(CNN) The meal replacement company Soylent offers grab-and-go drinks and bars designed to be consumed in place of sitting down for a meal -- but, for now, the company is urging customers to throw out its Soylent Bars.

The bars have been making some people sick , according to reports, and the company is investigating why "a small number of our customers have experienced gastrointestinal issues after consuming Soylent Bars."

It's offering customers full refunds.

"Until we are absolutely certain our products are safe, they will not be shipped," it said in a blog post . "We are deeply sorry if any customer had any negative experiences after eating a Soylent Bar."

Meanwhile, the rest of the world might be wondering: Why were people eating a single bar for dinner to begin with? The company aims to give customers a convenient way to consume the same sustenance and nutrition found in a balanced meal. Many meal replacement brands -- such as Soylent, Ambronite, SlimFast and Ample -- claim to be able to replace real food in our lives with ready-to-consume shakes or bars.

But are they actually effective?

It depends on what you're replacing

For the average healthy adult, meal replacement bars or shakes seem to be beneficial only when they are replacing an unhealthy food item in your daily diet, said Sharon Akabas, associate director of the Institute of Human Nutrition at Columbia University.

"The question is, what would the person do if they didn't eat the bar or drink the shake? Would they go to a fast food place and have a burger, fries and sweet beverage instead of the shake? In that case, any health specialist would say, go for the shake," Akabas said.

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Meal replacements can help with portion control, said Rachel Lustgarten, a registered dietitian at Weill Cornell Medical College in New York.

"This is something that comes up a lot for me in practice," she said. "Whole food, eating real food, tends to be the most satisfying way to go for most people. However, I am not down on meal replacements entirely, particularly for people looking to lose weight or control weight. For those clients, using meal replacements can sometimes be a great way to jump-start weight loss because they are portion-controlled, high in protein and low in sugar."

Additionally, meal replacements are sometimes used to help feed sick patients who can't keep any food down, said Charles Mueller, clinical associate professor of nutrition at New York University.

"There's a place for meal replacements when people are really having trouble eating for one reason or another, but for healthy, free-living people of any age, they should be eating whole food, and they should be taking the time and the effort to do so," Mueller said.

Otherwise, "you're missing out on the healthy bacteria; you're missing out on all the nutrients contained in food; you're missing out on the dietary fiber, which are what we call prebiotics, and literally, the roughage in the diet stimulates the gastrointestinal lining," he said. "Most meal replacements kind of don't do much for any of that."

Feeding the bacteria in your gut

Though many meal replacement products claim to offer the same amount of protein, carbohydrates, lipids and micronutrients that consumers would find in a whole food meal, they tend to lack the fiber and good bacteria needed to maintain the ecosystem of microorganisms, or microbiota, inside of you. That ecosystem is called your body's microbiome.

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"Your microbiome works closely with gastrointestinal immunity," Mueller said.

"Whole foods provide fiber, which is substrate or food for the microbiota. Food is full of bacteria, and you tend to develop a microbiome that profiles your diet wherever you live," he said. "There's a French microbiome; there's a Ugandan microbiome; there's a Mexican microbiome; you can go to Chile, and you find very specific profiles. We have very sophisticated technology now to classify the different types of organisms that live in our guts, and they are very dependent on how we eat, and they're very closely linked to chronic disease."

The human gut shelters a sophisticated community with trillions of microbial cells that can influence physiology, metabolism, nutrition and even immune function. Any change in diet can affect that community, Mueller said.

Solid foods also are an important part of a daily diet because they can stimulate the movement of muscles in the gastrointestinal tract , Mueller added. Many meal replacement products come in liquid form.

"Your gastrointestinal tract is a big muscle that likes to work out, and it likes to push and pull food, and when it doesn't get to do that all the time on a constant basis, it's not as healthy, just like a person who stops exercising," Mueller said.

'This isn't rocket science; it's harder'

There also is the act of sitting at a table and interacting with your food that can make a difference in how satisfied you feel during mealtime, which might not happen if you are consuming on-the-go meal replacement products.

"Meal replacements, because they're high in protein, tend to do a pretty good job in satisfying our physical hunger. They are quick and convenient, but some people might not feel as satisfied mentally since there is no chewing and actual eating involved," Lustgarten said.

"For some people, eating a meal of whole foods will feel more satisfying even if they feel just as physically full after having a meal replacement drink," she said, "and that tends to be more mental as opposed to just physical."

The benefits of fresh produce compared with meal replacements can be so nuanced that they have become a topic of ongoing research, Akabas said.

"This isn't rocket science; it's harder," she said. "We can send someone to space and bring them back in the same aircraft, and we still don't know what should go into a blender for complete nutrition for a long period of time for the general population."

Among the general population, though, a preference for meal replacements appears to vary by age.

Your age makes a difference

More recently, meal replacement brands -- such as Soylent and Ambronite -- seem to target busy young professionals who may not have the time to cook a traditional meal.

However, older adults are more likely to find satisfaction in a monotonous meal replacement diet, according to a small study published in the journal Physiology & Behavior in 2000.

The study involved 18 young adults and 14 older adults who completed questionnaires about their food cravings and preferences, as well as cognitive testing, and then were placed on a 19-day diet.

During the first seven days of the diet, the participants were asked to incorporate a vanilla-flavored nutritious beverage in their daily meals. Then, from the eighth to 12th day, they were instructed to consume only the beverage and nothing else. For the last week of the diet, the study participants returned to eating normal meals.

The participants were asked to keep food diaries, in which they recorded whether they experienced any cravings during the liquid diet.

After examining the diaries, the researchers found that the older adults reported significantly fewer food cravings than the young adults. In fact, older men reported almost no cravings at any time during the study.

Older adults were also more likely than the young adults, on average, to enjoy the vanilla-flavored drink and not mind the monotony.

"Complaints about chalkiness came primarily from younger adults, and the older adults didn't have those complaints," said Marcia Pelchat, an associate member emerita at the Monell Chemical Senses Center in Philadelphia and a co-author of the study.

"As people age, their sense of smell becomes less acute, and that explains why the younger adults say the beverages have a chemical or vitamin flavor, because that's primarily smell," she explained.

A meal (replacement) to start the day

Pelchat said there is one occasion, however, where satisfaction with food monotony seems common across all ages.

"Breakfast: People are much more likely to eat the same thing for breakfast every day than they are for lunch or for dinner, and you might think that they get bored, but they don't," she said.

Some meal replacement brands even market their products as morning food items, such as Carnation Instant Breakfasts.

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"Sometimes people think starting a program like this has to be all or nothing and that they have to replace all their meals, which can sometimes set people up for failure," said Lustgarten, the registered dietitian.

"You don't need to commit to doing this every single day. This might be your breakfast on the weekdays or during lunch when you have a meeting," she said. "If people are feeling confused about how to implement this, talk to your doctor or get a referral to see a dietitian."

No matter how you might implement meal replacements in your daily routine, experts say to make sure you are getting all of the vitamins and minerals you need.

'Food is a part of culture'

"There certainly is a lot of science behind meal replacement for a few different reasons. In some cases, it can help people with a limited ability to digest and absorb food, so hospital-based patients, and there's a long history of meal replacements for weight management," said Akabas, the nutrition expert at Columbia University.

"But they have often gotten a bad name because of manufacturing's role in making claims that are too good to be true," she said. "We have no single food that includes the nutrients that would be needed to sustain someone across the lifespan of any significant period of time, so anyone who makes a claim otherwise is over-reaching."

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Plus, there's more to food than just nutrients, Pelchat said.

"I would hate to see us reduced to being a society that just consumed nutrients to survive," she said. "Food is a part of culture, and the meal plays so many important roles in communication and customs. The history of a culture is really reflected in consuming food. ... It would be a shame to lose all of that."