Was just curious on what would happen if you were to only eat pure fat sources for maybe a 48hr period. Has anyone ever tried? Are there any known risks/benefits to a pure fat diet?

I've eaten 90% fat (by calories) for months at a time. Occasionally for brief periods (a few days) I went to 100%.

I did this to treat migraines. I wouldn't recommend this unless you have some health problem you want to treat with extreme ketosis. It's not a healthy diet.

A 100% fat diet would not be possible for long periods (months) because it lacks essential nutrients including amino acids. But a 90% diet can be sustained indefinitely because it contains enough protein to supply the body's rock-bottom needs.

That's 90% of calories. By weight, that comes out to about four parts of fat to one part of protein and carbs. This sort of diet was invented in the 1920s by doctors at the Mayo Clinic as a treatment for epilepsy. Nowadays it's often called "the Hopkins diet" because after it fell into disuse for several decades, doctors at Johns Hopkins hospital made it a popular treatment again.

You asked about risks and benefits. The main benefit of 90% fat diets is that they are the only way to achieve extremely high levels of ketosis for unlimited periods of time. Therefore they can be useful for treating some kinds of neurological disease and possibly some kinds of cancer. In many cases, though, less-extreme levels of ketosis (achieved, for example, by substituting a few tablespoons of MCT oil per day into an otherwise normal diet) might be equally effective, healthier, and less unpleasant.

As for risks, this type of diet is often prescribed for children with epilepsy, and for them, the most serious known risk is stunted growth. The second most serious known risk is kidney stones. When doctors put a patient on this kind of diet they often prescribe large doses of potassium citrate as a prophylactic against kidney stones.

Another concern is the omega 3-to-6 ratio. Because you're eating enormous quantities of fat, over a long period of time (months), the ratio may matter more than usual. For this reason, I mainly eat fat from grassfed cows or buffalo. I think it's the safest fat.

Another reason why I eat mostly cattle fat is that it's the only fat I can eat in such large quantities without feeling slightly sick. Coconut oil especially makes me feel sick in large amounts.

In the beginning, it can be hard to force yourself to eat enough fat. Two tips: Heavy cream is the most palatable fatty food (at least for me), and butter contains a natural emulsifier so it's easy to mix with other foods.

If you eat 100% fat for just 48 hours I don't think you need to worry about health effects. The worst that could happen is constipation or diarrhea and, if you're not already ketoadapted, symptoms of carb withdrawal. Several people here made jokes about diarrhea but in my experience, constipation is much more likely than diarrhea, at least with animal fat. On a diet of pure animal fat the stool becomes whitish and small.

If you continue to eat extremely high levels of fat for longer than 48 hours, your body will start to behave as if it's fasting and you will experience symptoms of fasting. These can include hairy tongue, orthostatic hypotension, lethargy, fatigue, decreased appetite, muscle wasting, etc.

After a while you would probably reach extremely high levels of ketosis. On a 90% fat diet, my plasma level of beta-hydroxybutyrate (one of the three ketones) rises above 6 mmol/L. That's the average over a week's time. Peak levels go as high as 7.7. These numbers are incredibly high. They are comparable to numbers reached by people who go several weeks without food of any kind.

New stuff in reply to comment below:

In response to Invisible Ink's question below, I'm adding some info about recipes. Most people on these diets are kids, and kids are fussy about food, so their parents go to enormous lengths to make meals as similar to convention meals as possible. Most of these people aren't paleo, so they end up making fake versions of fake foods. Here's a link with recipes:

Conventional recipes for a 90% fat diet

When you look at recipes for kids, keep in mind that kids usually consume more calories per unit of body weight than adults, so there's more room for carbs in a kid's diet than a typical adult's diet. I'm pretty sedentary and in my diet, there is almost no room for carbs.

My meals are much simpler. When I was following this diet rigorously, I would usually melt 45 g of grassfed tallow in a sautee pan, add 52 g of beef and maybe some bone broth, cook it for a short time, and eat it sort of like soup. This meal contains 500 calories. Sometimes I'd add spices. Here's a photo to show the macronutrient ratio. The picture is misleading because I kept the fat clean to make the picture clear. In real life I'd mix bone broth and juice from the meat into the fat to make it more like soup:

When I'm in the mood for something more normal, I might melt a very large amount of grassfed butter in a pan, scramble a couple of eggs in it (mixing them with the butter), and add ground cayenne pepper.

On this kind of diet I frequently crave fibrous vegetables, and I eat as much salad as I can. Unfortunately, there's little room for carbs in this diet, and even celery and lettuce have to be rationed.

The picture probably looks pretty disgusting, but I've gotten used to eating fat and it tastes okay to me.

In fact -- this will probably sound strange -- sometimes I simply mix a few tablespoons of room-temperature tallow with cumin and sesame seeds and eat it like mashed potatoes. (My tallow is usually soft and almost fluffy at room temperature -- see rendering instructions below.)

As you can see, I'm not much of a cook, but I do know one useful thing. I've learned to render beef fat so it tastes as good as possibe. The trick is to use the lowest possible temperature. I've tried many ways of rendering and here's what tastes the best. Chop up the fat into very small pieces and put in a large pot of hot water that isn't boiling. (This doesn't work with suet, only with mushy body fat. Suet requires boiling and many hours.) The water should be very warm but not close to boiling. I've never used a thermometer but the water is at a temperature where I can jab my finger in and out of the water without pain. That's hot enough to render the fat. (Don't try the finger trick after the fat has begun to melt!) Don't cook the fat too long -- a couple of hours is enough. (Hard fat requires a longer time.) When most of the fat has come out of the tissue and floated to the surface, use a strainer to remove the solid material. (Save the solid material and use it, for example, in chili.) Then put the entire pot in the refrigerator overnight. The next day, you will have a solid disk of pure yellow fat on top of the water. Remove it and cut the disk into pieces. You can put them in plastic bags in the freezer and use the pieces as is. Optionally, you can purify the fat further by melting it at low temperature in a clear Pyrex container and then pouring it through a paper towel in a strainer into bottles. Pour slowly and watch what's inside the Pyrex container. When most of the melted fat has been poured, you'll see some water in the container that is about to spill out. Stop at that point. The reason I recommend a clear Pyrex container is so you can see the water. If you take this second step, the fat will contain no visible moisture and can be stored for very long lengths of time.

Reference:

The largest source of scientific information about ultra-high-fat diets is the medical literature about ketogenic diets for epileptic children. These diets have been used and studied for nearly one hundred years. In these papers, 90% fat diets are usually called "the Johns Hopkins diet" or "the classic ketogenic diet" or sometimes just "the ketogenic diet." Here's a good recent review paper about them:

The Ketogenic Diet: From molecular mechanisms to clinical effects