Last week, a friend asked me what diet he should follow for his goal to reach a healthy body weight. Developing a good answer to his question ended up being a great exercise in consolidating my health and fitness knowledge into something sustainable and easy to remember, so I’ve decided to share it here as well.

The diet needed to fit three criteria: it needed to be easy to sustain for the rest of your life, it needed to be simple to remember, and it needed to take into account how you eat not just what you eat.

Most people who “go on diets” only lose weight temporarily, and in many cases, end up gaining back more weight than they initially lost. Massive lifestyle alterations can work well temporarily, but they’re typically abandoned later and don’t lead to a lasting change in health.

It’s no good to lose weight for a few months just to gain it back. You have to figure out how to change your lifestyle in such a way that you maintain a lower weight permanently. Any highly rigid diet, complicated diet, or diet that requires you continue buying products from the creator is not going to work in the long term.

And while it needs to be sustainable, it also needs to be simple. There are plenty of diets that are amazing, but that are impossible to remember and leave you confused about what to do at a restaurant or in the grocery store. While they work well for a while, the cognitive load grows to be too much and you give up on these as well.

Finally, for a diet to be effective, we have to consider what we eat as well as how we eat, the latter of which is unfortunately usually left out of the equation. Frequency and pace in eating make a big difference but are typically ignored.

Here’s what I came up with to recommend to him, and to start following myself. I call it the “A Priori Diet” since it makes sense on its own if you can unlearn all food-marketing that’s happened in the last 75 years. If I told you to sit in a room and to try to think of what a human should probably do for their diet, armed only with the knowledge of what a human is, what the natural world is like, and without any influence of modern food marketing or fad diets, you would get pretty close to this. Most nutrition research is simply telling us that everything we’ve added in recent history is terrible for us.

The diet only has five rules. I’ll list them here, then explain the logic behind each one:

Rule 1: Consume like it’s 2,000 years ago

The simplest check for if a food might be bad for you is if it was introduced in recent history. Our bodies did not evolve to consume Oreos, potato chips, or hamburger buns, they evolved to eat what we could find in the wild and cook relatively easy.

And I do mean “find and cook,” so while you could argue that “well, I could find some sugar and make some flour and make cookies…” you know you’re lying to yourself. Just look at any food and ask yourself “could I go out into the wild, or a small village, find this, and cook it?” If it doesn’t fit that rule, don’t eat it.

Now, the additional sub-rule here is to only consume what you could find and cook 2,000 years ago, in as close to the way you would have found it as possible.

You could certainly have found cows and pigs and killed them, but would they be corn-fed and pumped full of antibiotics? No, certainly not, so you need to get as close to naturally raised foods as possible.

This is where bread, fruit, and sweets become problematic. Commercial wheat is nothing like it was 2,000 years ago, nor are most fruits. If you can find pre-modernity grain or fruit go for it, but nothing in a U.S. grocery store is likely to qualify.

To find “near natural” meat and produce, an easy heuristic is how big is the company I’m buying this from? A massive company like Perdue has reached the economies of scale where it makes sense to destroy the quality of their chicken to save money, but a local farmer hasn’t. The smaller the producer you’re buying from, the less likely they’ve added a ton of nastiness and the more likely you’re getting close to “state of nature” food.

Finally, you also only want to only drink things that you could have gotten 2,000 years ago. That leaves us with water, tea, coffee, wine, and beer, though the wine and beer need to meet the same “historic quality” rules as foods. Most non-cheap wine will get close to that standard, but it’s rare for beer outside of places like Germany and the Czech Republic.

Rule 2: Eat till you’re 80% full

This rule comes from the Blue Zones research and is an extremely simple way of managing the amount you consume without counting calories.

As the researchers were studying populations that lived to 100+, they noticed that many of them (particularly in Okinawa, Japan) ate slowly enough that they could stop eating when they felt they were nearly full.

The nearly full is important: it takes about 20 minutes for you to feel full from what you’re eating, so if you tend to scarf down 1,200 calories of Chipotle in 5 minutes like me, you’ll completely miss the “hey that’s enough Barbacoa, amigo” signal your brain would otherwise send.

Since counting calories is simply a way to measure the quantity of what you’re eating, this “eat till you’re 80% full” rule achieves the same goal while being much less demanding. Counting calories is a massive pain and you’ll quit doing it, so simply saying “I’ll stop eating when I’m 80% full” makes quantity management much easier.

Finally, if you’re having a hard time telling when you’re 80% full, eat slower.

Rule 3: Fast 16 hours a day, or 1 day per week

Continuing along the theme of “eating as if you didn’t have all these modern options,” you’ll do well to fast regularly.

How you fast is up to you. You could do daily intermittent fasting where you consume all of your food within 8 hours then fast for 16 hours. I’ve been doing this for 6 years now and love it, usually only eating between 1pm and 9pm. But you could also do a 24 hour fast once a week, say on Sunday.

If you want to get the full benefits of fasting, try adding in 3 or 5 day fasts every couple months. These are particularly good for cleaning out your system, and they’re really not all that painful. You might find you enjoy them.

Why does fasting make sense? First, pre-modernity we would not have had access to food 24/7. We would have regularly gone for hours or days without anything to eat, especially men while out hunting. Your body is designed to be completely functional for periods on no food, but the vast majority of us have lost this ability as the result of eating “3 square meals a day.”

The idea of eating regularly is mostly a marketing gimmick, primarily by breakfast cereal companies. Your body does not store energy from food then use it, it uses food to replenish expended energy. You don’t eat then go hunting, you hunt and then eat your prey.

And if your reaction is “oh but if I don’t eat for a few hours I get hangry,” that’s not a reason you can’t do it, that’s a sign that you’re addicted to eating regularly. If someone said “I need heroin every few hours or I start losing my mind” I don’t think we’d say “Ah, I guess they just naturally need heroin. Nothing to be done about it!”

Trust me, after a few weeks of fasting to some degree, you’ll wish you had started sooner.

I should note that some women’s cycles can get messed up by intermittent fasting. I’m not very steeped in the details since I’m a guy, so if you experience any issues with that I’d suggest shortening your fasting window or looking into how else you can mitigate the side effects. But women should have had to do some fasting too… so I’m inclined to believe it’s something you can adapt to as well. 

Rule 4: Prioritize veggies, fats, and meats

While you could, technically, get 100% of your calories from wild rice and not be breaking any of the rules, it’d be hard to argue that you’re “being healthy.”

Vegetables, fats, and meats would have been the preferred sources of calories to wild grains and starches since they’re easier to get and much more calorically dense relative to your efforts. Spending a day collecting rice would result in very little food, spending a day collecting nuts would yield much more. And while fruits could make it in here, they’ve been altered so heavily to be made sweeter that you can’t really get “old fruits” anymore.

When designing or choosing a meal, opt for some combination of vegetables, fats, and meat. Fat here means nuts, good oils, avocados, eggs, butter, and unless you’re East Asian, some milk and cheese. Always opt for “full fat.” And when possible, vary your veggies, fats, and meats so you’re not having the exact same things all the time.

Rule 5: Cheat on two meals, or one day, per week

Depending on how big of a change the last four rules are to your current diet, you may feel completely overwhelmed by the thought of making this big of an adjustment. Or, you may recognize that while you can stick to this 90% of the time, you’ll end up cheating occasionally, which is totally natural.

Is it better if you don’t cheat? Yes, but since it’s bound to happen eventually, it’s better to plan for it, an idea that comes from basic psychology but also the cyclical ketogenic diet and Tim Ferriss’s “Slow-Carb Diet.” Not only does planning for your cheats help mitigate the psychological drain of thinking you have to be always on, it allows you to occasionally shock your system so you don’t become completely unable to tolerate sugar.

Even if you think “I’m tough and don’t need a cheat day!” pick one day or two meals each week (the same ones each week) where you can do whatever you want. If you get to it and don’t want to go crazy, you don’t have to, but plan for it just to give yourself the comfort of knowing the restrictions don’t last till you die. When cheating, eat everything you’ve been missing without worrying about quantity or quality but be sure to keep it constrained to that meal or day. This will help a ton with getting over the terrifying thought that you can never eat Oreos again.

Aaand go. Put it on a notecard (or just memorize it, it’s not that hard) and repeat until you die. Unlike the latest fad diet, this has been tested by millions of years of biology and is unlikely to change in the future.