There are currently multiple definitions of malnutrition. Some focus on only undernutrition, others include overweight and obesity, and some include energy and protein intake, while others look at micronutrient deficiencies. These varied definitions look at malnutrition from different perspectives that could be based on anthropometrics (measurements of body weight and size), medical imaging to assess muscle mass, markers of inflammation, and micronutrient deficiencies. There are a lot of factors to consider when defining malnutrition, such as insufficient intake, poor absorption, interactions between different nutrients, individual needs, medical conditions, and the effects of medications.^[1]

The World Health Organization (WHO) includes 3 types of malnutrition in their definition: undernutrition, micronutrient-related malnutrition, and overweight and obesity (overnutrition). Undernutrition includes several subcategories of malnutrition, namely wasting (low weight for height), stunting (low height for age), and underweight (low weight for age). These categories are often noted in childhood development but can also apply to adults. Micronutrient-related malnutrition could refer either to a deficiency of certain nutrients due to insufficient intake or to an excessive intake of certain micronutrients. Lastly, overweight and obesity are a result of an excess intake of energy. In some cases, this can still involve micronutrient deficiencies. Some definitions of malnutrition also include health conditions — such as diabetes and heart disease — that can be a direct result of overweight or obesity.^[2]

The signs and symptoms of malnutrition are diverse because there are many different types of malnutrition.

In undernutrition, the initial symptoms might be a lack of interest in food, fatigue, irritability, and poor concentration. There might be unintentional weight loss, a low body-fat percentage, and sometimes mental health conditions like anxiety or depression. Muscle weakness could result in difficulty with normal tasks like getting dressed or taking a walk. Very low intake or poor absorption of food could result in low blood sugar levels and electrolyte disturbances.^[1]

In micronutrient malnutrition, the symptoms could be related to the inappropriate levels of a specific vitamin or mineral. For example, a low level of vitamin B12 could cause peripheral neuropathy,^[3][4] and a high calcium level could cause abdominal discomfort and bone pain,^[5] while a low iron level might cause severe fatigue.^[6]

In overweight and obesity, malnutrition occurs due to an excess of energy intake, and this can cause joint pain and fatigue. It can also have a negative effect on mental health. Chronic conditions like type 2 diabetes, high blood pressure, and high cholesterol can develop in people with overweight and obesity, and this increases the risk of cardiovascular disease, stroke, and chronic kidney disease.^[7]

Since there are no universally recognized diagnostic criteria for malnutrition, the diagnosis is not always straightforward.

There are several sets of criteria currently in use for the diagnosis of malnutrition.^[8][1] The Global Leadership Initiative on Malnutrition (GLIM) criteria include unintentional weight loss, low BMI, reduced muscle mass, decreased intake or absorption of food, and inflammation.^[9] The American Society for Parenteral and Enteral Nutrition (ASPEN) criteria are insufficient energy intake, weight loss, loss of subcutaneous fat, loss of muscle mass, accumulation of general fluid, and decreased functional status (assessed by grip strength).^[10] The European Society for Clinical Nutrition and Metabolism (ESPEN) criteria for a malnutrition diagnosis are either a BMI less than 18.5 or an unintentional weight loss paired with either a low BMI or a low fat-free-mass index (FFMI).^[11] There are many similarities between these diagnostic tools. BMI is often used as a starting point because it is easily determined and provides quick information. Other gauges of body composition, like a DEXA scan or CT scan, might be more accurate but require equipment and financial resources.

The current malnutrition diagnostic criteria do not include the overweight and obesity (overnutrition) or micronutrient types of malnutrition. However, overweight and obesity are commonly diagnosed according to BMI, although it’s not always accurate, e.g., in the case of athletes. A BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher is considered obese. Measuring the waist circumference can also be an indicator of the distribution of excess adipose (fat) tissue.^[7]

Micronutrient malnutrition can occur in both undernutrition and overnutrition. Blood tests for micronutrient deficiencies are ordered based on symptoms, physical examination findings, and diagnosis. For example, testing for a vitamin B12 deficiency would be helpful in someone who is using medication for stomach ulcers and experiencing numbness in the feet and hands (peripheral neuropathy).^[12]

The treatment for malnutrition will differ depending on what type of malnutrition is diagnosed.

In undernutrition, treatment consists of rehydration and slowly increasing food intake. This could involve feeding through a tube, an intravenous line, or through oral intake, depending on the situation and the severity of the malnutrition. For overweight and obesity, physical activity and behavioral therapies are important in treatment. Increasing physical activity increases the amount of calories burned and works in tandem with calorie-restricting diets for weight loss.^[13] In some cases, medications or surgical interventions might also be recommended. In both undernutrition and overweight and obesity, if there is a deficiency of a certain vitamin or mineral, it might be replaced through an oral supplement, an intravenous supplement, or fortified foods.^[14]

Most of the people who are diagnosed with malnutrition can benefit from the education and support provided by a dietitian.^[14]

Which supplement(s) is (are) used for malnutrition depends on the type of malnutrition.

Supplements are often a part of the treatment of undernutrition. While the main focus of treatment is improving nutrition through dietary interventions, increasing caloric and micronutrient intake can be difficult through diet alone. Supplements can make it easier to take in more calories, such as using meal replacement shakes to increase calorie intake. Taking oral supplements can help to treat specific vitamin or mineral deficiencies, such as calcium, vitamin C, or vitamin B12. Zinc has also been shown to increase appetite rapidly in people with zinc deficiency and could be an option in people with malnutrition.^[15][16][17] In more severe cases, malnutrition might be treated in a hospital and involve tube feeding, ongoing supplementation through an intravenous line, and regular monitoring of blood tests and body weight.^[14][18]

In overweight and obesity, supplements are sometimes helpful as well. Using whey protein can help increase protein intake without adding a lot of extra calories. People with overweight or obesity can also have micronutrient deficiencies that can be replaced through vitamin and mineral supplementation. There are very few supplements that have a significant effect on weight loss. Weight-loss supplements often contain ephedrine, caffeine, and green tea extract. These may reduce appetite and assist with weight loss but can also have significant side effects, and weight-loss benefits tend to be small.^[19][20][21] Weight-loss supplements are not usually part of the initial treatment for overnutrition.^[22]

There are herbal remedies that are sometimes used to increase appetite in people with undernutrition-type malnutrition.

Cannabinoids have been looked at as a way to stimulate the appetite in undernutrition, especially for people who lose their appetite due to chemotherapy drugs. However, the evidence shows that this is not consistently effective, and so it is not currently recommended. While THC and its analogs may have appetite-stimulating effects in certain cases,^[23][24][25] some studies have found that cannabidiol (CBD) products might actually decrease appetite.^{[26][27][28][29]}

In some studies, ginger has been found to increase feelings of satiety and help with weight loss,^[30][31] but it can also reduce nausea, which might help increase food intake.^[32][33]

For people with overweight and obesity, several different dietary ingredients have been studied for weight loss. A 2021 meta-analysis found that chitosan, glucomannan, and conjugated linoleic acid could assist with weight loss of up to 1.9 kilograms (around 4 lbs).^[34] Other supplements have been commonly studied for weight loss, such as green tea extract, garcinia, and L-carnitine, but there is limited evidence to support their effects on weight loss.^[19][20][21]

Undernutrition-type malnutrition can be caused by a decreased food intake, a decreased absorption of food, or an increased demand for nutrients.

A decreased food intake might be due to a lack of available food, a poor appetite, or a medical condition. A decreased absorption might be due to gastrointestinal conditions such as inflammatory bowel disease^[35] or gastritis.^[36] Other medical conditions can affect the absorption of nutrients as well, especially conditions affecting the liver, pancreas, or gallbladder.^{[37][38][39][40]}

In some situations there is an increased demand for nutrients. Wound healing after an injury or an operation will increase the energy needs of the body, while burn wounds can increase fluid and electrolyte requirements.^[41][2]

Overweight and obesity are caused by prolonged periods of eating more calories than the body needs and can use. The extra calories are stored as fat. Factors that affect weight and appetite include genetics, physical activity, nutrition habits, socioeconomic factors, and mental health conditions.^[42]