Hidden Side Effects Of The Most Popular Diets

Before you take a medication, you probably are warned of the side effects. Potential downsides like diarrhea, depression and migraines are listed on the label. But when you turn to a diet to treat some aspect of your health (whether it be your weight, mood or gut health) you aren't always awarded the same courtesy. People often see going on a diet as something common and casual. It's just a diet! What's the big deal? People go on diets all the time. But diets come with their consequences, too. You're just not always warned about them.

You're probably often faced with information about the benefits of various diets. You might have read about the best diets for weight loss or the best foods to eat for your heart. Maybe your coworker told you all about how the newest diet fad cured her acne or helped improve his mood. However, you're much less often faced with information about the drawbacks of following a diet. But that doesn't mean these downsides don't exist.

"Overall, dieting for the sake of weight loss or manipulating your body shape or size is not a harmless pursuit," says Cara Harbstreet, MS RD LD, of Street Smart Nutrition. "Yes, we can agree that there are many methods that can potentially result in short-term weight loss. No one is denying that there is efficacy behind some (not all) of these methods. However, what we need to also question is the long-term efficacy of dieting and the potential side effects."

So just like you would before taking a new medication, it's important you read up on the side effects of the new diet you're considering. Before you decide to try it, you want all the information necessary to make an informed decision. Here are some hidden side effects of a few of the most popular diets — and of all diets in general.


Many diets discourage consumption of carbohydrates, including the Atkins diet, low-carb paleo diets and other diet plans. On these diets, there are some carbs consumed, though the majority of food eaten contains mostly protein and fats instead. Low-carb diets have been found to promote weight loss in numerous studies. Do your research on whether low-carb will promise the benefits you are looking for in your eating plan. However, it's also important to do your research on the alternative side; here are some of the side effects of low-carb diets. 

Low-carb: Low energy levels

Carbohydrates are the body's preferred source of energy; if you limit them, you may feel the effect. Sugars and starches are utilized in a number of processes in the body. According to Ginger Hultin, spokesperson for the Academy of Nutrition and Dietetics and owner of ChampagneNutrition, lethargy is often a side effect of low-carb dieting.

Low-carb: Difficulty concentrating

Your brain prefers to use glucose in order to function properly, says registered dietitian Cheryl Mussatto. "If carbs are strictly reduced, the brain will be cut off from its main energy source, which can drastically alter brain functioning. One such change can occur with serotonin, a chemical produced by the brain. Serotonin regulates our sleep cycle, mood and appetite, all of which will be noticeably altered along with experiencing brain fog."

Low-carb: Hormonal changes

Every person's carbohydrate needs are different, says registered dietitian Jillian Greaves. Certain low-carb diets may restrict carbs to a level that's too low. "If you're overly restricting carbohydrates, this is a form of stress on the body that can disrupt normal endocrine function," Greaves says. Your endocrine system, responsible for your hormones, is related to many other systems in the body. "This disruption may contribute to cravings, an irregular or stopped menstrual cycle, hypoglycemia, mood swings, anxiety, chronic fatigue, suppressed immune function and thyroid disturbances."

Low-carb: Less muscle growth

If you aren't eating enough carbs, you might see fewer results after strength training at the gym. "Carbohydrates are important for building lean muscle, which is key for maintaining a healthy metabolism and for bone health," Greaves says. Your body needs carbohydrates to undergo a process called "muscle glycogen resynthesis," which is an integral part of building muscle. Without an adequate supply, this process is going to occur much more slowly, which could stall your progress.

Low-carb: Binge eating

Your body prefers to use carbs for a number of basic brain functions; so eating too few of them can cause your brain to fight back. "Cutting out carbs through any low-carb diet (including paleo and keto) can set a person up for binge eating in part because when we don't eat enough carbohydrates, our body releases a brain chemical called neuropeptide Y," says registered dietitian Julie Dillon. "This chemical's job is to tell our body to eat carbs — and eat them now." When a person's brain is flooded with this chemical, it can result in an animal-like instinct to go crazy on carbs — attack a plate of brownies or eat an entire pizza, for example. "It can feel like every cell in your body is demanding carbs, which can lead you to feel that you lack willpower," Dillon explains. "But it has nothing to do with discipline. Rather, this neurochemical is trying to save your body from experiencing fainting, dizziness or worse!"

Low-carb: Lower alcohol tolerance

You might have heard the common advice to load up on carbs before a night of drinking. There's actually some science that supports this theory. Studies show that high-carb meals eaten before drinking can reduce blood alcohol content. If you cut down on carbs, you may find that you feel the effects of drinking more severely.


Low-fat diets have been popularly recommended for decades. These diets involve limiting foods with both unsaturated and saturated fats. Whether or not these diets are effective or healthy is highly controversial, especially in recent years, as evidence of the health benefits of fats accumulates. However, conclusions have not yet been drawn by the medical community. Regardless of whether or not there are benefits to a low-fat diet, here are some side effects you should know about.

Low-fat: Increased hunger

"Fat is a bit slower to digest than protein and carbs, so when you cut back on the fat in your diet you could find yourself feeling hungrier faster," explains Hultin. Eating meals with multiple nutrient groups, including fats, helps your body to remain satiated until your next meal. 

Low-fat: Vitamin deficiencies

Certain vitamins, including vitamins A, D, E and K, are fat-soluble, meaning that your body needs fats in order to absorb them. If you do not have enough fats in your body, the vitamins you eat may pass through your system unabsorbed. This can result in nutrient deficiencies, which may cause other negative health effects.

Low-fat: May harm cholesterol levels

While it's true that too many saturated fats may not be great for your cholesterol, too little fat overall may also have a negative effect. Studies show that low-fat diets were associated with more of the "bad" cholesterol that could damage your arteries. Certain types of fatty foods, including olive oil, nuts and seeds, may actually help keep your cholesterol down and keep your heart healthy.

Low-fat: Poor brain health

High-fat foods such as avocados, nuts and oils are some of the top foods neurologists recommend eating for brain health. Your brain, which is actually made up of 80 percent fat, relies on dietary fats to function. If you don't eat enough of this nutrient, you may miss out on some of the brain-boosting benefits of dietary fats including better memory, lower risk of dementia and Alzheimer's, and better focus and concentration.


The Whole30 is a fad diet based on a book by Melissa and Dallas Hartwig that advocates for eliminating major food groups such as grains, legumes, soy and dairy, as well as added sugars and alcohol, for 30 days. It is similar to the paleo diet, but more restrictive. The diet became extremely popular and continues to receive thousands of tags on social media each year.

Whole30: Poor relationship with food

"This is far and away the top diet that people cite when they seek my support for disordered eating and healing their relationship with food," says Harbstreet of Street Smart Nutrition. "Often, this comes after multiple rounds of the 30-day program where they initially wanted to eat more healthfully but became increasingly disordered with each successive round. It's harmful because it's promoted as a 'lifestyle' not a diet, yet the food rules are rigid and inflexible and lack evidence to back up the sensational claims." The most common effects Harbstreet sees in patients are orthorexic behaviors — meaning an unhealthy obsession with "healthy" eating. Additionally, those who complete the 30 days of dieting often report feeling large amounts of anxiety once the 30 days are over. Transitioning back into eating normally without fearing certain foods can be extremely stressful for the dieter.

Whole30: Social isolation

Though it may not be the first thing you think of when you think of your health (what with all the restaurant food and alcohol involved), your social life is an integral part of your well-being. Since the Whole30 diet is so restrictive, it can eliminate (or at least complicate) the possibility of participating in many social outings and occasions. This is also true of other restrictive diet plans such as intermittent fasting, which only allows you to eat during certain time windows, and other diets that eliminate large food groups. "Dinner dates, drinks with friends and snacks at ball games or movies all become obsolete if you're keeping a strict schedule," says Hultin. "As a registered dietitian, I want my clients to have flexibility with their food and eating schedules, so it's important for someone to think about their lifestyle when considering a diet."

Keto diet

The keto diet is a popular diet trend based on the idea that if you eat a small enough number of carbohydrates, your body will be forced to burn fat for energy in a state called ketosis. On the diet, you can eat large amounts of fatty foods and animal proteins, but can only eat very small amounts of foods that are sources of sugar, such as fruit, grains and vegetables with carbohydrates like sweet potatoes or carrots. Whether or not the keto diet is actually safe, many have turned to the diet (which originated as a last-resort treatment for epilepsy in children) in order to lose weight. However, this diet can cause a number of other reactions in the body. Here are some side effects you should know.

Keto diet: Risk for heart health

The keto diet is built around the idea that by eating mostly protein and fats and an extremely low number of carbs, the body will be forced to burn fat for fuel. However, this level of restriction is difficult to achieve. "Most people are probably not actually getting into ketosis, and are eating a lot of dietary fat without burning fat for fuel," explains registered dietitian Glenys Oyston. "The potential for harm to heart health can be high, especially if people are including a lot of saturated fats." Many keto dieters eat large amounts of foods high in saturated fats such as bacon and butter, while skimping on heart-healthy foods such as fruit and whole grains.

Keto diet: Constipation

Dietary fiber, which is found in whole grains, fruits and many other carbohydrate-rich foods eschewed by the keto diet, is essential for maintaining proper digestion. Cutting many of these foods from your diet to remain in ketosis may back things up. One study testing the efficacy of the keto diet to manage epilepsy in children showed that a majority of patients on the keto diet complained of constipation.

Keto diet: Weaker bones

"The keto diet may weaken bones," says Oyston. "Ketone bodies may make the blood too acidic, causing calcium to be leached from the bones to neutralize the acidity." Ketone bodies are released into the bloodstream when the body is in ketosis. This lack of calcium can put the dieter at risk for weak bones that result in bone fractures or, eventually, osteoporosis.

Keto diet: Acid reflux

"The keto diet may be hard on digestion, as many people find a high-fat diet aggravates acid reflux," says Oyston. Eating large amounts of cheese, red meat and these other common heartburn triggers could become really painful.

Keto diet: Bad breath

Some people who try the keto diet notice that their breath smells like nail polish remover. This is a side effect of ketosis that can be seriously unpleasant. Besides ketosis, other scenarios that may cause this chemical change include fasting, starvation, prolonged and intense exercise, alcoholism and untreated Type 1 diabetes. Some people also notice an unpleasant vaginal odor that occurs for similar reasons. This is a side effect referred to as "keto crotch." It's thought by some gynecologists and dietitians that the same chemicals that cause "keto breath" cause a pH change in the vagina that causes the smell, though this theory has never been thoroughly studied.

Keto diet: Keto flu

Many who try the keto diet experience a set of symptoms referred to as the "keto flu." In some studies, the keto flu afflicted over a third of those who attempted the diet. Symptoms vary, but they generally include nausea, vomiting, constipation, diarrhea, headache, irritability, weakness, muscle cramps and soreness, dizziness, poor concentration, stomach pain, difficulty sleeping and sugar cravings. Reportedly, the symptoms subside after a week or two, but some people don't shake them off as quickly.


Many people believe that limiting their calorie intake to a certain number will help to prevent overeating and aid in weight loss. Limiting calories imposes a limit of available energy to the body. Some believe that this will then influence the body to burn fat cells instead of calories from food. While this may be true in the short term, there are some aspects to calorie counting and restricting food intake that they may not be taking into account. "The hidden side effects of dieting and energy restriction may not become apparent for months or years afterwards, so this approach is especially worrisome when it's treated in a very cavalier fashion," says Harbstreet. Here are a few of the potential consequences to consider before you try it.

Low-calorie: Slower metabolism

Many people believe in the philosophy "calories in, calories out," but this theory fails to take metabolism into account. "Calorie restriction triggers our bodies' survival mechanism," explains Alissa Rumsey, MS, RD, nutrition therapist and owner of Alissa Rumsey Nutrition and Wellness . "When you eat too few calories, your body thinks you are starving so it decreases your metabolism and increases fat storage." This habit is one of many that could slow down your metabolism over time.

Low-calorie: Food preoccupation

The Minnesota Starvation Experiment, conducted in the 1940s, revealed groundbreaking information about how calorie deprivation affects the human brain. "It demonstrated that a lower calorie intake provokes the mind to overly think about food," explains Dillon. Even years after the study had ended and participants again began eating a higher number of calories, participants who had been limited to diets of 1500 calories per day found they felt fixated on food. Some participants even changed their careers, eventually becoming chefs. "Dieting provokes the brain to dream about food and consume thoughts. We believe this is a necessary evolutionary response to not eating enough," Dillon says. "This food obsession is designed to keep us alive yet in the modern world keeps us distracted from what we really want to do in life."

Low-calorie: Hormonal changes

Low-calorie diets may seem like a benign way to prevent overeating, but research shows that they have their consequences. Some studies show that low-calorie diets result in an increase in the stress hormone cortisol. Cortisol is released in many scenarios wherein your body is under duress — lack of sleep, eating tons of sugar and drinking excessive amounts of alcohol, for example. Too much cortisol in your body can cause some serious negative health effects over time.

Low-calorie: Loss of hunger cues

If you're restricting your intake to a certain number of calories, it's likely that you're making your decisions about what and when to eat based on numbers, rather than whether or not you're actually hungry. "This keeps you very externally focused, which means you could lose sight of your body's own hunger and fullness cues," says Rumsey. To keep these signals working properly, it's important to pay attention to and respond to them.

Low-calorie: Obsessive behaviors

Calorie counting is a form of food tracking, and it involves keeping tabs on what you're eating every day, whether in your head, an app or some form of food diary. "This can trigger obsessive behaviors, guilt and anxiety around food," says Rumsey. These behaviors can have detrimental effects on mental health over time or even lead to disordered eating, she explains.

Low-calorie: Feeling cold

Even moderately limiting your calorie intake could affect your body temperature. If you feel cold all the time, not eating an adequate number of calories could be the cause. In a study of middle-aged men and women, those who consumed an average of 1,769 calories per day had a significantly lower body temperature than those who consumed over 2,300 calories, regardless of physical activity. Additionally, dietary fats (often found in high-calorie foods limited by low-calorie diets) are used by the body to keep warm. Limiting both calories and fats can result in colder body temperatures.

Dieting in general

While specific diets have their individual side effects, all diets share a few important side effects. Before you pursue weight loss by limiting your food choices with any diet, here are a few side effects you should be prepared to face.

Dieting in general: Depression

"Research suggests going on any restricted diet places a person at a higher risk for experiencing depression," Dillon says. "Mood is regulated by hormones that require enough calories, carbs and fats through food to be adequate." Any diet that limits these things could affect your mood.

Dieting in general: Weight cycling

Weight cycling occurs when a person repeatedly loses and regains weight, which often results in a net weight gain over time. "Long-term diet research (two years or longer) suggest that most people regain all the weight lost during a diet, whether they stick to the diet or not," Dillon says. "In fact, many regain more weight than was lost in the first place." Weight cycling is not only stressful for the dieter; it's also connected to other health complications such as increased insulin, blood sugar, blood pressure and cholesterol. It also is connected to a slower metabolism.

Dieting in general: Increased inflammation

Weight cycling, mentioned above, has also been linked to increased inflammation in studies. "Inflammation is a necessary human function. When our body has to do extra work more inflammation occurs," Dillon explains. "However, inflammation can make you feel drained or foggy and harms health. Short-term research suggests many diets lower inflammation, but research looking six months out or more shows that inflammatory markers increase."

Dieting in general: Worsened body image

You might think that going on a diet would make you feel better about your body, but research shows quite the opposite. A study of college students showed that for men and women, dieting (even diets described as "normal" in severity) resulted in an increased concern with weight and a lower self-esteem. Another larger study showed that both men and women who reported dieting behaviors were less likely to have a positive body image. Instead of dieting, consider trying one of these simple ways to feel better about your body right now.

Dieting in general: Trouble sleeping

When your body is feeling hungry or deprived of certain foods, it's less likely to get quality sleep. Studies have connected restricted dieting behaviors with poor sleep quality. In one study, even a short period of dieting (four weeks) resulted in a significant decrease in the amount of time women spent asleep and a greater difficulty falling asleep. Sleep is an important part of health and a lack of sleep can result in inflammation, increased disease risk and other unappealing (and serious) side effects.

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