As you may recall, moderation refers to having the proper amount of a nutrient—having neither too little nor too much. A healthy diet incorporates all nutrients in moderation. Low protein intake has several health consequences, and a severe lack of protein in the diet eventually causes death. Severe protein deficiency is a rare occurrence in children and adults in Canada. The Acceptable Macronutrient Distribution Range (AMDR) for protein among adults is between 10 and 35 percent of kilocalories, which is a fairly wide range. The percent of protein in the diet associated with malnutrition and its health consequences is less than 10 percent, but deficiencies in calories and other micronutrients often accompany this. In this section, we will discuss the health consequences of protein intake that are either too low to support life’s processes or too high, thereby increasing the risk of chronic disease. In the last section of this chapter, we will discuss in more detail the personal choices you can make to optimize your health by consuming the right amount of high-quality protein.

Health Consequences of Protein Deficiency

Although severe protein deficiency is rare in high-income countries, it is a leading cause of death in children in many low-income countries. There are two main syndromes associated with protein deficiencies: Kwashiorkor and Marasmus. Kwashiorkor affects millions of children worldwide. When it was first described in 1935, more than 90 percent of children with Kwashiorkor died. Although the associated mortality is slightly lower now, most children still die after the initiation of treatment. The name “Kwashiorkor” comes from a language in Ghana and translates to “rejected one”, or “the sickness the baby gets when the new baby comes.” The syndrome was named “Kwashiorkor” because it occurred most commonly in children who had recently been weaned from the breast, usually because another child had just been born. Subsequently, the older child was fed watery porridge made from low-protein grains, which accounts for the low protein intake. Kwashiorkor is characterized by swelling (edema) of the feet and abdomen, poor skin health, growth retardation, low muscle mass, and liver malfunction. Recall that one of protein’s functional roles in the body is maintaining fluid balance. Diets extremely low in protein do not provide enough amino acids for the synthesis of albumin. One of the functions of albumin is to hold water in the blood vessels, so having lower concentrations of blood albumin results in water moving out of the blood vessels and into tissues, causing swelling. The primary symptoms of Kwashiorkor include not only swelling, but also diarrhea, fatigue, peeling skin, and irritability. Severe protein deficiency, in addition to other micronutrient deficiencies, such as folate (vitamin B9), iodine, iron, and vitamin C, all contribute to the many health manifestations of this syndrome.

Children and adults with marasmus neither have enough protein nor enough calories in their diets. Marasmus affects mostly children below the age of one in low-income countries. Body weights of children with Marasmus may be up to 80 percent less than that of a healthy child of the same age. Marasmus is a Greek word, meaning “starvation.” The syndrome affects more than fifty million children under the age of five worldwide. It is characterized by an extremely emaciated appearance, poor skin health, and growth retardation. The symptoms are acute fatigue, hunger, and diarrhea.

Kwashiorkor and marasmus often coexist as a combined syndrome termed marasmic kwashiorkor. Children with the combined syndrome have variable amounts of edema and the characterizations and symptoms of marasmus. Although organ system function is compromised by undernutrition, the ultimate cause of death is usually infection. Undernutrition is intricately linked with the suppression of the immune system at multiple levels, so undernourished children commonly die from severe diarrhea and/or pneumonia resulting from bacterial or viral infection. The United Nations Children’s Fund (UNICEF), the most prominent agency with the mission of changing the world to improve children’s lives, reports that undernutrition causes at least one-third of the deaths of young children. As of 2008, the prevalence of children under the age of five who were underweight was 26 percent. The percentage of underweight children has declined less than 5 percent in the last eighteen years despite the Millennium Development Goal of halving the proportion of people who suffer from hunger by the year 2015.

Health Consequences of Too Much Protein in the Diet

An explicit definition of a high-protein diet has not yet been developed but some suggest that diets high in protein are considered those that derive more than 30 percent of calories from protein. Many people follow high-protein diets because marketers tout protein’s ability to stimulate weight loss. It is true that following high-protein diets increases weight loss in some people. However, the number of individuals that remain on this diet is low, and many people who try the diet and stop, regain the weight they had lost. Additionally, there is a scientific hypothesis that there may be health consequences of following high-protein diets in the long term, but ongoing clinical trials are scheduled to examine this hypothesis further. As the high-protein diet trend arose, so did the intensely debated issue of whether there are any health consequences of eating too much protein. Observational studies conducted in the general population suggest diets high in animal protein, specifically those in which the primary protein source is red meat, are linked to a higher risk for kidney stones, kidney disease, liver malfunction, colorectal cancer, and osteoporosis. However, diets that include lots of red meat are also high in saturated fat and cholesterol and are sometimes linked to unhealthy lifestyles, so it is difficult to conclude whether the high protein content is the culprit.

High protein diets appear to only increase the progression of kidney disease and liver malfunction in people who already have kidney or liver malfunction, and not to cause these problems. However, the prevalence of kidney disorders is relatively high and underdiagnosed. In regard to colon cancer, an assessment of more than ten studies performed around the world, published in the June 2011 issue of PLoS, purports that a high intake of red meat and processed meat is associated with a significant increase in colon cancer risk.^[1] Although there are a few ideas, the exact mechanism of how proteins, specifically those in red and processed meats, cause colon cancer is unknown and requires further study.

High-protein diets can restrict other essential nutrients. High-protein diets are not recommended because they restrict healthful foods that provide essential nutrients and do not provide the variety of foods needed to adequately meet nutritional needs. Individuals who follow these diets are therefore at risk for compromised vitamin and mineral intake, as well as potential cardiac, renal, bone, and liver abnormalities.^[2]

As with any nutrient, protein must be eaten in proper amounts. Moderation and variety are key strategies for achieving a healthy diet and must be considered when optimizing protein intake. While the scientific community continues its debate about the health consequences of too much protein in the diet, you may be wondering just how much protein you should consume to be healthy. Continue reading to find out more about calculating your dietary protein recommendations, dietary protein sources, and personal choices about protein.