calcium and iron. Their high amounts of omega-3 fatty acids have been associated with everything from reducing inflammation and heart disease to warding off depression.

Too much of a good thing: These types of fish contain relatively high levels of mercury, and while this is particularly concerning in pregnant and lactating women, it's not good for anyone to ingest too much mercury, Parker said.

Stick to this: No more than 6 ounces of large fish weekly.