Negative Side Effects of a Low-Carb Diet

Reducing your carb intake can have beneficial effects on your weight and overall health, especially when you cut out unhealthy simple carbs from refined grains and sugar. Depending on how much you cut out, you may experience some low-carb side effects as your body adapts. These can be uncomfortable, but they typically subside after the first week or two.

Headaches are a side effect of a low carb diet. (Image: Peopleimages/E+/GettyImages)

It Depends on Your Diet

The severity and scope of side effects you experience when you cut your carb intake depend on a few factors:

  • How much you reduce your carb intake
  • Your diet before you reduced your carb intake
  • Your individual sensitivity to the reduction in carbs

If you were previously eating somewhere close to the National Academy of Medicine's recommended daily intake of carbs for adults — 45 to 65 percent of calories, which on a 2000-calorie diet amounts to 225 to 325 grams — and you suddenly cut that down to 20 grams of carbs a day, you're likely going to experience more intense symptoms than if you only dropped to 75 grams of carbs daily. Your body has to adjust to the difference in carb intake, and the more dramatic the change, the more dramatic the reaction.

The same is true if you used to eat a lot of sugary, processed foods, such as sweets, pastries, white bread, sugary cereals and snack foods, and if you drank soda or other sweetened beverages. These foods actually have addictive characteristics similar to drugs, according to a study published in February 2015 in PLoS One. As a result, you could experience a type of withdrawal when you quit these unhealthy carbs. On the contrary, if you ate a pretty healthy diet with a lot of complex carbs, you probably aren't going to experience such extreme, low-carb diet side effects.

Lastly, everyone is different. Your reaction to cutting carbs might be worse or better than a friend's reaction. This has a lot to do with the two factors already discussed, but it also has to do simply with your own chemical makeup. Just like some people are more sensitive to alcohol or lack of sleep, some people may be more sensitive to carb restriction.

Low-Carb Diet Side Effects

Low-carbohydrate diets come in a few varieties. A moderate reduction in carbs, say 100 to 150 grams of carbs per day, probably isn't going to cause any noticeable side effects. If anything, you may feel a little fatigued for a few days. A larger reduction to between 50 and 100 grams of carbs could cause more pronounced effects, such as:

  • Headache
  • Fatigue
  • Changes in bowel habits — either constipation or diarrhea
  • Irritability

But these days, many people aren't just going low-carb, they're going very low-carb. The ketogenic diet has become increasingly popular as a way to shed fat quickly, and it's also touted for increasing energy, improving heart health and aiding blood sugar control, among many other proposed benefits. On a ketogenic diet, you drop your carb intake super low and increase your fat intake significantly. According to Harvard Health Publishing, fat can make up as much as 90 percent of total calories on a keto diet.

This shift will throw most people's bodies for a loop. In addition to the side effects above, you may also experience:

  • Nausea
  • Vomiting
  • Dizziness
  • Insomnia
  • Reduced exercise tolerance

Collectively, these symptoms are referred to as the "keto flu." While not a medical diagnosis or an actual illness like the flu, it can knock you out for a few days to a week — or more — in a similar manner. For many people, the experience is enough to send them running for a bowl of pasta.

Causes of Keto Side Effects

According to Dr. Marcelo Campos, a Harvard Medical School lecturer and clinical assistant professor at Tufts School of Medicine, no one is quite sure what causes these symptoms. As mentioned earlier, the impact that such a drastic drop in carbs — your body and brain's main (preferred) source of energy — can have on your basic physiology is bound to make you feel pretty icky.

There are other potential explanations:

Dehydration: A quick decrease in carbs can cause fluid loss. If not adequately replaced, you'll feel the effects of dehydration, including fatigue, headache and dizziness.

Electrolyte loss: Fluid loss can also cause losses of electrolyte minerals like sodium, potassium, magnesium and calcium. Electrolytes regulate your body's water balance, muscle function and blood pH, among other processes. Side effects of an electrolyte imbalance may include muscle spasms and muscle weakness.

Campos also hypothesizes that keto flu could be caused by changes in the gut microbiome, detoxification or an immune system reaction.

Curing the Keto Flu

Whatever the cause of the keto flu, it's typically short-lived. Most people report that symptoms are at their worst for the first three days, but then get better and dissipate after the first week. However, in some cases, it could last longer than that — up to a month. Most people can handle the side effects for a week, but any longer than that can be not only unappealing but also disruptive to daily functioning.

Although there isn't a "cure," there are a few steps you can take to potentially lessen its impact:

Increase fluid intake: According to the Mayo Clinic, men need 15.5 cups of fluids each day and women need 11.5 cups. Some of these fluids come from your food, but the majority should come from drinking water and unsweetened beverages.

Replenish electrolytes: Low-carb sources of electrolytes include vegetables, cheese, nuts and seeds. You can also sprinkle a little extra sea salt on your meals.

Eat more frequent meals: This will give your body a more steady stream of nutrients and energy.

Reduce your carb intake gradually: Rather than drastically cutting carbs, reduce your intake a little each week. This will give your body time to adapt to the dietary change, and you may experience fewer and milder symptoms.

If your symptoms don't disappear after a week or they worsen, consider adding more carbs back into your diet and paying your doctor a visit.

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