How did low-carb fad influence the allocation of resources?

Through the allocation of income and production.

Explanation:

Low carb is a diet in which carbohydrates, ie more calorie foods, for example pasta or pasta in general, are replaced by foods high in protein and fat. This diet influences income allocation because part of what was consumed in carbohydrates will be reallocated to consumption and other foods. In this sense, it reallocates resources to production, as demand for carbohydrates will decrease, increasing resources for the production of other foods.

The "Low Carb" fad diet influenced the allocation of resources by influencing the population to put funds toward buying Low Carbohydrate/High Fat foods.

People were encouraged that studies found that foods high in carbohydrates were a cause of obesity. This affected the economic demand and price of many of these "high carb" products (rice, white bread, pasta) making them less expensive and less in demand.