Which of the following is a minor side effect of following a very low calorie diet?

Following a very low-calorie diet can lead to a range of physiological changes, and gallstone formation is indeed recognized as a potential minor side effect. When caloric intake is significantly restricted, the liver can produce more cholesterol, which may lead to the development of gallstones. This occurs because gallstones often form when there is an imbalance in the substances that make up bile, and a rapid loss of weight often seen in very low-calorie diets can contribute to these imbalances.

In contrast, the other choices do not accurately represent minor side effects associated with a very low-calorie diet. For instance, increased energy is usually not a response to such diets, as energy levels often drop due to insufficient caloric intake. Similarly, muscle gain is unlikely; in fact, muscle loss can occur due to insufficient protein and overall energy, which are critical for muscle maintenance. Vitamin deficiency is a concern, but it is typically a significant concern rather than a minor side effect, as the lack of essential nutrients over time can lead to severe health issues. This highlights why gallstone formation is a more appropriate selection as a minor side effect of following a very low calorie diet.