The gut microbiota (GM) is a collection of microorganisms living in the digestive tract of humans, which if unbalanced, may have a role in the development of certain disorders such as type 2 diabetes and obesity. A number of factors can imbalance the gut microbiota, one of the main being diet. This review paper of 190 papers aimed to summarise the relationship between GM, diet and modifiable diseases such as type 2 diabetes and obesity. Dietary components and the role of carbohydrates, protein and fats in shaping the GM were discussed. It was determined that carbohydrates have the greatest influence, with simple carbohydrates such as the sugars fructose and sucrose having a negative impact and the more complex forms being beneficial. Diet types were also reviewed. Vegetarian and vegan diets appear to increase the diversity of the GM, the Mediterranean diet changes the species balance, and the Western diet imbalances the GM causing diseases such as heart disease. Interestingly the literature points towards a negative impact of the gluten free diet. Diseases such as type 2 diabetes, obesity and increased fats in the blood all display an imbalanced GM causing increased energy harvest from food and disruption of various energy pathways in the body. It was concluded that a balanced diet rich in fruit, vegetables, fibre and healthy fats can promote GM diversity and activity. This study could be used by health care professionals to understand the importance of certain dietary components to promote GM diversity in order to reduce the risk of diseases such as obesity and type 2 diabetes.