Upvoted this question, it is excellent and I'm honoured to be answering it. I read all other answers first as you should if you're serious about q&as.

Okay qualification first. I was given two years to live 15 years ago because of my consistent 220/120 blood pressure. I've tried all the types of blood pressure reducing medication but with no effect. I don't smoke and gave up drinking and smoking in 1994 but started again last year as I would rather live my final years however many they may be doing what makes me happy. But I think my rules of thumb is what have kept me alive when the doctors wrote me off (And are still doing).

I have many rules of thumb. The milk marketing board, the many food marketing boards like meat and veg are all very powerful and convince us we need 5,7,9,12 (I'm different countries)fruit and veg... and meat and milk and eggs and chocolate and breakfast cereal a day and we buy it.

Don't buy it.

Breakfast is the most profitable meal of the day so skip it if you want a healthy bank balance.

All dairy products produce a mini high the same as heroin.

Meat can change your mind and make you risk your life while simultaneously shortening your life anyway.

Everything edible has a built in reflex defence attack mechanism that stops you from living solely on it, everything evolves that way for survival. Eat only rabbits and you die of vitamin deficiency, eat only anything and you will die of something it evolved to kill you for eating it. So to rule number one.

1. Don't eat much and vary it. As my mum says 'everything in moderation'.

2. Less processed the better if they remove something or add something then we haven't evolved to expect that.

3. Don't eat breakfast. This is mine as I do intermittent fasting and eat and drink alcohol up to going to bed so my fasting window starts as I go to sleep so must continue as I wake up most mornings. (Some days require a fuck it Eat and cup of tea, not only because I have an autistic daughter and my wife is leaving me)

4. Don't eat fruit. Fruit contains fructose, one of the worst sugars for your body and most addictive. It's defence mechanism.

5. Give in to what you crave a little now and then. Your body is amazing and has evolved to know what is in each of the foods you have ever tasted and what you are running low on. Also life Isn't just about health and efficiency.

6. Try and eat a rainbow as the vitamins, minerals and micro/macro-biome/nutrients that your body needs to be healthy is all colours.

7.etc Future edits to be filled in when I'm not so on my drink.

Nikki's answer is really good; Michael Pollan said this in a book and the MOOC Stanford Introduction to Health & Nutrition.

Rules of thumb

Enough calories gets enough protein, enough fiber so ya poop, enough fat to absorb vitamins A, D, E and K and ... follow the national dietary recommendations:

• 2 servings of protein (e.g., one cup of beans)
• 3 pieces of regular fruit, where two small fruits equal one "regular" (e.g., two kiwis)
• 4: iodine,¹ B₁₂ fortified plant milk³ and fat⁴
• 5 servings of vegetables
  • How to eat more vegetables? Try this goal: new recipe that uses a different kind of leafy green vegetable this week (spinach, broccoli, Romaine lettuce, bok choy, Swiss chard, collards or kale).

• 3 servings of fruit (easy) or 2 fruit + 8 dried apricots
• 5 servings of vegetables--dark leafy greens 2 times/week
• 1/2 cup mixed nuts/day ≈ 55% fat
• 1 cup of protein

---

Protein - One cup cooked beans = two servings - legumes (peanuts, chickpeas, lentils, peas)

Nuts - 1/4 cup almonds (1.15 oz) - 1/8 cup pecans (0.52 oz) - 1/8 cup walnuts (0.52 oz) = 2.19 servings

Plant milk - 1.75+1.75 = 3.5 serving of soy milk

Veggies - 1 serving/cup green leafy salad - 2 more servings for lunch - 2 servings for supper - two whole carrots - 1/3 medium avocado - cup broccoli - cup potato - cup tomato - 8 Brussels sprouts - 1/2 cup cooked or cup raw mushrooms

Fruit - Select two: apple, banana or two kiwis + dried mangos/apricot (with the nuts)

What to avoid

• Processed foods, but certain types are beneficial like extra virgin olive oil, soy milk and other plant milks (to name a few)
  • Non-ionized salt (or little salt if you use iodine drops or have a vegan vitamin with iodine)
  • Added sugars (especially in drinks), while it doesn't matter with natural sugar in fruit and pure fruit juices

Bonus

reddit.com/r/nutrition/comments/gl724u/eli5_how_would_you_summarize_the_purpose_of

CalculatedVegan.wordpress.com

1. calculatedvegan.wordpress.com/iodine
2. calculatedvegan.wordpress.com/b12
3. → https://i.imgur.com/AwnFbdN.png
4. Fat soluble vitamins are A, D, E and K.

A common rule of thumb I've always heard is by Michael Pollan, a food author:

Eat food, not too much, mostly plants.

Specifically:

1. Don't eat anything your great grandmother wouldn't recognize as food. "When you pick up that box of portable yogurt tubes, or eat something with 15 ingredients you can't pronounce, ask yourself, "What are those things doing there?" Pollan says.
2. Don’t eat anything with more than five ingredients, or ingredients you can't pronounce.
3. Stay out of the middle of the supermarket; shop on the perimeter of the store. Real food tends to be on the outer edge of the store near the loading docks, where it can be replaced with fresh foods when it goes bad.
4. Don't eat anything that won't eventually rot. "There are exceptions -- honey -- but as a rule, things like Twinkies that never go bad aren't food," Pollan says.
5. It is not just what you eat but how you eat. "Always leave the table a little hungry," Pollan says. "Many cultures have rules that you stop eating before you are full. In Japan, they say eat until you are four-fifths full. Islamic culture has a similar rule, and in German culture they say, 'Tie off the sack before it's full.'"
6. Families traditionally ate together, around a table and not a TV, at regular meal times. It's a good tradition. Enjoy meals with the people you love. "Remember when eating between meals felt wrong?" Pollan asks.
7. Don't buy food where you buy your gasoline. In the U.S., 20% of food is eaten in the car.

Source: WedMD

Nature is revealing the best practice to eat correctly. Here are my observations:

1. Women's body starting to cleanse right before pregnancy to create the ideal state for fetal development.
2. When the body is clean the sensory organs can provide enough information to be able to decide what can be eaten and what to avoid.
3. Eating right is starting by preparing the time and space for it. Drink clean water before, light a candle, bring calm and peace to the table and give time to others, then bless the food and anyone who contributed.
4. Stay present, don't talk and do not let anyone or anything to distract your attention from the process of eating.
5. Don't drink too much after, and don't do any drugs (sugar, caffeine, alcohol and nikotin included) to let the digestive system do its job.
6. Avoid produced food and artificial compounds, eat local and make yourself or leave it to your loved ones.
7. Stay conscious about the effect of food on your body (energy level, gastrointestinal disorders) and pay attention to the final product (stool and urine)
8. Compare your observations to the recommendations by the Blood Type Diet and based on the Ayurveda Dosha and do your own research by trying out different eating habits until you find what really suits you.

These are my observations in nature and in archaic sacred societies, where people are connected with nature and live healthy.

Egészséget kívánok!

Except for vitamin B12 and vitamin D, you're going to get everything you need provided you get most of your calories from a wide variety of fruits, vegetables, and grains. What is not good is to use a supplement to correct for a deficiency (except for vitamin D and vitamin B12) and then think that you've plugged all holes in your diet. The essential vitamins, minerals, amino acids,etc. are just the tip of the iceberg; for optimal health a lot more compounds are needed than can be found on any list of RDAs of vitamins, minerals, amino acids and essential fats.

We need to consider here that science hasn't yet identified all the compounds our bodies need. E.g. celiac patients who's intestines are so damaged that they need to get supplements via IV, only thrive when given whole food extracts. If they only get a mix of the known nutrients they don't do as well. The difference is then due to the likely large number of compounds in whole foods that has yet to be identified as necessary for the human body.

Suppose that 200 years from now scientists will have identified every last compound you need to eat with their RDAs. If there are 1000 items on that list and today we only have an incomplete list of, say, 25 items with their RDAs, then it's obviously impossible to check if a diet is adequate today. However, there is a statistical trick you can use to guess whether a diet is likely to be deficient according to the unknown list of 1000 compounds.

This works by checking if the 25 compounds in a diet are coming from a wide variety of food sources, here you pay attention to how close different plants are related to each other and also the similarities in the entire profile of the 25 compounds. So, two food sources that are rich in calcium should be considered to be more different if they come from different plants that have different profiles for the other compounds.

Suppose that your diet is not so optimal according to the above criterion, a few of the 25 compounds only come from 3 reasonably independent sources. Then it's a forgone conclusion that your diet will lack many of the 1000 compounds of the unknown list. If you put the compound that comes from the largest number of independent sources on top and below that the compound that comes from the next larger number and so on, then item number 25 of the old list will appear somewhere at the bottom of the new list, but it's likely not going to be the last item of the new list. So, it's quite likely that you are missing quite a few nutrients that are necessary for optimal health.

An effective way to boost the quantity of nutrient intake is to eliminate all sources of empty calories like refined sugars and fats and to get all your calories and essential fats from whole food sources. So, no cooking oil should be used, one should instead eat nuts and seeds. Also, by exercising a lot one can increase the calorie requirement thereby boosting the nutrient intake.

I ran my personal best of 2:49 at the Boston Marathon after one year of training under a pretty strict vegan diet. (I'm still vegan but running less these days). My diet wasn't complicated at all, I had three rules of thumb:

• A smoothie consisting of various types of fruit combined with soy milk, a spoonful of 'good' fats (mostly flax) and some extra hemp protein. This in the morning and/or after training. I still eat my fruit and use some flax seed oil, but as I'm not training hard, no extra protein.
• Main courses followed different combinations of 'a grain, a green, and a bean'. Without much hassle, this allowed me to combine different proteins, carbs, fibers and micronutrients. I often ate my greens raw, and I tried not to eat the same combinations too often in a short time.
• I took my B12 more or less each day and had my blood tested for deficiencies.

This is just what worked for me.YMMV. I did some research at the time, but I can't be bothered right now to digg up any references. What was important for me was to keep it simple and to rely only on the absolutely necessary substitutes (B12). There's no point in a sophisticated and scientifically approved diet that you won't realistically implement.

I am not sure if the French can claim to have started the 5 a day rule - it seems to be fairly universal. As long as you eat a balanced diet including foods that contain protein (beans are an excellent choice here), carbs are also needed, then you should be alright. Eating your five a day is a great way to get vitamins, but what about that all-elusive vitamin B12? You state that you are vegetarian/vegan, so eggs, are they off the menu or not, if not then there is a good source of vitamin B12, and the same goes for cheese. Here is a great table for reference. Here is what the vegan society says about B12. And a bit more info about finding a vegan B12 here.

What about the vitamin D? Well, sunlight is a great start (avoid burning obviously), but if you don't get enough then try this great chart (too much to print here).