- Starting the
keto diet will often come with symptoms including fatigue and constipation. - However, stick with it and you may have increased energy levels and fewer hunger pangs.
- The keto diet can help you lose weight but is not recommended over more sustainable diets.
Starting the keto diet comes with a number of challenges — for starters, it's highly restrictive.
Note: The ketogenic diet is a very low-carb, high-fat diet that forces your body into a metabolic state called ketosis, where you burn fat for fuel instead of carbs. The keto diet can lead to weight loss that may be difficult to maintain long-term given the diet's restrictive nature.
"It takes a good base of nutrition knowledge and the ability to read labels to successfully follow a ketogenic diet," says Emilie Vandenberg, a registered dietitian nutritionist at The Ohio State University Wexner Medical Center. "You must know the macronutrient composition of most foods. It can also be difficult to give up or avoid certain foods."
Medical term: Macronutrients are the nutrients from food that your body needs in the largest amounts to function properly. They include protein, fat, and carbohydrates.
That said, this diet does promise some potential benefits, including improved blood sugar control for patients with type 2 diabetes, weight loss, and reduced seizures in people with epilepsy.
Below, experts reveal some key considerations to keep in mind before starting the keto diet including how you'll feel soon after starting the diet as well as possible
1. Digestion
The most common gastrointestinal side effects for the keto diet include diarrhea, acid reflux, nausea, and constipation, says Vandenberg.
Constipation often happens because the keto diet prohibits whole grains and starchy vegetables as well as limits fruits to very small servings of only low-carb fruit. All of these foods are a good source of fiber, which makes your stool easier to pass.
Paula Doebrich, MPH, a registered dietitian with her own private nutrition practice, says the keto diet typically involves the following (though ratios may vary slightly):
- Fat: 70% to 90% of total calories
- Protein: 1 g/kg of body weight (so 68 grams for a 150 lb person)
- Carbohydrates: Remaining calories (usually somewhere between 100-200 calories or around 50 grams)
Gastrointestinal "symptoms can persist for as long as someone is on the diet," says Vandenberg. "Others may adjust to the diet changes and GI symptoms may resolve."
2. Keto flu
Within the first week of starting the keto diet, some people begin experiencing a cluster of symptoms known as the keto flu.
The symptoms tend to resemble those of the traditional flu and can include:
- Headache
- Brain fog
- Nausea
- Vomiting
- Weakness
- Irritability
- Muscle cramps
- Dizziness
- Difficulty sleeping
Fortunately, Doebrich says this is usually just a temporary side effect of severely decreasing your carbohydrate intake, and it should subside after a week or so.
Vandenberg advises decreasing carbohydrates very gradually over a few weeks and staying well hydrated in order to mitigate these symptoms.
3. Energy levels
Experts say it's normal to experience some short-term fatigue after you first start the keto diet. This is because your body is slowly adapting to a different metabolic state that doesn't rely on carbs for fuel.
"As your body adjusts to using fat as fuel instead of carbohydrates, your energy levels may return to normal," says Doebrich, adding that this fatigue will usually last one to three weeks.
However, Doebrich says that people who regularly engage in high-intensity exercise may continue to feel fatigued after workouts if they're not consuming enough calories. Since the keto diet is very high in fat, which tends to be extremely filling, you may stop eating before you get an adequate amount of energy to power you through those workouts.
4. Blood sugar levels
Carbohydrates are broken down into sugars in the body, which means they can cause blood sugar spikes, especially if they're simple carbs.
Since the keto diet drastically decreases your carb intake, it's been shown to help improve blood sugar levels, particularly in people with type 2 diabetes.
So, if you're accustomed to the crash from blood sugar spikes, then you may experience increased energy levels and higher alertness after starting keto.
Important: The keto diet is not advised for patients with type 1 diabetes, as ketosis can increase the risk for certain complications, like dyslipidemia, hypoglycemia, and diabetic ketoacidosis, which can lead to diabetic coma.
Research findings on the effects of the keto diet in type 1 diabetes patients have been mixed — a 2019 review noted that long-term outcomes are still unknown, and there's still no consensus on the acceptable level of ketosis in patients with type 1 diabetes.
5. Heart health
Research on how the keto diet affects heart health has yielded mixed findings. Some studies suggest that it can trigger spikes in "bad" LDL cholesterol — which is linked to an increased risk of cardiovascular disease. Other studies have found that the keto diet decreases cardiovascular risk factors, like obesity.
People on the keto diet consume a lot of fat. And if that fat is in the form of saturated fat — like red meat, bacon, cheese, butter, and coconut oil — it may raise levels of LDL cholesterol.
Opting for more unsaturated fats such as olive oil, nuts, seeds, and avocado is one way to minimize these risks, says Vandenberg.
Note: If you are at a higher risk of heart disease or cardiovascular disease, you should talk to your doctor before starting the keto diet.
6. Weight loss
A 2019 review found that very-low-carb diets are "not superior to other dietary approaches for weight loss."
Most studies on the link between the keto diet and weight loss have been conducted in a controlled environment, and the participants usually have a metabolic condition like insulin resistance or type 2 diabetes, says Doebrich — so it's hard to know what a regular, healthy dieter can expect.
Additionally, Doebrich says the rapid initial weight loss is mostly water weight.
"Glycogen retains water, and when you decrease your intake of carbs, glycogen stores get used as a fuel source, causing rapid weight loss," she says. "Weight loss has been reported to peak around five months but is not easily sustained after that."
Note: About 1 to 2 pounds per week is the recommendation for healthy and sustainable weight loss. If you're losing more than that, it may be difficult to keep that weight off.
7. Hunger pains
It's not uncommon to experience hunger pangs when starting the keto diet, as your body adjusts to drastic changes in your carb intake.
While Doebrich says these hunger and cravings are to be expected in the beginning, it's important to monitor whether or not this side effect goes away as your body adjusts to your new diet.
Fortunately, this side effect appears to be short-term — in fact, a 2015 review found that ketogenic diets may ultimately promote appetite suppression.
To curb these hunger pangs and cravings make sure you're getting as much protein and fiber as the diet allows and drink plenty of water.
8. Nutritional deficiencies
Because the keto diet cuts out grains, as well as a number of fruits and vegetables, there is a risk of certain nutritional deficiencies.
"There tends to be very little vitamin C, potassium, magnesium, calcium, or folate in this diet," says Doebrich. "Also, the lack of fiber could have an impact on the gut flora — and the microbiome is known to play an important role in immunity, endocrine function, inflammatory processes, and even brain health."
Eating a wide variety of vegetables, fruits, and protein sources can help you to avoid many of these deficiencies. Vandenberg also recommends the following supplements to fill these nutritional gaps:
- Multivitamin
- Fiber
- Calcium: 500 to 1,000 milligrams per day in two doses
- Vitamin D: 1,000 to 2,000 International Units
- Potassium citrate: Talk with your physician to find out if this is necessary
9. Bone loss
A small 2020 study found that elite athletes doing intense training on the keto diet showed increased signs of bone breakdown and decreased signs of repair than athletes on a diet that included more carbs.
Not only that, but after reintroducing more carbs into their diets, those athletes' bone health only partially recovered.
Most studies on the keto diet and bone health have been conducted in athletes. However, Doebrich notes that since vitamin D and calcium deficiencies are super common in the general population, it's important to ensure you're getting enough of these bone-strengthening nutrients after starting the keto diet.
10. Who should avoid the keto diet
The keto diet isn't for everyone. People with the following medical conditions should avoid it, according to Vandenberg and Doebrich:
- Disordered eating (or a history of an eating disorder)
- Type 1 diabetes
- Hypertension
- Kidney disease
- Cardiovascular disease
- Pancreatic conditions
- Kidney stones
- Osteoporosis
- Low body weight
- High cholesterol
Athletes should also be cautious about trying the keto diet, says Doebrich. Especially if you regularly participate in strenuous or high-intensity exercise, it's advisable to talk with your doctor about your own unique nutritional needs before starting this diet.
Insider's takeaway
While the keto diet may help to promote weight loss and blood sugar control, it also comes with a multitude of challenges, risks, and side effects.
Fatigue, digestive issues, and "keto flu" symptoms — which come on within the first week of starting the diet — are usually temporary.
However, it's important to fill any necessary gaps in your diet to avoid nutritional deficiencies long-term. Also, talk with your doctor about any lingering symptoms that don't go away after a month.
"The ketogenic diet should not be undertaken alone," says Vandenberg. "It should first be discussed with your doctor and a dietitian that specializes in the ketogenic diet."