It is very easy to tell when myself or a client hasn't had enough food on training day, or the day before.
Energy lacks, and basic exercises seem extremely difficult. Sometimes the hour needs to be cut short, and more time spent stretching because there isn't much fuel left in the tank.
One big reason why this may happen comes down to FOOD!
Many of my clients are in the process of losing weight, at the same time as increasing fitness. It's definitely a tough battle on figuring out how much to eat and how much to exercise when balancing these goals.
To start of I like to keep to a few simple rules.
#1: Depending on when you train you want a good source of complex and simple carbs, as well as a fat source about 1-3 hours before training. These two fuels will provide you with continuous fuel while exercising. If you exercise right when you wake up I suggest a simple sugar (fruit) and a fat. I find a piece of fruit and almond butter to work great!
#2: During exercise (if I am training extremely heavy) I like to include some coconut water, bring a veggie juice, or put a small amount of pure fruit juice in a bottle of water. This keeps the sugars and electrolytes maintained in my body while I push it to the limits!
#3. After workouts the most important thing to replenish at first is your carbs. Simple and complex. I generally enjoy squashes/sweet potatoes and sometimes rice or quinoa. This is also a great time for some protein to rebuild those muscles! This isn't necessarily the best meal for a fat source.
Timing of these meals is extremely important. The rest of the day I don't focus too much on what I am eating, I try to make most of my meals primarily veggies with some protein, with added nuts/seeds/oils included.
When you aren't getting enough carbs around workout time this I find is when you suffer the most!
Hope this helps a bit, more question?