A smartphone is the perfect diet and exercise aid. You always have it with you, so you can quickly enter data without having to carry around a food diary or relying on your memory at the end of the day. Dozens of health-related smartphone apps are available, but here are a few standouts that our editors and writers have found especially useful.

1: MyFitnessPal

The MyFitnessPal app is great. You set it up with your age, height, weight, normal activity level, and your goal weight (or the weight you want to maintain), and it calculates your base calorie requirements. Then you enter the food you eat each day, along with cardiovascular and strength building exercises you perform, and the app provides you with a running total of your net calorie intake and how many you have left (or have gone over).

The app also gives you a running breakdown of fat, cholesterol, sodium, potassium, carbs, fiber, sugars, protein, and various vitamins. This makes it easy to adjust your eating to ensure that you’re getting a balanced diet, not just a low-cal one.

You can download MyFitnessPal for Android, iPhone, and BlackBerry. It’s free, but it does contain advertising. You can also enter information using your desktop or laptop computer via the MyFitnessPal Web site. Your information will be synchronized between the Web site, your phone, and other devices (such as an Android tablet).

2: RunKeeper

RunKeeper is another handy little app — and it’s not just for runners. You can select from a number of activity types: running, cycling, walking, swimming, hiking, downhill skiing, cross-country skiing, elliptical, and several more.

RunKeeper uses the GPS in your phone to track your time, distance, speed, and your rate of climb to more accurately calculate the calories burned. It also gives you a nice map of your route with mile markers, so that you can see where you’ve been. If you want, you can share your route with other members of your “street team” — other RunKeeper users you added or that the service picked up from your Facebook friends list or email contacts list. You can also view a list of nearby RunKeeper users (based on your GPS information).

RunKeeper lets you configure who can see which parts of your information; for example, you might want to share your activity summary with everybody but allow only members of your street team to see your route map. You can also send fitness alerts (like when you set a new personal record) to your email address or social network.

RunKeeper has apps for Android, iPhone, BlackBerry, and Windows Phone 7. The basic functionality is free, though it does require registration. There’s also an Elite subscription that costs $4.99/month or $19.99/year. It lets you compose advanced reports and broadcast activities and races live.

3: Meal Snap

For those who don’t want to be bothered with entering the names of their foods into a database, Meal Snap makes it easy to get a rough estimate of your calorie count by simply taking a photo of your meal with your smartphone’s camera. Meal Snap uses the photo to match to its database (which contains about half a million food items) and then sends you a range of calories for the food item you photographed. It also provides other nutritional information, such as proteins, fats, and carbohydrates.

This is not nearly as precise or as accurate as the calorie count you get from the MyFitnessPal database, since the latter is based on measured portion sizes. However, it’s helpful for giving you a rough idea of your calorie count. MealSnap is available only for the iPhone, and it’s $2.99.

4: Nutrition Menu

Nutrition Menu is an iPhone app that serves as a mobile compendium of nutritional data. It has calorie information on common foods (more than 92,000 food items) and meals served in most major U.S. restaurants (41,000 restaurant menu items). It also has calorie-burning information for many types of exercises.

Nutrition Menu allows you to track your daily weigh-ins and provides a Food Score calculator. Its journal features show your progress via daily summaries, and you can write notes and import the information into a spreadsheet.

5: healthycloud

healthycloud tracks it all — nutrition/calorie intake, blood pressure, weight and body metrics, blood sugar, water intake, cholesterol results, blood lab results, body temperature, and more. If you don’t want to track all this information, you can choose which items to track.

Keep your iPhone handy, and as you snack throughout the day or grab lunch, track your intake. healthycloud is available for iPhone and iPad and costs $3.99.