The title is something of a misnomer. There is no such thing as 'the' proper diet for every individual. Nevertheless, all humans are similar enough that there are broad categories, and many specifics, that are correct for almost anyone.
Despite all the fads of the last 30 years or more, it remains true - backed by a large amount and variety of nutritional research - that a good diet is the old-fashioned 'balanced diet' that has remained largely unchanged for 60 years or more. The keyword deserves repeating: balanced.
There are fad diets that emphasize protein over carbohydrates, or fruits one day with meat the next or eating vegetarian exclusively. All these may have valid elements, but they almost all tend to go too far in one direction or another.
Everyday, at regular intervals, a person interested in optimizing health should eat daily meals consisting of fruits and vegetables, grains, dairy and a protein source. Of course, there will be exceptions for those with special dietary needs. Some people, for example, can't process dairy products. Others are sensitive to peanuts or other things.
But the four traditional food groups, in the proper proportion, remain the undisputed recommendation of every reputable nutrition scientist. The reasons are that studies continue to support the notion that these supply the compounds needed by the body. From those it can perform muscle maintenance, proper electrolyte balance, cellular repair and other essential activities along with the needed energy to carry out all of them.
Nature, as discovered by science, determines what the body needs - not marketing.
Insoluble fiber, for example, (as gained from fruit, vegetables and grains) isn't readily digested. As a result it helps digestion and in cleansing the digestive system.
Certain vitamins (D, B, E, K) and minerals (lithium, calcium, postassium) are needed for carrying out the thousands of biochemical reactions critical to proper health. Sodium and potassium, in moderation, are used by the heart muscle in order to keep pumping blood through the body.
