What Are the Side Effects of a Low-Carb Diet?

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A low-carb diet can look different for everyone. For instance, many people consume more carbohydrates than their bodies need, and reducing carbohydrates can bring them back into an ideal moderate carbohydrate intake. However, some individuals prefer to minimize carbohydrates further to control their blood sugar or lose weight. 

While cutting carbs is not necessary to lose weight, some people prefer to limit the number of carbs they eat because it helps them feel better overall. The best diet for you is one that you can sustain for the long term or that enables you to become healthier. Before beginning a low carbohydrate diet, it is essential to discuss the options with a health care professional.

In general, a low carbohydrate diet restricts carbohydrate intake to 130 grams or fewer each day. Very low-carb diets may restrict carbs to 5 to 10% of total calories.

Low-Carb Diet Side Effects

The types of side effects you might feel while embarking on a low-carb diet depend on your physiology, your current eating patterns, and how much you reduce your carbohydrate and overall calorie intake. Here are some of the most common side effects of a low-carb diet.

Low-Carb Diet Side Effects

  • Constipation
  • Fatigue
  • Headaches
  • Muscle cramps
  • Bad breath
  • Reduced athletic performance


While any change to your regular dietary patterns can cause alterations to your bathroom habits, constipation is a common complaint from those on a low carb diet, especially very low carb diets like keto.

One reason for this is a lack of fiber due to minimal grains and beans, which are high-fiber foods that are also reasonably high in carbohydrates and so often avoided on low-carb diets.

While sometimes constipation can remedy itself as the body adjusts, keeping hydrated, taking a fiber supplement, or increasing low-carb, high-fiber foods in your diet can help. If constipation persists, speak to a health care professional.


Fatigue is a common symptom of low-carb diets because carbohydrates are the body’s preferred fuel source. Limiting carbohydrates to amounts below what our body requires for optimal energy can lead to fatigue, especially as the body adapts to the new diet.

Low-calorie diets, in general, can also lead to fatigue, so be sure that you are not eating too few calories to support your lifestyle. If weight loss is your goal, a calorie deficit is required, but one that is too extreme can have the opposite effect, reducing your total daily energy expenditure and slowing your metabolism.


If you are lowering carbs, you will also be reducing your sugar intake. Those who may be used to eating more sugar can lead to withdrawal symptoms like headaches. Studies show that headache severity increases when starting a low-carbohydrate diet. 

Muscle Cramps 

If you aren’t getting enough of certain minerals, such as potassium and magnesium, in your diet, you may experience muscle cramps. Potassium, sodium, and magnesium are responsible for muscle contraction and several other vital processes related to muscle and nerve function.

Whole grains are an excellent source of these minerals, and excluding them from your diet may lead to deficiencies and resulting side effects. When you lower carbohydrates, your body will carry less water due to how carbs interact with glycogen and water storage. Losing a lot of body water may also result in a loss of these minerals.

Bad Breath

Bad breath (halitosis) is an often-reported side effect of low-carbohydrate diets. Sometimes bad breath is combined with a foul taste in the mouth.

On very low carbohydrate diets, your body switches to ketones for fuel in the absence of preferred glycogen. These ketones are removed through urination and exhalation. The exhalation of ketones may be the cause of bad breath.

Reduced Athletic Performance

Low carbohydrate diets provide little in the way of glycogen energy stores, the body’s preferred source for fueling activity. You may notice a substantial drop in your ability to perform during exercise, sports, and training activities. 

For instance, a study in the journal Nutrition & Metabolism reported a reduction in VO2 max (amount of oxygen measured in the body during exercise) peak and peak power.

Post-Diet Side Effects

When you decide to end your low-carb diet, you may also experience side effects as your body adjusts. The most common of these include:

Weight Gain

Typically from water weight, you may experience an initial increase in weight. Carbohydrates increase the amount of water stored in your body, so as water increases, so will your weight. This is not fat gain and is not unhealthy weight gain.


Increasing your fiber and carbs may cause an adjustment period that leads to bloating. As well, increased body water may look like bloat. This should stabilize, and it’s important to keep hydrated as you adjust.

Fatigue and Irritability

Particularly if you increase simple carbs and sugars, you may experience blood sugar spikes, and crashes can lead to feeling tired and irritable. Try to stick to high-fiber complex carbs, and consuming fat and protein to slow digestion.


If you are used to the satiating effects of a higher fat diet to replace the lack of carbohydrates, you may feel hungry. As well, if you choose simple carbs or sugar, this can lead to blood sugar spikes and leave you feeling hungry shortly after you've finished eating.


Those with diabetes or people on blood pressure or blood sugar moderating medications should not go on a low-carb diet without guidance from a health care practitioner. Medications may need to be adjusted when trying a low carbohydrate diet, and this should be monitored.

Short-term weight loss and blood sugar control are often reasons why people begin a low-carb diet. However, more studies need to be done regarding the long-term risks involved in following this diet.

A long-term study on the effects of low carbohydrate diets over time revealed that they could be risky, potentially increasing your risk for premature death, coronary heart disease, stroke, and cancer. 

Pregnant and breastfeeding mothers should likely avoid low-carbohydrate diets. If you are considering lowering carbs when you’re pregnant or nursing, speak to a health care professional first.

A Word From Verywell

While a low-carb diet is suitable for some people in certain situations, there are risks involved. It’s wise to speak to a health care professional about any dietary changes you plan to make, especially if you take medication, have diabetes or high blood pressure, or are pregnant or breastfeeding.

Know that a low-carbohydrate diet isn’t needed to lose weight and is not superior to any other diet that creates a calorie deficit. Food quality is most important for healthy nutrition and weight management.

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Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Rachel MacPherson, BA, CPT
Rachel MacPherson is a health writer, certified personal trainer, and exercise nutrition coach based in Montreal.