There is no "one size fits all" diet that will positively ensure success for everyone. However, the success or failure of any diet depends on how well a person/client adheres to it and this in turn, to some extent depends on the macronutrient composition of the diet.
Macronutrients are the protein, carbohydrates ("carbs"), and fats that make up the food we eat.
Protein - Aids and is necessary, for growth, repair and renewal of cells. Almost 50% of the protein in the body is in the form of muscle, and substantial amounts are in the skin, and the blood too. Protein also provides the body with energy.
Protein is made of of twenty (20) different amino acids, often referred to as "building blocks." Digestion of protein begins in the stomach, where it is broken down into amino acids, and is then absorbed into the bloodstream through the small intestine.
Those amino acids that are absorbed in the intestinal walls/cells can be used for energy, or to create new proteins: hormones and digestive enzymes as examples.
In addition, amino acids that are not used in the intestinal cells will pass on to the liver, to be processed and sent on to other parts of the body.
As a source of energy, proteins (amino acids) aren't usually used. In a typical diet sources of energy look like this -
Fatty Acids = 40-45%
Carbohydrates = 40-45%
Proteins = 10-15%
The minimum protein intake for a sedentary, generally healthy adult is 0.8 grams per kilogram of body-weight. This equates to approximately 56 grams a day for men, and 45 grams a day for women.
Some experts suggest higher intakes of protein, especially in trained individuals. 1 gram of protein per pound of body-weight is a common recommendation.
Some recent studies have shown that some ultra-endurance athletes (ex. Iron Man Participants) have benefitted from increased protein intakes (1.2 - 1.4g per kilogram of body weight), as did resistance exercise, strength athletes, and some of the bigger body-builders (1.2 - 1.7g/kg of body weight). However, whether ultra-endurance exercises, strength training, or body-building, when an individual trains more and protein use becomes more efficient, the amount of protein needed to maintain muscle mass may drop slightly.
What types of protein?
Animal sourced proteins such as meat, poultry, fish, eggs, milk, and cheese. Many plant foods contain all the amino acids too.
I would suggest that it is best to consume a variety of foods as sources of protein.
Additionally, some, especially athletes, may choose to use a protein supplement/powder.
While protein powders can be useful when used at the right time, excessive use or use on days when no exercise is performed just means extra calories, which, if not offset by activity, will result in fat gain.
Whey protein powder is the most popular base of protein, as it is the source that has the highest proportion of amino acids that have been found to facilitate the healing of damaged muscle tissue. Whey protein comes in three (3) forms: isolates, hydrolysates, and concentrates. Personally, I have also benefitted from the use of other protein powders, where the source of the protein was vegetable (peas).
When it comes to ingesting protein, I still prefer "clean eating." That is lean, whole, fresh foods.
Carbohydrates include starches and sugars, as well as the fiber found in fruits, vegetables, pulses (the edible seeds of plants in the legume family), and grains. As one of the three (3) macronutrients they are one of the three main ways in which the body obtains calories, which equals energy. The digestive system breaks carbohydrates down into one of three simple sugars: Glucose, Fructose, or Galactose.
Carbohydrates are sent to the liver after digestion through the small intestine and bloodstream as carbohydrates and monosaccharides. The liver keeps what it needs for energy transfer and glycogen storage, and sends the rest out as glucose.
Glucose is essential to life. It is the preferred fuel of the brain and central nervous system; they both benefit greatly from a continuously available supply.
In order to supply the brain with enough glucose, the dietary guidelines committee recommends a minimum intake of 130g. of carbohydrates daily. However, there are some variables to consider: other macronutrient intakes, specific goals, and activities, and body size. A highly active individual might benefit from a higher carb intake, while someone with a goal of decreasing body-fat may consume fewer carbs to encourage rapid fat loss.
Minimally processed carbohydrates, found in whole foods, is what I would suggest to a client. These types of carbs have a higher concentration of micronutrients (vitamins and minerals), they have more fiber, and typically a higher thermic effect than refined carbs (white bread, white pasta, cookies, processed cereals, etc.). Refined carbs typically contain added sugars and have a lower fiber content. Temporary elevations in blood sugar occur, as does LDL ("bad") cholesterol, inflammation, and insulin resistance. My advice would be to resist, or at least reduce, the consumption of refined carbs, and stick with minimally processed sources of carbohydrates. Stick with whole food sources of carbs!
Carbohydrates provide another benefit - FIBER!
Fiber comes in two different forms: soluble, as found in oats, beans, peas, nuts, fruits (bananas, oranges, apples, etc.) and vegetables (tomatoes, carrots, etc.). Then there are insoluble fibers, as found in root vegetable skins, dark green leafy vegetables, fruits, seeds, and nuts.
High fiber content has been shown to be beneficial for gut health and to help reduce the risk of heart disease, strokes, type II diabetes, and colorectal cancer.
As good sources of fiber, I once again recommend whole plant foods. Carbohydrates are an important part of our diets --- overall, about 50% of our energy should come from carbs. However, as with the other macronutrients, we need to think about the number of carbohydrates in our diet.
Fats - The nature of fat depends on the types of fatty acids it is made up of. All dietary sources of fat contain both Saturated and Unsaturated fatty acids, but they are often described as either "saturated fat" or "unsaturated fat" based on the proportions of fatty acids present.
Saturated Fat comes mostly from animal sources of food, such as butter, cheese, fatty red meats, processed meats, and full-fat dairy products. These types of sources of fats tend to raise LDL "bad" cholesterol levels and, high intakes have been linked to an increase in risk of heart disease, stroke, and even type II diabetes.
The potentially "healthy" fats are the unsaturated ones. These include monosaturated and polyunsaturated fats, both of which have been shown to reduce blood cholesterol levels when used to replace saturated fats in the diet.
Monosaturated Fats are found in high amounts in oils, like olive and peanut. This type of fat is also found in avocados, nuts, and seeds, such as pumpkin and sesame seeds.
Polyunsaturated Fats are found mostly in plant-based foods, and oils, such as sunflower oil, sesame oil, soy oil, and spreads made from these, as well as in flaxseeds, pine nuts, sesame seeds, sunflower seeds, and walnuts. Omega-3 Fatty Acids are a type of polyunsaturated fatty acid and have been shown to reduce the risk of heart disease.
The three important Omega-3's are Alpha-Linolenic Acid (ALA), Docosahexaenoic Acid (DHA), and Eicosapentaenoic Acid (EPA). ALA is found in inland plant sources: chia, flax, hemp & walnuts for example, while EPA and DHA come from marine sources: fish oils, oily fish (sardines, salmon, mackerel), and algae. This is why dietary guidelines recommend regular consumption of oily fish.
Fat is the most concentrated form of dietary energy (providing 9.44 kcal/g. compared to 4.18 kcal/g. for carbs, and 5.65 kcals/g. for protein). Foods that are high fat typically have high calories. That is why low-fat diets are promoted to consumers as much for weight-loss as for cholesterol control.
Almost all natural foods contain some amount of fat, as both plants and animals use fat as the most economical way to store energy. Fat is needed for growth, proper nerve function, brain function, healthy skin, transporting the fat-soluble vitamins (A, D, E & K) through the bloodstream to where they are needed, and to form hormones. Some essential functions are performed by dietary fats that we cannot produce in our bodies (we cannot form the Omega-3 fatty acid ALA or the Omega-6 fatty acid Linoleic Acid, so we actually need some fat in our diet to survive. So, too much fat can be detrimental to our health, but a certain amount is required to maintain good health.
Knowledge, where macronutrients are concerned is an ideal place to start because they influence things like - the ability to do work, perceived energy levels, recovery from exercise, chronic disease progression, body composition, appetite and satiety, and more. The personal trainer with this knowledge is more prepared to develop a program balanced for optimal results and encourages good eating habits through a well-balanced diet. The success or failure of any diet will depend on the adherence to that diet, including the foods that the personal trainer recommends, palatability, convenience, satiety (feeling of fulness), and macronutrient content.