What percentage of fat, carbs, and protein make up a healthy diet?

I would recommend that 30% of your calories should come from fat, 50% from carbs, and 20% from protein. That percentage represents the percentage of calories. Remember that fats provide 9 calorie/gram versus protein and carbs which provide 4 calories per gram. Choose fats that are not saturated, trans-fasts, or hydrogenated oils. Get your protein from lean meats and legumes. Try to eat at least 50% of your carbs from whole-grain sources. Avoid sugary drinks and sweets.

I am a big believer in adjusting peoples nutrition first based on their body type then base it on activity levels.

If someone is insulin sensitive yet is doing a ton of cardio they still need ample amounts of carbohydrates.  Insulin sensitivity usually means a diet that has more dietary fat will help but if that person is very active and doing tons of cardio then their need for carbohydrates as a fuel source will be much much higher.

The percentages will vary also but below are some general thoughts.

Lets break down the body types a bit.

Endomorph - More pair shaped and carry a majority of fat around their stomach and through their midline.  They should  not eliminate carbohydrates but should reduce them slightly, keep moderate levels of protein and increase healthy fats.   

Mesomorph - The natural muscular person that can usually gain muscle and lose fat easily.  The type of bodytype that almost everything works for.  This bodytype will be able to usually handle good amounts of calories, proteins, carbs and fats.  Moderate amounts of protein and healthy fats are recommended and more carbs than an endomorph.

Ectomorph - The naturally skinny, long and lean with long limbs is the description of the body type here.  This body type generally has a hard time adding weight and can handle a very large percentage of carbs.  

There is also blends of body types.  You have meso-endo morphs as an example that add muscle easily but also add fat.  You also have ecto-meso morphs which add muscle but are longer and leaner and have a hard time adding weight.  You also have just full blown people that just fall in the categories listed.

Activity will determine the most.  Sedentary people that do not workout do not need as much protein as someone who heavily weight trains.  People in the gym who just lift weights and do not do large amount of cardio will not need as much carbs.  

As with everything there are baselines to follow but needs to be adjusted based on the individual and a number of factors. 

That’s a great question, and there are lots of opinions out there! So let’s assume that we aren’t talking about an ultra endurance athlete, or a body builder. For the average person a good guideline would be 40/30/30. I like this guideline because it’s easy to remember and gives you a little wiggle room up or down.

40% carbs. 
High fiber, fresh foods are the goal here. As much as possible, avoid empty carbs….cookies, chips etc. Things that really have no nutrient value.

30% protein.
A serving size of protein (as big of a deck of cards) at each meal should be good.

30% fat. 
This often surprises people, but it really isn’t a large quantity. It’s just that fats are denser in calories, and of course, some of these fat calories will be in the carbs are proteins. Avoid trans fats and stick to your better fats found in things like nuts, olive oil and avocadoes.