Can an extremely low fat diet cause major joint pain especially in the back and cold appendages?

Yes: Very low-calorie diets are often deficient in necessary fat and protein, causing muscle loss which can lead to joint pain, particularly in postural muscles. Cold hands and feet can be seen in many disorders, including vascular conditions, thyroid disease, but also in anorexia and malnutrition. Proper weight loss should aim for no more than 1-2 pounds per week, otherwise there is risk of regain.

Low vitamin levels?: Low levels of fat-soluble vitamins in the body could be one rare problem on extremely low-fat diet. If you are thinking about calcium levels and back pain, there is no evidence of adverse effect in the area of causing low calcium levels. In short low-fat diet should not be associated with low-back pain or joint pains. Dietary fat intake should be limited to 30% of total energy intake.