Unlike a car, which just needs one type of fuel, the human body requires three energy sources from food: protein, fat and carbohydrate. Over the past few decades, conflicting and confusing recommendations have come from a wide variety of sources about how these three calorie sources should be balanced in the adult diet.

Question: Why has there been so much confusion about something so basic?

Answer: Part of the problem is the common assumption that “one size fits all.” Recommendations that work for the normal, healthy, average American don’t work for people who have health problems, are inactive or exercise at high levels, or are experiencing the challenges of the last decade or two of life.

Q: How do you figure out what proportions of protein, fat and carbohydrate make sense for you?

A: First, forget about a specific percentage of your calories coming from protein, fat and carbohydrate. Instead start with determining the amount of protein your body needs. Protein in the diet is much more than just part of the calorie supply. Even though the most obvious use of protein is in building muscle tissues, protein is also critical for the structure of healthy bones and for thousands of other functions.

Q: How are protein needs determined?

A: Body weight is the major determinant of protein recommendations. For the normal healthy average adult, the daily recommendation is set at 0.8 gram of protein for each kilogram of body weight (0.36 gram/pound). This protein consumption was enough to keep most individuals in protein balance (same amount consumed as lost each day). However, these studies used people who were relatively sedentary and were obtaining adequate amounts of calories and all essential nutrients. Many other things affect protein needs. For those with little or no animal protein foods in the diet, research indicates that daily protein needs increase by about 20 percent to 1.0 gram per kilogram body weight (0.45 gram/pound).

Colin Wills’ research studying a large national data set (the National Health and Nutrition Examination Survey) shows that most individuals who had both a normal body mass index and normal body fat consumed closer to double the protein recommendations.

During dieting for weight loss, other research indicates that consuming double or even triple the usual protein recommendation is needed to enhance fat loss and reduce loss of lean mass. A good deal of research also concludes that seniors require more protein per kilogram of body weight to reduce the common loss of lean mass with age.

Q: How much fat is needed in the diet?

A: In general, to absorb fat-soluble vitamins and meet essential fatty acid requirements, a minimum of 30 grams of fat a day is needed. Some research also has indicated that consuming 50 grams of fat in a single meal can affect blood vessels in a way that puts the heart under increased stress.

Q: How much carbohydrate is needed?

A: Under normal conditions the brain of the average adult uses at least 130 grams of carbohydrate per day, or about 520 calories’ worth. Additional carbohydrate is needed for physical activity, especially higher-intensity types of physical activity.

Basically, meeting calorie needs with a variety of reasonably wholesome food from across all the food groups is likely to provide the best balance of protein, carbs and fat without obsessing about it.

Alan Titchenal, Ph.D., C.N.S., and Joannie Dobbs, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa. Dobbs also works with University Health Services.