That's in your day to day diet, not during exercise. Obviously an excess intake of sugar over time is going to have negative effect
What we're discussing here is refueling during exercise. When we do intense exercise we get our energy from blood sugar (glucose) which is converted to glycogen. When glycogen levels are depleted we feel tired and the quickest way to top your glycogen level back up is a sugary snack or a sports drink (I mean a propper sports drink, eg lucozade not monster etc)
Eating a banana gives you that glucose needed with the added benefits of potassium along with the glucose, fibre, vitamins B6 and C. Unripened bananas contain carbs and ripe bananas contain sugars. Look up "gut bacteria fibre and glucose" and find studies that might help you recharge your energy levels. Electrolytes replaces what you lose from sweating, if you have a camel pouch / pack, add in soluble electrolytes to your water, you can add some flavouring or get flavoured electrolytes.
The best way to fuel up I have found is to have a full english, my prefered style is Streaky Bacon, Chipolata Sausages, Free range eggs, Mushrooms, Plum tomatoes and toasted sourdough bread. One of those breakfasts is more than enough to carry me through the day with a light lunch around 1pm and a treat on the way home of a burger and discuss the days play over a bit of junk food intake.
If you look up the vitamin and mineral content of a full english, you realise quickly why its the best start to the day.
Bacon and sausages contains : Vitamins
B1, B2, B3, B5, B6 and B12. 89% of the RDA for selenium. 53% of the RDA for phosphorus. Decent amounts of the minerals iron, magnesium, zinc and potassium.
Eggs :
vitamin B2 (riboflavin), vitamin B12, vitamin D, selenium and iodine. They also contain vitamin A and a number of other B vitamins including folate, biotin, pantothenic acid and choline, and other essential minerals and trace elements, including phosphorus.
Mushrooms :
zinc, magnesium, copper, potassium, selenium, phosphorous, and a number of B vitamins and antioxidants. They are also a fairly good source of fiber.
Plum Tomatoes : vitamin C, potassium, folate, and vitamin K.
Sourdough Bread :
iron, manganese, calcium, B1-B6, B12, folate, zinc, potassium, thiamin, niacin, riboflavin, selenium, iron, manganese, magnesium, phosphorus, and vitamin E
Regular pan bread like supermarket bread : proteins, vitamin B complexes and minerals such as
calcium, phosphorus, manganese, magnesium, zinc and copper
So a full english will set you up for the biggest part of your day and all you need to do to top up when you feel the need.
So next time someone tells you that your full english is bad for you, you can reel off all the vitamins and minerals that you are getting to benefit your body.