This story is part of a series on finding the healthiest menu options at some of the most popular fast food restaurant chains.

(CNN) So what's so special about Subway, you might ask? After all, a sandwich is a sandwich -- right? Well, yes and no.

Yes, you can choose from pre-designed sandwiches at the chain, including the typical turkey, ham and roast beef varieties. But what we really like about the sub stop is that any menu item can be customized to meet your diet preferences: breads, meats and cheeses, veggies and dressings. You can think of it as one big sandwich buffet -- that is, without having to worry about portion control.

Here's more food for thought: If you think of Subway solely as a sandwich chain, you might be surprised to learn that from the time the first store opened in 1965, salads have been on the menu. We're not talking about new culinary creations but simple adaptations. For example, a sub can quickly morph into a salad if you ask for the sandwich to be prepared as a salad. The ingredients are pretty much the same (except for the bread), just in different proportions (i.e. more veggies, including spinach, and olives). Chopped salads were introduced in 2012.

The same rule applies to breakfast:. If you want the egg whites and cheese from an egg and cheese breakfast sandwich to sit on top of a bed of leafy greens, all you have to do is ask your sandwich artist.

Also worth mentioning: More than 80% of Subway restaurants offer soup, though varieties can differ according to location.

Most of our sandwich picks come from Subway's Fresh Fit menu, which includes sandwiches that are low in saturated fat, are sensible in calories and have less sodium than some of the other sandwiches.

When it comes to breads, the chain's nine-grain wheat bread and multigrain flatbread are richest in whole grains and fiber. The standard build for all sandwiches and salads contains five fresh veggies -- lettuce, tomato, cucumbers, peppers and onions -- but you are welcome to add or subtract toppings as you wish.

Sandwich condiments such as light mayonnaise and mustard add flavor but will contribute small amounts of sodium.

Though most vegetables will not make a dent in your daily sodium budget, pickles do contribute 115 milligrams. Cheese contributes sodium, too: American cheese has the most, at 200 milligrams per serving.

Salad dressings and sauces deliver more sodium than most condiments, but the oil and vinegar dressing is sodium-free.

To limit added sugars, pair your salad or sub with water or unsweetened tea instead of sugar-sweetened beverages.

Below are, by my estimation, the best Subway options if you're focused on healthy choices within the limits of the menu. We break it down by our picks for kids, athletes, drivers, vegetarians and vegans, as well as low-calorie, low-sugar, low-salt, gluten-free and low-carb options. (Note: All sandwiches are 6 inches long, unless otherwise noted.)

For kids

Our picks: Roast beef mini sandwich on nine-grain wheat bread; or Veggie Delite mini sandwich on nine-grain wheat bread with shredded Monterey cheddar; and apple slices and low-fat milk

At Subway, kids can enjoy the same sandwiches as grownups, but in smaller sizes. A "Fresh Fit for Kids Meal" includes a Fresh Fit mini sub, which is a 4-inch sandwich that is low in saturated fat and calories.

Our top pick is the roast beef mini, as it's the most protein-rich of the four kids' sandwich options, and it's the lowest-sodium meat option. For a vegetarian kids option, choose the Veggie Delite sandwich with shredded Monterey cheddar, which has less than half the sodium of processed American cheese.

The kids meal comes with apple slices and a choice of low-fat, calcium-rich milk or fruit punch; we recommend the milk for kids' growing bones.

For vegetarians

Our picks: Veggie Delite sandwich on multigrain flatbread with shredded Monterey cheddar (toasted); or salad with egg whites, cheese, avocado and oil and vinegar dressing

Whether you're vegetarian or simply in the mood for a meatless meal, Subway will satisfy your needs. The chain offers a Veggie Delite sub, a combination of lettuce, tomatoes, green peppers, cucumbers and onions on bread. If that doesn't sound exciting, give it a tasty upgrade by ordering it with multigrain flatbread, which is rich in fiber and whole grains, and ask for it to be toasted with shredded Monterey cheddar.

You can also get a lighter, vegetarian version of a Cobb salad by ordering an egg white and cheese sandwich as a chopped salad. Add guacamole if it's available to boost heart-healthy fats, and top with oil and vinegar dressing.

For vegans

Our picks: Veggie Delite sandwich on Italian bread with vinaigrette; or black bean soup

Vegans who visit Subway can enjoy the Veggie Delite sandwich with vinaigrette dressing for a filling, fiber-rich meal. For vegans who avoid honey, we recommend the Veggie Delite sandwich on Italian bread; however, for those who include honey as part of a vegan diet, we recommend the nine-grain wheat bread, which is richer in whole grains. (Note: Sourdough and roasted garlic are also vegan breads, though not all chains carry them.) Vegans can choose from toppings including mustard, oil, vinegar, pickles, jalapenos and sweet onion sauce.

Another satisfying vegan option offered at some locations is the low-fat black bean soup, with 12 grams of protein and 15 grams of fiber. You can even combine the soup with the sandwich, depending on your appetite. Also worth noting: If you are near one of the 400 or so Subway locations that offer it, you can order a vegan vegetable patty sub, though its sodium count is on the higher side, at about 1,000 milligrams.

For calorie counters

Our picks: Turkey breast salad; or Black Forest ham salad with honey mustard dressing; or turkey breast sandwich on nine-grain wheat bread with honey mustard sauce; and apple slices

We love that Subway can satisfy calorie counters with its wide assortment of salads, which can come chopped or unchopped, depending on your preference. For waistline watchers who find small portions a bit challenging, the salads offer volume: 2½ cups of veggies, including spinach, tomatoes, green peppers, red onions and cucumbers, for very few calories.

The turkey breast and Black Forest ham salads are lowest in calories, with only 110 calories each. You can even add honey mustard dressing without significantly boosting the calorie load (it has only 60 calories per serving). If you prefer a sandwich, turkey comes out on top again as the lowest-calorie non-vegetarian option on the menu; adding honey mustard sauce brings the total sandwich count to 310 calories.

Add apple slices to your salad for a sweet crunchy snack, and your total meal count is only 205 calories.

For the sugar-sensitive

Our picks: Turkey breast sandwich with Monterey cheddar cheese on toasted multigrain flatbread; or oven-roasted chicken salad; or tuna salad with oil and vinegar dressing

With the exception of sugary beverages and desserts, sugars are not a concern at Subway. Most of the chain's sandwiches and salads have less than 10% of calories from added sugar. Our top low-sugar pick is the turkey breast sandwich with cheddar cheese on toasted multigrain flatbread, which has only 5 grams of sugar. Other low-sugar options include the oven-roasted chicken salad and the chopped tuna salad, each with only 6 grams of sugar.

If you're watching sugars, you'll want to steer clear of the sweet onion dressing, which contributes 16 grams, or 4 teaspoons, of sugar per serving. Top either salad with sugar-free oil and vinegar dressing instead.

Additionally, pair your salad or sub with water or unsweetened tea instead of sugar-sweetened beverages. And try to tame your cravings for cookies: Just one chocolate chip cookie delivers 18 grams, or 4½ teaspoons, of sugar.

For the salt-sensitive

Our picks: Veggie Delite salad with oil and vinegar dressing; or Veggie Delite sandwich on nine-grain wheat bread with oil and vinegar dressing; or oven-roasted chicken salad with oil and vinegar dressing; and apple slices

As is typically the case, a meatless meal, such as Subway's Veggie Delite sandwich or the related salad with oil and vinegar dressing, will naturally be lower in sodium. However, there are other options for those looking to limit their salt intake. One we like, with fewer than 300 milligrams of sodium, is the oven-roasted chicken salad with oil and vinegar. You can add avocado if it's available, as it's essentially sodium-free.

However, we caution you to be careful with condiments such as ranch dressing and buffalo sauce (each deliver 400 milligrams of sodium), American cheese (it adds 200 milligrams) and pickles (115 milligrams). Pair your meal with apple slices for a small potassium boost; the mineral will help blunt sodium's effects on blood pressure.

For the gluten-sensitive

Our picks: Rotisserie-style chicken salad with vinaigrette; or black bean soup; or broccoli and cheddar soup

If you're avoiding gluten in your diet, Subway's salads are the way to go. The chopped salad with rotisserie-style chicken topped with Subway's own vinaigrette -- a blend of vinegar, oregano, garlic and pepper -- is one of our favorites. It's packed with vegetables, including folate-rich spinach, and provides 23 grams of lean protein.

Other options with no gluten-containing ingredients include the black bean and broccoli and cheddar soups, which deliver 12 and 5 grams of protein, respectively.

Also worth noting: Approximately 5% of Subway restaurants offer gluten-free bread, so you can ask for your sandwich to be made with it if your local chain offers it. (Note that even when gluten-free bread is available, sandwiches are not labeled "gluten-free" due to the risk of cross-contamination with products that include gluten in the store; an individual with celiac disease would have to be extra cautious.)

For the athlete

Our picks: Rotisserie-style chicken sandwich on nine-grain wheat bread with honey mustard sauce; or roast beef sandwich on nine-grain wheat bread with mustard; and Gatorade

Whether you favor weights, sprints or long cardio routines, Subway's rotisserie-style chicken sandwich can meet your workout needs. It offers 29 grams of protein and 45 grams of carbs to fuel and repair muscles. We also like the roast beef sandwich on nine-grain wheat bread, which is similar in terms of nutritional value.

Athletes with bigger appetites can order a footlong sandwich instead of 6 inches, if more calories and carbs are desired.

Additionally, strength trainers can take advantage of the chain's "double meat option," which gives you two times the amount of protein in the meal. For example, doubling the amount of roast beef brings the protein count of the sandwich from 25 grams to 41 grams (the bread contributes some protein, too). The sodium count also increases significantly, though, so we recommend proceeding only if you can afford the extra milligrams.

Grab a Gatorade for a post-workout beverage to replenish electrolytes lost in sweat.

For the road warrior

Our picks: Oven-roasted chicken sandwich on nine-grain wheat bread with honey mustard sauce; or turkey breast sandwich with Swiss cheese on toasted multigrain flatbread; and apple slices and Baked Lay's chips

Subway's sandwiches are great for grabbing on the go. We like the oven-roasted chicken sandwich with honey mustard sauce, which has 23 grams of protein and 5 grams of fiber -- a nutrient combo that will keep you sated on the road. If you're not in a rush, order a turkey sandwich with shredded cheddar, and ask for it to be toasted on multigrain flatbread.

For a snack during your travels, apples are still the healthiest option, though we understand if you want to grab some Baked Lay's, too.

For the carb-conscious

Our picks: Carved turkey sandwich without bread; or club with scooped-out bread; oil and vinegar dressing

Those cutting carbs have a few options at Subway: you can order a salad (though most dressings and veggies still contain carbs), order a sandwich sans bread or get your bread scooped.

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Keep in mind, the more you scoop, the more you save: A Subway club with scooped-out bread still has 39 grams of carbs (compared with 46 grams in the non-scooped version), a number based on the minimum amount of bread removed.

We would typically save the carved turkey sandwich for November, as it's higher in sodium than the other sandwiches. However, in addition to slashing 40 grams of carbs, omitting the bread brings the sodium count down to 620 milligrams from 890 milligrams. Olive oil and vinegar dressing adds flavor without any carbs.

For breakfast

Our picks: Egg whites with Swiss cheese, spinach and tomato on multigrain flatbread

If you have a hunch that breakfast at Subway consists of breakfast subs, well, you're correct. You won't find oatmeal or even yogurt on the morning menu.

The point of differentiation for the chain's breakfast offerings comes down to the way you order your eggs (both eggs and egg whites are available), what type of bread you choose and whether you add cheese or meats to your morning sandwich.

We favor the egg white sandwich with Swiss cheese, spinach and tomato on multigrain flatbread, which is packed with protein, carbs and fiber to get your day off to an energizing start.