Intermittent Fasting and Exercise: How to Time Your Workouts

1. Know what research on intermittent fasting and exercise shows

The trial closest to a real 16:8 gym routine comes from Italy. For eight weeks, 34 men who already lifted regularly either ate three meals at 1, 4 and 8 pm, fasting the other 16 hours, or ate the same calories at 8 am, 1 pm and 8 pm. Both groups followed the same program, three sessions a week between 4 and 6 pm. Bench press and leg press strength held in both, and so did lean mass and the muscle size of the arms and thighs. Fat mass fell more in the 16:8 group. Their testosterone also dropped, and eight weeks is too short to know what that means over a year.

The TREAT trial shows the other side. Across the United States, 116 adults with overweight or obesity either ate only between noon and 8 pm or ate three structured meals a day for 12 weeks, with no training program. The 16:8 group lost about the same weight as the three-meal group. Among the 50 people measured in person, lean mass in the arms and legs dropped slightly more with 16:8.

So lifting and eating enough kept strength and muscle in place in the first trial, while changing meal times alone cost a little lean mass in the second. Both studies are small, and results vary from person to person.

2. Pick a training time that fits your window

Most people on 16:8 eat from about noon to 8 pm, which puts a 6 am workout deep in the fast and a 5:30 pm class inside the window. Both can work. A 2018 meta-analysis of 46 studies compared exercise after a meal with exercise after an overnight fast. Eating first gave a small boost to aerobic sessions longer than 60 minutes and made no overall difference to shorter ones. The few studies on intervals and sprints mostly found no difference. Fasted sessions raised fatty acids in the blood afterward, a sign the body was drawing more on fat.

Your routine	Eating window	Train fasted?	First meal after
6 am gym before work	Noon to 8 pm, or 9 am to 5 pm on lifting days	Yes, for sessions under an hour	Breakfast at 9 after lifting, lunch at noon after cardio
30-minute run at 12:15 on your lunch break	1 pm to 9 pm	Yes, at the end of the fast	Lunch at your desk at 1 pm
5:30 pm spin or boot camp class	Noon to 8 pm	No, it falls inside the window	Dinner by 7:30 pm
Saturday run or ride of 90 minutes or more	7 am to 3 pm that day	No, eat one to three hours before	A full meal within an hour

Once you pick a slot, keep it most days. FastTrack starts and stops the fast automatically on the schedule you set, so when a workout feels flat you can see how many hours you had been fasting.

3. Lift inside your eating window when you can

For strength work, the Italian trial is the best guide: the men trained between their 4 pm meal and their 8 pm dinner, with 20 g of whey protein 30 minutes after each session. Most studies in the fed versus fasted review tested endurance exercise, so it says little about heavy squats at 6 am.

If 6 am is the only slot your job allows, you have two workable options:

• Train fasted and eat soon after. Many lifters move the first meal to 8 or 9 am on gym days.
• Shift the whole window to 9 am to 5 pm on lifting days, and back to noon to 8 pm on rest days.

Plenty of people lift fasted and feel fine on heavy sets, then fade on long leg days. Keep a simple log, and if your main lifts drop for two weeks in a row, move the session into the window. Daily protein and regular lifting matter more than the clock, as our guide to intermittent fasting and muscle loss explains.

4. Keep fasted cardio under an hour

A 30 to 45 minute run or brisk walk before your first meal is where fasting and exercise fit most easily. In the meta-analysis, sessions under 60 minutes went about the same fed or fasted, so if the 12:15 loop around the office park feels good on an empty stomach, there is no reason to eat first.

Fasted cardio for fat loss is a separate question. In a four-week trial, 20 young women on a calorie-reduced diet did an hour of steady cardio three times a week, half after an overnight fast and half after a meal replacement shake. Both groups lost weight and fat, with no difference between them. Choose fasted cardio because it suits your schedule and your stomach.

Longer sessions are where food pays off. For a 90-minute Saturday run, eat one to three hours before, such as a bagel with peanut butter and a banana, and move your window earlier that day. Hard intervals have little research behind either choice; if your legs feel empty halfway through, put them inside the window.

5. Eat a protein-heavy meal after you train

The meal after your workout matters more than any powder. A 2018 review on how much protein one meal can use for muscle building suggested about 0.4 g per kilogram of body weight per meal, over at least four meals, to reach 1.6 g per kilogram a day. A 16:8 window rarely holds four meals, so each of yours has to carry more. For a 170-pound person (77 kg), the review's figures of 0.4 to 0.55 g/kg per meal work out to about 31 to 42 g, and three meals of around 40 g get close to 1.6 g/kg.

Post-workout meals in the 30 to 45 g range:

• After the 6 am gym: three scrambled eggs and a cup of plain Greek yogurt.
• After the lunchtime run: a burrito bowl with chicken, black beans and rice.
• After the evening class: a 6-ounce salmon fillet with a baked potato.
• After a long Saturday run: a turkey and cheese sandwich with a glass of milk and fruit, since you need the carbs back too.

Our 7-day intermittent fasting meal plan builds every lunch and dinner around that much protein. If you are unsure what a plate holds, scan it in FastTrack to see its estimated protein.

6. Drink water and add salt on fasted training days

By 6 am you have usually gone eight hours or more without a drink. Have a 16-ounce glass of water when you get up and take a bottle to the gym. Black coffee before a morning session suits most people, and it does not break the fast.

Salt matters when you sweat heavily, on a hot July run or in a 90-minute class. A pinch, about 1/8 teaspoon, in a 16-ounce bottle of water covers most fasted sessions without adding calories. Sugary sports drinks break the fast, which is fine inside your window or during a long effort where you want the fuel. Our guide to electrolytes while fasting explains how much sodium makes sense. If you have high blood pressure or kidney disease, or take a water pill, ask your doctor before adding salt.

7. Know who should not train fasted, and when to stop

Fasted training suits healthy adults who already handle their fast well. Check this before your first fasted session:

Your situation	Train fasted?	What to do
You take insulin or tablets that lower blood sugar, such as glipizide	No	Exercise and fasting both lower blood sugar, so agree on a plan with your doctor
You are pregnant or breastfeeding	No	Skip fasting unless your doctor agrees to a plan, and eat before exercise
You have felt dizzy or fainted during a fasted workout	No	Train after a meal, and see a doctor if you fainted
You are in your first two weeks of 16:8	Easy sessions only	Lift after eating until the fast feels routine
You are on a fast of 24 hours or more	Walking and stretching only	Save hard training for after you eat
You are under 18 or have had an eating disorder	No	Skip fasting unless a doctor guides it

During any fasted session, stop and eat if you feel unusually dizzy or shaky, or your vision blurs, and sit down first if you feel faint. Fainting, chest pain or a racing heartbeat that does not settle need medical help right away. With a medical condition or medication, see your doctor before any fast longer than 24 hours. Water-only fasts over 72 hours belong under medical supervision.

The short version

You can keep training on 16:8. Lift inside or close to your eating window and follow the session with 30 to 45 g of protein. Fasted cardio under an hour works for most people, and longer sessions go better with a meal beforehand. Drink water before fasted workouts and salt it on hot days. If you take blood sugar medication or are pregnant, talk to your doctor before training on an empty stomach.