Nutrition psychology is the psychological study of the relationship between dietary intake and different aspects of psychological health. It is an applied field that uses an interdisciplinary approach to examine the influence of diet on mental health. Nutrition psychology seeks to understand the relationship between nutritional behavior, mental health and general well-being. It is a sub-field of psychology and more specifically of health psychology, and may be applied to numerous related fields, including psychology, dietetics, nutrition, and marketing.

Nutrition psychology assesses how nutrition affects psychological functions, and how psychological choices and behavior influence nutrition and health.

Nutrition psychology addresses dietary quality and its relation to various components of mental health.

Nutrition psychology is a field that is still in its early stages of development. Despite the growing interest and demand for nutrition psychology, there is a lack of research studies on this topic. Expanding the field will require a proliferation of peer-reviewed research. Recent studies suggest that probiotics and prebiotics may support mental well-being by promoting a balanced gut microbiota. These dietary components help regulate inflammation and neurotransmitter activity, which are linked to mood and cognitive function.

With obesity being a continually growing problem in many countries, nutrition psychology is gaining importance and popularity in society today. As it has grown, nutrition psychology has directly and indirectly influenced research on dieting, food labels, the way food is marketed, food technology, obesity, and the attitude of the public towards food, among other topics.

Some research discusses the idea of "food faddism", which is loosely defined as, "the idea that too much weight is put upon the influence of food and diet on overall health and that claims, whether good or bad, are often exaggerated." This idea of food choices having extreme consequences is thought to be deeply ingrained into culture, possibly stemming from the story of Adam and Eve eating the forbidden fruit.

In 1990, the U.S. Food and Drug Administration (FDA) required that nutrition labels be put on food products in the United States. The thought behind doing so was to provide consumers with the necessary information to make educated decisions about the foods that they purchased. Since that time, nutrition psychologists have done research on how these labels influence how consumers choose what foods to buy. These studies have shown mixed results concerning the effects of nutritional labeling. According to the research, the average consumer does tend to read the labels and take the information into consideration, in part because companies have begun producing foods with more health-conscious ingredients. However, many of these potential health benefits are overshadowed by the continuing increase in obesity and deaths related to obesity in the United States over the last few decades.

Because of the misinformation and easy access to processed and prepackaged foods, people are more likely to choose them over fresh food or healthier packaged foods. That can lead to health-related diseases and obesity. Those that are uneducated on the topic of nutrition and how to read nutrition labels are the most at risk for this. When informed on nutrition labeling, there is an impact on one's health, body weight, and overall energy intake. Many food companies market their food products to make them seem more nutritious than they actually are, falsely leading people to believe that they are a healthy option. That can link to obesity because some may be intaking a large amount of a product they think is a healthy option. They can do that by making the product packaging look a certain way, or putting misleading words on them. For example, people may have more incentive to buy a product if the packaging looks nicer and is colorful or has pictures on it. Also, false claims can be made on the packaging such as made with real ingredients, boosts immunity, no high-fructose corn syrup, or lowers cholesterol. In reality, the food items have processed ingredients, these claims are not true, and it does not mean the other ingredients are healthy. Food companies will also say their product has nutritional benefits that others do not, just to get consumers to buy theirs. Many people would like to be more knowledgeable about nutrition labeling. However, it is hard for some when they have to compare many different components they don't want like sugar, cholesterol, sodium, or fat. It is easier for those with background knowledge or interest in nutrition to determine labels.