A ketogenic diet, also known as keto, very low carb, or LCHF (low carb, high fat), is where you force your body to produce ketones in the liver to be used as energy. On a high carbohydrate diet, your body produces glucose and insulin. Glucose is the easiest molecule for your body to convert and use as energy, so if available, your body will choose it over any other energy source. Insulin is produced to process the glucose in your blood by taking it around your body. Since the glucose is used by your body as it's primary energy source, your fat is not needed and stored instead.
By using lowering your intake of carbs (preferably to under 20 grams), the body is put in ketosis. Ketosis is a natural process where the body produces ketones from the breakdown of fats in the liver. The end goal of a ketogenic diet is to force your body into the metabolic state known as ketosis. You don't have to starve to get here, but you do have to be VERY low carb. But if you take away the carbs and increase your fat, your body will burn ketones as the main energy source.
I guess the next question I always get is... What do I eat?
Ideally, your macronutrient ratios should be around 5% carbs, 20-25% protein, and 70-75% dietary fat. This is really what works for you. Some are successful with 80-85% fat. Some eat up to 10% carbs. Some advocate for higher protein. But the graphic below, in my opinion, is the sweet spot.
Your carbs should mostly come from vegetables, nuts, and dairy. DO NOT eat wheat (breads, pasta, cereals), starch (potatoes, beans, legumes) or fruit (berries are ok on occasion). Dark and leafy greens are best choice. Fatty cuts of meat are great (leave the skin on the chicken!). Put grass-fed butter on meat and veggies. In the beginning if you need a snack, eat cheese or almonds. But the longer you are keto, the less you need to snack. You're simply not hungry as often as you used to be. The high fat curbs your appetite and keeps you so satisfied.
Now, some get very caught up in macros and tracking. There are some sites that will provide calorie calculators (ruledme.com has a great one) and some people track religiously. I did at first. But with keto, you really don't HAVE to track calories or macros. Many people who have been super successful do not. But in the beginning, I think it's helpful until you have a grasp on WHAT to eat and how much. Overeating is overeating. You want to get to a point where you are eating when you are hungry, and stopping when you are full. Eventually, calories won't matter. Your metabolism will be burning strong despite low calories (more info on that another day).
Just eat lots of GOOD fat. Keep your protein moderate. And stay under 20 g of carbs. You'll be good.
There is endless diet dogma and misconceptions about being keto or very low carb. Don't listen to the negativity or bad information. Even from doctors. They are studies that were published 30+ years ago that show high amounts of fat and low carbs are good. It's high fat and high carbs that are bad for your body. And the low fat/high carb recommendations are just bad information that has led us into an obesity epidemic. Don't believe the hype. Educate yourself on this lifestyle and you might even teach your doctor a thing or two. If you have Type 1 diabetes or pre-existing kidney problems, DO get in touch with your doctor before starting keto because in these cases it may put a strain on your kidneys. But don't worry about high cholesterol or heart disease... keto (and any high fat, low carb diet) has been well known and documented to help improve these conditions.