Sugar

Stimulates receptors in the brain that are also activated by drugs such as heroin and morphine. This is how sugar can act as an antidepressant and why it is also addictive. In a study conducted by the Mayo Clinic it was found that people with the highest carbohydrate intake were 1.9 times more likely to suffer from cognitive impairment than those who consumed the least carbohydrates. 940 Participants from the age of 70 to 89 provided information on their dietary habits for a year and had no cognitive impairment beyond normal age-related decline when the study began.  After four years 200 of these people had begun to show mild cognitive decline. Individuals that had a specifically high sugar intake had a 150% risk of cognitive impairment compared to those with the lowest sugar intake. Those with the highest fat and protein intakes had 42% and 21% reduced risks (respectively) of cognitive impairment after this time period.

http://www.newswise.com/articles/view/594916/?sc=rsmn

Overconsumption of sugar leads to an increased risk of obesity, type 2 diabetes, heart disease, high blood pressure, hormonal imbalance (particularly oestrogen and testosterone, though there are others), infertility, arthritis, osteoporosis, dementia, Alzheimer's, depression and many more. Sugars tend to have very high glycemic indices. A high glycemic index means that the substance being described is metabolised rapidly by the body and causes a large, rapid increase in blood sugar levels. Foods with a low glycemic index still contribute to raised blood sugar levels but they cause changes that are longer lasting and much more gradual, i.e. there is an increase in blood sugar, but not a 'spike'. These increases in blood sugar levels stimulate the production and secretion of insulin. Insulin is a hormone that transports the sugar into cells where they are metabolised to create ATP.

In a study on two groups of rats who each received water containing fructose but only one group received omega-3 fatty acids it was shown that fructose has detrimental effects upon the rat brain (rats have similar metabolic functioning to humans) and that omega-3 fatty acids were neuroprotective (they protected the rat brain from these effects). The rats that didn't receive omega-3 had a decline in synaptic activity, their neurons "had trouble signalling each other, disrupting the rats' ability to think clearly and recall effectively". These same rats also had insulin resistance which may be connected this cognitive decline.

http://www.medicalnewstoday.com/releases/245469.php

Cancer cells rapidly metabolise glucose, they have an extremely high number of insulin receptors through which glucose enters their cell surface membrane. Breast cancer cells, for example, have around 17 times the insulin receptors of normal cells.
It is also estimated that around a teaspoon of sugar per day can dramatically increase your levels of inflammation.

Sugar ingestion also causes an increase in body fat as this sugar is converted to fats for storage. It also raises bad cholesterol and paradoxically stimulates appetite. In another study two groups of rats were either fed normal rat food or processed food from the supermarket. Those fed the normal rat diet put on weight slowly and stopped eating when full. The rats fed the supermarket food ballooned in size in a matter of days and ate a lot more. This is thought to occur by the suppression of leptin, a hormone that, when produced, communicates to the brain that the body has eaten enough. A knock-out mouse without the leptin gene will simply keep on eating and become very overweight.

Fructose is a particularly unhealthy sugar, it is 7 times more likely to form Advanced Glycation End-Products (AGEs) than glucose. Elevated levels of AGEs are signs of many diseases, for example, diabetes and degenerative eye disease. Also, fructose does not suppress ghrelin, a hormone that makes us feel hungry, therefore a high fructose diet keeps us hungry. Chronic fructose exposure alone is enough to cause metabolic syndrome, an umbrella term encompassing obesity, type 2 diabetes, lipid problems, hypertension, and cardiovascular disease. Fructose consumption is also detrimental to liver function as only the liver can metabolise fructose. Also, approximately 30% of fructose is stored as fat.