My mother always told me "eating that chocolate bar will give you pimples." Is there any truth to the claim?
I did find an article on about.com that verifies that this is a common myth, and suggests that chocolate bars do not cause acne: Does Eating Chocolate Cause Acne?
There is a study demonstrating that cocoa, which differs chocolate from other sweets is not responsible.
Study in question is Fulton JE Jr, Plewig G, Kligman AM. "Effect of chocolate on acne vulgaris." The Journal of the American Medical Association. 1969 Dec 15;210(11):2071-4. Cited in PubMed; PMID 4243053.
The 1969 study was aimed at determining whether chocolate and acne are related. Two groups were given chocolate bars. One group was given chocolate bars with zero cocoa content, and the other group was given chocolate bars with a huge cocoa content. The two groups fared equally well as far as their acne was concerned. However, the chocolate bars both had sugar, milk, etc. The study was only testing cocoa in particular.
However, what's nowadays called "chocolate bar" has actually very little chocolate at all. It's mainly sugar in various forms. For example Mars bar is 60% sugar (35g of 58g).
Sugars are known to induce acne:
high-glycemic diets, with their higher carbohydrate and sugar contributions, generally do give rise to increased plasma levels of insulin. High-glycemic diets also promote increased blood levels of another substance: free insulin-like growth factor (IGF-1).
IGF-1 is thought to promote acne in a similar way to insulin, by promoting follicular obstruction through inducing androgen-mediated sebum production, but also hyperkeratosis, a thickening of the outer layers of skin, and epidermal hyperplasia, a proliferation of skin cells. A study of post-adolescent women, ages 20–25 years, found increased levels of IGF-1 in those suffering from acne.
So given that it's fair to say that eating excess of candy bars (aka "chocolate" bars) could induce acne.
