With nearly 39% of adults worldwide classified as overweight, the diet industry has never been stronger (1).

Diet foods, such as those labeled “low-fat,” “low-calorie” or “fat-free,” are specifically marketed to people looking to shed excess weight.

However, many diet foods may do your waistline more harm than good.

Here are 21 diet foods often considered healthy that may, in fact, cause weight gain.

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1. Smoothies and Protein Shakes

Smoothies and protein shakes are all the rage on social media and in the wellness community.

While some smoothies and protein shakes are nutritious and exceptionally healthy, others are loaded with calories and sugar.

For example, some premade smoothies contain nearly 14 teaspoons (55 grams) of sugar in just one bottle (450 ml) (2).

What’s more, certain protein shakes pack almost 400 calories per bottle (450 ml) (3).

Smoothies and protein shakes can easily be consumed too quickly, stuffing your body with excessive calories and sugar.

2. Low-Fat Flavored Yogurt

Fat is a filling nutrient that amplifies foods’ flavors.

When fat is removed to reduce the calorie content of certain products, sugar is usually added to heighten flavor.

Many low-fat yogurts are loaded with added sugars, which isn’t good for weight loss or overall health.

For instance, 1 cup (225 grams) of Yoplait low-fat vanilla yogurt contains over 7 teaspoons (29 grams) of sugar (4).

Interestingly, full-fat dairy may be a better choice than low-fat dairy.

In an 11-year study in 8,238 women, those who consumed more high-fat dairy products gained less weight than women who consumed low-fat varieties (5).

3. Fresh-Pressed Juices

Many people drink fresh juices made of fruits, vegetables or a combination of both in order to improve health or boost weight loss.

Although not all juices are high in sugar and calories, most fruit juices are.

Drinking fresh fruit juice regularly can contribute to excess calorie consumption, which may cause you to gain weight.

Stick to juices that contain mostly non-starchy veggies like kale and low-sugar fruits like lemon to control your calorie intake.

4. ‘Healthy’ Sweeteners

With many people cutting white sugar from their diet, alternative sweeteners marketed as “healthy” have become increasingly popular.

Agave, coconut sugar and date sugar are just some of the many sweeteners available.

Even though these products are often deemed healthy, going overboard with any sweetener — even natural ones that aren’t as processed as white sugar — can contribute to weight gain.

Agave, for example, is higher in calories than table sugar and extremely high in fructose, a type of sugar that can contribute to insulin resistance and fat accumulation (6).

Because any type of added sugar can cause weight gain, it’s important to limit your total consumption of sugar — including alternative sweeteners.

5. Low-Calorie Cereal

When trying to lose weight, you might reach for low-calorie cereals to start your day.

While these breakfast foods may be low in calories, they are often loaded with added sugars.

Plus, many low-calorie cereals lack the protein and healthy fats that help you feel satisfied.

A study in 30 men demonstrated that a breakfast of eggs and toast provided greater fullness and led to significantly fewer calories consumed throughout the day than did breakfast cereal (7).

6. Dried Fruit

Dried fruit is packed with fiber, vitamins and minerals.

However, since dried fruit is smaller and sweeter than fresh fruit, it can be easily overeaten.

Plus, a serving of dried fruit holds more sugar and calories than an equal amount of fresh fruit.

Sticking to a 1/4-cup (50-gram) portion when snacking on dried fruit is an excellent way to avoid consuming too much.

Although dried fruit is convenient, fresh fruit is a healthier option.

7. Packaged Diet Foods

From diet cookies to fat-free chips, grocery store shelves are brimming with packaged diet foods.

While these items may be tempting, the majority of them are unhealthy.

Many diet foods contain preservatives, unhealthy fats and artificial sweeteners that may harm your body.

It’s best to replace these packaged, overly processed foods with nutrient-dense, filling options.

8. Flavored Coffees

It’s well known that caffeine acts as a mild appetite suppressant, which leads many people to up their coffee intake when trying to drop weight (8, 9).

Although coffee has many health benefits, you should refrain from certain coffee drinks when trying to lose weight (10).

Many such beverages — including lattes, frappes and cappuccinos — are abundant in calories and sugar.

For instance, a Starbucks Venti Cinnamon Dolce Latte made with skimmed milk — and without added whipped cream — crams in 280 calories and 12 teaspoons (50 grams) of sugar (10).

Even though a daily latte may seem harmless, sugary coffees could sabotage your weight loss efforts.

9. Premade Salads

Salads chock full of fiber-rich vegetables can be very weight-loss-friendly.

On the other hand, those doused in high-calorie dressing or topped with unhealthy ingredients are not.

Premade salads, such as those in grocery stores or fast food restaurants, can be very high in calories, sugar and unhealthy fats.

Making your own salad with nutritious ingredients is a better choice.

10. Protein Bars

Many people rely on protein bars for a quick, convenient energy boost.

Although some protein bars are healthy and nutritious, others bulge with calories, sugar and artificial ingredients.

For example, one PowerBar ProteinPlus Chocolate Brownie product contains over 6 teaspoons (24 grams) of sugar alongside 330 calories (11).

A more nutritious, whole-foods-based snack can offer the same calorie and protein content — with much less sugar.

11. Diet Soda

Diet soda is often viewed as a healthy beverage because it contains 0 calories.

However, research ties diet soda consumption to weight gain — not weight loss.

A study in over 2,000 people indicated that those who drank diet soda had larger waist circumferences than those who did not.

What’s more, those consuming diet soda were more likely to have high blood sugar and high blood pressure than people who abstained (12).

Diet soda has also been found to negatively impact gut bacteria, thereby increasing your risk of diabetes and other metabolic diseases (13).

But keep in mind that association does not equal causation. While a few studies have associated a high intake of diet soda with weight gain, there is no solid evidence that it is a cause of overweight or obesity.

12. Granola

Granola is a filling breakfast food beloved by many health-conscious people.

However, while granola can contain nutritious ingredients like oats, nuts, seeds and coconut, many are saturated with added sugars.

To control your sugar intake, choose granolas with no more than 6 grams of sugar per serving.

Better still, make your own granola at home by baking oats, cinnamon, nuts, coconut, coconut oil and dried fruit at a low temperature.

13. Sports Drinks

Sports drinks can be beneficial for athletes and anyone participating in prolonged, intense workouts.

However, these beverages are simply unnecessary for the average person.

Sports drinks can be filled with sugar and may contribute to excess calorie consumption.

Plus, any type of sugary beverage can spike your blood sugar levels, which can lead to insulin resistance and weight gain (14).

For example, a study in over 7,500 children and teens noted that those who regularly drank sports drinks weighed significantly more than their peers (15).

14. Diet Peanut Butter

Diet peanut butter is lower in calories and fat than regular peanut butter.

Although this may seem like a good choice for weight loss, diet peanut butter regularly harbors unhealthy oils and added sugars.

Natural peanut butter made with limited ingredients is a better choice for weight loss.

Research shows that limiting added sugar may promote weight loss, so choosing natural peanut butters with no added sugar is the best choice (16).

15. Low-Calorie Condiments

Low-calorie condiments like salad dressings and ketchup may be a hidden source of added sugars that can contribute to weight gain.

Surprisingly, many low-calorie dressings are packed with sugar.

For example, just 2 tablespoons (31 grams) of Ken’s Steakhouse Lite Honey Mustard dressing contain 2 teaspoons (8 grams) of sugar (17).

Other condiments that tend to be high in sugar include barbecue and tomato sauces.

16. ‘Healthy’ Sweets

Many desserts and candies are marketed as healthy alternatives to more calorie-laden sweets.

Although they may contain fewer calories than traditional desserts, low-calorie baked goods, candies and other confections can be packed with sugar and artificial ingredients.

In order to decrease calories while maintaining taste, manufacturers replace fats with sugars or artificial sweeteners.

Added sugars can contribute to weight gain. At the same time, artificial sweeteners like sugar alcohols can cause digestive symptoms like bloating, diarrhea and gas (18).

Plus, consumption of artificial sweeteners like aspartame and sucralose is linked to an increased risk of obesity and may even encourage sugar cravings and dependence (19).

17. Low-Fat Foods

Many people try to cut out high-fat foods when attempting to lose weight.

However, research shows that this can be counterproductive.

One review determined that low-fat and nonfat foods contained more sugar than regular versions of the same foods (20).

Consuming even small amounts of added sugar can lead to weight gain, blood sugar problems and an increased risk of heart disease (21, 22, 23).

What’s more, dieting and dietary restraint likely increase the chances of future weight gain (24).

18. Frozen Yogurt

Frozen yogurt is a popular dessert that’s widely considered healthier than ice cream.

Because frozen yogurt is often associated with health, it’s frequently over-consumed.

Many frozen yogurt establishments let you fill your own cup, making portion control difficult.

Additionally, the enticing, sugary toppings offered at most frozen yogurt shops can stuff your dessert with even more calories and sugar.

To keep your intake under control, opt for the smallest yogurt cup available and choose natural toppings like fresh fruit, unsweetened coconut and nuts.

19. Sushi

Sushi can be a healthy or unhealthy meal depending on what it holds.

Rolls filled with ingredients like tempura shrimp or sweet sauces can be packed with calories.

Choose sushi rolls that contain healthy ingredients like fresh vegetables, avocado, fresh fish or grilled shrimp and opt for brown rice over white to boost your fiber intake.

20. Coconut Water

Coconut water is one of the most popular natural beverages around.

Even though coconut water provides vitamins, minerals and antioxidants, it does contain sugar and calories.

One cup (240 ml) of coconut water has 45 calories and 6 grams of sugar (25).

While coconut water is much lower in calories and sugar than beverages like juice and soda, it’s best to limit your consumption of any sweet drink.

21. Diet-Specific Processed Foods

Many grocery stores offer diet foods targeting vegans and vegetarians, as well as those following low-carb meal plans.

These products are often filled with artificial ingredients and added sugars that can negatively impact health.

Additionally, these specialty items are often expensive, which can add up if you are eating these types of foods daily.

Focusing on whole, unprocessed foods rather than convenient, packaged foods is always healthier — no matter your dietary preferences.

The Bottom Line

Though many diet foods are branded healthy, they may destroy your weight loss efforts.

Products like smoothies, frozen yogurt and low-fat snack foods can negatively impact your health and even cause you to gain weight.

What’s more, research shows that dieting is not the best way to lose weight (26).

Following a whole-foods diet rich in healthy fats, proteins and fresh produce is the best way to keep weight off for good.

But keep in mind that association does not equal causation. While a few studies have associated a high intake of diet soda with weight gain, there is no solid evidence that it is a cause of overweight or obesity.