Studies show that diets containing relatively low levels of carbohydrate are more effective for weight loss than those containing high levels of carbohydrate, especially refined carbs (sugars). The problem is that as a triathlete undergoing relatively high training volumes, you need plenty of carbohydrate to fuel that training, otherwise you’ll end up feeling tired and your training quality will suffer.
Some studies have shown that muscles can be fully replenished with glycogen by consuming 7-10g of carb per kilo of bodyweight following training (e.g. 490-700g of carb for a 70kg athlete) and this has led to a general recommendation among some sports scientists.
At lower training volumes, however, this blanket recommendation can easily lead to weight gain. Even at the lower end of the 7-10g range, 490g of carbohydrate (for a 70kg athlete) contains nearly 2,000 calories. By the time you’ve consumed the required 100g or protein or so and some essential dietary fat, your daily calorie intake is going to be pushing 3,000kcals.
A better way is to use calorie burn – easily calculated using a good heart rate monitor – and consume approximately 1g of carbohydrate for every 4 calories of expended energy.
Regarding carb type: the gentle energy release of slow-releasing carbs found in brown pasta, beans and oats are less likely to result in blood sugar spikes, which can lead to weight gain. These should form the mainstay of your diet. But because your muscles benefit from rapid replenishment during the post-exercise ‘window of opportunity’, some quick-release carbs (e.g. sugars in carb drinks) are particularly effective when
taken straight after training.