The body is continually remaking itself to optimize. Taking bone and muscle from unused areas and strengthening used areas. A reduced-calorie diet can interfere with this process if it also leads to reduced activity. A supply of protein and minerals is not enough to maintain strength when calories are reduced. Activity levels must also be maintained.
As weight is lost, some loss of overall strength is to be expected because the body has an easier fight against gravity. In which case weight training and increased activity can be used to maintain overall strength.
However, realize that strength is relative. If a person weighs 300 pounds and then drops to 200 pounds they will typically lose some strength but will find that they are able to move much easier with greater endurance. However, when they weighed 300 pounds they could likely deadlift more weight than when they weigh 200 pounds. Unless they increase activity levels and strength training.