There's two schools of thought on detoxing; one that claims it isn't needed and unnecessary as the body is effective at doing its' own detox (if it didn't we probably wouldn't last very long). The other group claims that benefits can be derived from it and it is probably something that most people could do with from time to time.
Personally I can see both points ~ yes, the human body is excellent at cleaning the blood, renewing iron supplies and filtering toxins for removal. However this process becomes impaired by external factors such as diet, stress, alcohol etc etc, it still gets done but not as effectively. For someone that drinks coffee and/or alcohol moderately, has the odd bit of takeaway food, is exposed to stress of any degree (wouldn't think there would be many of us that aren't) and trains hard on a regular basis then it can have its' place. It doesn't need to be too drastic in terms of what you have to take and there are a variety of options, but doing a mild detox once a year could offer some people potential benefits. It makes sense that if you body is being run on clean blood then it will be able to utilise the nutrients from food better, you'll have a more optimal blood flow to the heart as well as the quality of your skin usually improves for a lot of people.
I do agree with the basic principles of fasting although there are different degrees of fasting and often a lot of them are quite subjective. To me a fast is going without food but with freshly juiced vegetables & fruit being allowed. Probably the most common combination of vegetables to juice is carrot, celery and beetroot; fantastic for cleansing and building the blood as well as helping to lower BP.
Carrots are well known for their high phytonutrient content, beta carotene being the main one everyone associates them with. They have been shown to potentially inhibit the growth of cancer cells (studies have mainly been done on colon cancer but much more testing is needed), this seems to be mainly ocurring from a few of the more abundant polyacetylenes. Vision health has been well linked to them over the years as well as helping to lower the risk of cardiovascular disease due to the high concentrations of antioxidants present.
Celery is low in calories and again packed with phytonutrients, fibre and Vitamins A and C. Fantastic for the kidneys and renal funtioning, as well as being used for lowering BP due to it's magnesium, potassium and calcium content.
Beetroot is rich in phytonutrient pigments called Betalains; these are nitrogen rich, water soluble plant pigments which have many roles in the body including protection against CVD and being a natural anti-inflammatory. Beetroot has been shown in studies to increase glutathione peroxide as well as other antioxidant enzymes which can lower homocysteine levels in the blood (homocysteine can damage artery walls which over time can lead to artherosclerosis). As long as homocysteine is converted back in methionine or cystathione then the build of it shouldn't occur, but the availability of TMG in beets can further help lower the levels that are in the blood. Betroot also contains nitrate in the juice which converts to Nitric Oxide in the body ~ great for athletes, especially BB'ers.