Intermittent fasting has become shorthand for everything from 16:8 eating windows to multi-day fasts. The evidence supports the first; the second is less clear. For men over 40, intermittent fasting (IF) can be effective for fat loss and metabolic health—but only if it's implemented correctly and only if the man is a suitable candidate.
The benefits aren't magical. They're straightforward. The common mistakes are equally straightforward to avoid.
The Evidence Base
Time-Restricted Eating (TRE) and Insulin Sensitivity
Sutton et al. (Cell Metabolism, 2018) examined the effects of early time-restricted feeding (eating within an 8-hour window, 8 am to 4 pm) in men with prediabetes. Results:
- Improved fasting glucose
- Improved insulin sensitivity (measured via HOMA-IR)
- Improved 24-hour blood pressure
- Modest improvements in body weight and fat mass
The mechanism: Concentrating eating into a shorter window reduces total meal-stimulated insulin elevation and gives the pancreas and liver longer periods of lower insulin. This improves insulin sensitivity over time.
This isn't about calorie restriction per se (though IF often leads to modest calorie reduction). It's about the metabolic benefit of longer fasting periods.
Lean Body Mass Preservation
A common concern: Does IF cause muscle loss? Moro et al. (Journal of Translational Medicine, 2016) examined trained men doing 16:8 intermittent fasting during a resistance training program.
Results: Lean body mass was preserved. Fat loss occurred. Training performance didn't decline.
The key: These were trained men eating adequate protein. IF doesn't inherently cause muscle loss. Poor protein intake causes muscle loss. IF just makes it easier to accidentally under-eat protein if you're not mindful.
Testosterone Implications
Acute effects. Short-term fasting (8–16 hours) may acutely raise testosterone slightly. Studies show modest increases in LH and testosterone in the fed-to-fasted transition. This is a transient effect, not a sustained benefit.
Prolonged fasting. Extended very-low-calorie fasting (< 800 kcal/day) or multi-day fasts suppress LH pulsatility and reduce testosterone. Röjdmark et al. (1987) showed that just 24 hours of complete fasting suppressed LH pulse frequency and amplitude, with testosterone declining as a result.
Practical implication: 16:8 intermittent fasting, if it results in adequate total calorie and protein intake, doesn't suppress testosterone. Very low-calorie fasting or multi-day fasts do.
16:8 Intermittent Fasting: The Practical Protocol
For busy men over 40, 16:8 (16-hour fast, 8-hour eating window) is the most practical and evidence-supported protocol.
Implementation:
Option 1: Skip breakfast, eat lunch at 12 pm, dinner at 7 pm, done eating by 8 pm. Fast from 8 pm to 12 pm the next day. This is straightforward for most men and doesn't require waking up hungry.
Option 2: Eat breakfast at 8 am, last meal at 4 pm. Fast from 4 pm to 8 am. This is less practical for men who work through mid-afternoon or have evening commitments.
Option 1 is more sustainable.
Adherence rules:
- Start with 12–13 hours of fasting if 16 feels extreme. Progress gradually.
- During the fasting window, black coffee, tea, and water are fine. Anything with calories breaks the fast.
- During the eating window, eat normally. No bizarre restriction or elaborate meal composition needed.
Who Shouldn't Fast Intermittently
Lean men in a calorie deficit. A man at 12–14% body fat trying to get leaner whilst training should not add IF to an already low calorie intake. The combination tends to impair recovery, suppress testosterone, and reduce training performance.
Men with a history of disordered eating. IF can trigger or exacerbate restriction patterns. If you have a history of binge eating, food obsession, or eating disorders, IF is probably not for you.
Men with adrenal insufficiency or chronic stress. Fasting raises cortisol acutely. For men with already elevated baseline cortisol (chronic stress, poor sleep, overtraining), IF can be counterproductive. Prioritise stress management and sleep first.
Men who feel terrible on IF. Some people genuinely don't tolerate IF well. They feel foggy, fatigued, or irritable. There's no shame in that. It's not a failure. Some bodies prefer frequent meals. Listen to yours.
Muscle Protein Synthesis Timing Considerations
A common myth: You must eat protein every 3–4 hours for optimal muscle protein synthesis (MPS). This isn't evidence-based. MPS is elevated for 4–6 hours after a meal containing adequate amino acids. With IF, you're simply shifting when those eating windows occur.
If you eat lunch at 12 pm and dinner at 7 pm, you've hit two MPS windows. That's sufficient for muscle maintenance and growth if the meals contain 30–40g of protein each.
The key: Adequate total protein intake over the day. 1.6–2.2g per kg bodyweight, regardless of meal timing.
Common IF Mistakes
Undereating during the eating window. Men often cut calories too aggressively with IF, thinking the fasting window will amplify effects. Instead, they just become chronically depleted, suppress testosterone, and hurt training performance.
Ignoring protein. During an 8-hour eating window, it's easy to eat inadequate protein if you're not deliberate. Track it for a week. Make sure you hit 1.6–2.2g/kg.
Extending the fast too long. 20:4 fasts (4-hour eating window) or 24-hour fasts are rarely necessary and often counterproductive. 16:8 provides the metabolic benefits without the downsides.
IF without structured training. IF combined with no resistance training will result in fat loss alongside some muscle loss. Add training and it shifts toward fat loss with LBM preservation.
Who Benefits Most From IF
- Men with insulin resistance or borderline type 2 diabetes
- Men trying to lose fat without elaborate calorie counting (IF creates a natural calorie deficit for most)
- Busy men who find meal prep time-consuming (skipping breakfast simplifies the day)
- Men already lean (< 15% BF) looking for a sustainable eating pattern, not aggressive fat loss
Practical 16:8 Example Week
Monday–Friday:
- 8 am–12 pm: Fasted (black coffee at 8 am)
- 12 pm: Lunch (grilled chicken, rice, vegetables, olive oil)
- 3 pm: Snack (Greek yoghurt, berries, nuts)
- 7 pm: Dinner (salmon, sweet potato, salad)
- 8 pm: Done eating, begin fast
Macros (example for 90kg man, 14% BF):
- Lunch: 40g protein, 60g carbs, 15g fat
- Snack: 20g protein, 20g carbs, 10g fat
- Dinner: 40g protein, 50g carbs, 20g fat
- Total: 100g protein, 130g carbs, 45g fat (roughly 2,100 kcal—appropriate for modest fat loss with training)
The Bottom Line
Intermittent fasting works for fat loss, primarily because it's an easy way to create a calorie deficit without obsessive tracking. It also genuinely improves insulin sensitivity and gives your metabolic machinery longer periods to rest between meals.
It's not magical. It's not suitable for everyone. And it's definitely not compatible with very low-calorie eating or extended fasting protocols.
For men over 40 who are reasonably trained, adequate in protein intake, and not already in a severe deficit, 16:8 intermittent fasting is a sensible, evidence-supported eating pattern.
References:
Sutton EF, Beyl R, Early KS, et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1434-1448.
Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290.
Related Guides
- Fasting Vs Breakfast Men Over 40
- Calorie Counting Guide Men Over 40
- Weight Loss Over 40 Men
- Testosterone Friendly Diet
Röjdmark S, Calissendorff B, Jeppsson S. Inhibition of the luteinizing hormone surge by prolonged fasting in normal women. Clin Endocrinol (Oxf). 1987;27(1):45-51.