By now, you’ve heard a ton of buzz about the ketogenic diet. This high-fat, low-carb eating plan is popular, but can also be extremely divisive. Your coworker says he lost XX-pounds — and eats all the bacon he wants. Your neighbor, on the other hand, said she felt so tired she stopped the diet after four days. If you’re thinking of trying keto yourself, you’ll best set yourself up for success by knowing — and preparing for — what lies ahead.
The goal of the keto diet (which encourages eating high-fat, moderate protein and is very low-carb) is for your body to enter ketosis or a state where it burns fat for fuel rather than carbs, the normal source of energy. Expect to be in ketosis 24–48 hours after starting the diet, depending on your carb intake, says Scott Keatley, RDN, of Keatley Medical Nutrition Therapy in New York City.
Some people have the wrong impression about keto, assuming it’s a diet where you just avoid grains and sugar. In reality, you also have to forgo legumes, fruit, starchy vegetables and most dairy in order to hit those super low-carb targets. Considering it’s so easy to eclipse the 20–50 gram daily carb limit (for instance 1 cup of blueberries has more than 20 grams), you have to make sure you’re tracking your carbs very carefully. That means things like counting every baby carrot every step of the way.
For all the stories you may hear about people feeling better than ever on keto, you may not expect what happens to so many in the beginning: the keto flu. “The keto flu is less of an illness and more of a feeling of lethargy,” says Keatley. Your body prefers to source its energy from carbohydrates, he notes. When it makes the transition to being a fat-burner, “your body is unable to supply the energy you need as quickly,” he adds. Expect this not-great feeling to stick around for a week or less until you adjust. Keatley advises adjusting your diet gradually (decreasing carb intake while upping fat) to make the transition smoother.
Remember how we said your body loves to run on carbs? Keto may have a negative effect on your gym sessions if your muscles don’t get the fuel they need. “Some research has had mixed results when it comes to endurance and strength on the keto diet, so it’s hard to know for sure if your workout will be impaired or improved on keto,” says Abbey Sharp, a registered dietitian and author of “The Mindful Glow Cookbook.” “My experience tells me most people thrive exercising with some quality energy from carbs, so, until your body is a pro at running on fats, it will likely be a tricky transition,” she says. One study in the journal Metabolism concluded that after 12 weeks of keto adaptation, endurance athletes lost more weight and body fat compared to a high-carb group without impacting their performance. However, the study also found the athletes said their energy dropped for the first 7–10 days, and then their performance suffered for 4–6 weeks. It concludes that if you’re an athlete going keto, you should time it right and avoid starting the diet 4–6 weeks before an event.
Most Americans don’t get enough fiber to begin with — and that’s before cutting out fiber-rich foods like beans, whole grains, fruits and veggies, as is necessary on keto. During the diet, where the focus is on meat and fats, which don’t contain fiber, it’s easy to miss out on the digestion-friendly nutrient. “Cutting down on these sources of fiber can lead to difficulty ‘going,’” says Keatley. No matter your diet, your goal should be about 25 and 38 grams of fiber daily for women and men, respectively. If you do get stopped up with constipation, Sharp recommends increasing your consumption of high-fiber vegetables (like avocado, broccoli and greens), drinking more water and upping your activity level.
Or anyone else, for that matter …
While someone may say they’ve never felt better and can think clearly, you may find the exact opposite. “Keto fans often suggest they feel and think better on keto, while others tell me they couldn’t get anything accomplished in the day because they felt in a fog, even after months of trying the diet,” says Sharp. Check in with yourself often and adjust where necessary. Consider working with a registered dietitian, who can help you plan a diet to feel your best, before embarking on keto. It’s also worth mentioning that what you eat is supposed to make you feel great; if keto makes every day a slog to get through and you’re miserable, it may not be right for you. (And, hey, that’s OK.)
Keep a water bottle by your side and sip often throughout the day. On keto, kidneys excrete more electrolytes and water, so it’s easy to get dehydrated, says Sharp. For the same reason, make sure to salt your food to get the sodium you need.
When you hear the word diet, you assume you’re going to go around hungry. While keto is restrictive, it is made up of mostly fat. “Fat is very satiating, so many people are surprised at how full they are,” says Sharp. Many people find it tough to eat enough fat throughout the day, she adds, so it might take some work to get your daily intake perfect. Fair warning.
One of the selling points of keto is the ability to eat what you want — as long as it falls under your carb allotment. That means you may see dieters eating lots of bacon, cheese and butter. This might sound like the opposite of deprivation since low-fat diets reigned in the past. However, your relationship with keto may be different than someone else’s. “Like most things, how easy it is to follow a restrictive eating plan like keto depends on the person,” says Lisa Moskovitz, RD. “Some find it a breeze and the only thing that works for them when it comes to weight loss and blood sugar control. Others feel constantly deprived, tired and even hangry as they struggle to avoid all the foods they love that give them pleasure with eating,” she says. For instance, for some people, a burger doesn’t taste complete without a bun or they like to pair whole-grain crackers with cheese, she says. If this diet doesn’t satisfy you, it won’t work long-term.
This diet appeals mostly to carnivores, says Moskovitz. Vegetarians need to do a little extra planning and focus on carb-free, plant proteins like nuts, seeds, as well as other sources such as eggs and cheese.” If you’re vegan, it might be slightly harder, and you might need to take a B12 supplement. Moskovitz recommends consulting with a registered dietitian first to make sure you’re hitting all the nutrients your body needs to function at its best.
Ultimately, this is the reason people try keto: fast results. “Keto was originally created as a treatment option for children with seizure disorders,” says Moskovitz. It’s also being studied for help with Type 2 diabetes. That said, “it has gained a lot of recognition, as people who attempt it for weight loss often notice results within weeks or perhaps days.” Many experts caution against losing weight quickly, as it tends to come back just as fast (especially if you don’t stick with keto for the long-haul). However, research on very low-carb diets (a category keto falls into) doesn’t prove gradual weight loss is better, according to a study in Critical Reviews in Food Science and Nutrition. One perk to dropping pounds quickly is getting good results may help you better stick to a program. Still, if you see keto as a short-term solution, know that returning to your previous diet will likely cause weight regain and a harmful yo-yo diet cycle.
While keto may be a trendy diet, it requires lifestyle changes that aren’t necessarily for everyone. “If you’re feeling constantly deprived, fatigued, afraid to eat out or hyper-focused on food all the time, chances are this diet is not the one for you,” says Moskovitz. Remember, “you want to find and embrace a sustainable, effective and healthy plan that easily fits into your lifestyle,” she says. Do your research, experiment and see what works best for you. Don’t be afraid to pivot and reach out to an expert, like a dietitian, to find what suits you the best.
Originally published January 2019, updated with additional reporting
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