Eating just one green olive will hardly bust your diet, as each piece of the brined fruit contains only 4 calories, according to the USDA National Nutrient Database. This is just an average, however, and large olives will have more calories. In addition, moderation counts -- although 4 calories isn't much, eating handfuls of olives at a time can still promote weight gain.
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More than 90 percent of the calories in green olives come from fat, as this food provides little protein or carbohydrates. According to the 2010 Dietary Guidelines for Americans, you should limit fat intake to 20 to 35 percent of total caloric intake. Therefore, olives are best consumed in small portions as part of a balanced diet.
The fat in olives is mainly monunsaturated, which is a healthy fat that can help reduce "bad" LDL cholesterol while increasing "good" HDL cholesterol, promoting clear arteries. In contrast, saturated fats from animal products may contribute to unhealthy cholesterol levels. Olives are very high in sodium, however, which can lead to high blood pressure. Therefore, people with cardiovascular conditions should eat them in limited quantities, if at all.