Protein After Fasting: How Much Do You Need and What Sources Work Best

Breaking a fast raises an immediate practical question: how much protein should you eat, and which sources will help your body recover and rebuild without overwhelming a digestive system that has been at rest?

This is a question Upton Sinclair wrestled with personally in 1911 — and modern nutritional science has since provided much clearer answers.

The Historical Context: What Sinclair Found

In The Fasting Cure (1911), Sinclair documented his own dietary experiments after extended fasting in considerable detail. After his first 12-day fast, he followed the traditional milk diet — warm milk in small, increasing quantities — and gained 32 pounds over 24 days, which he described as an extraordinary experience of physical and mental restoration.

But milk was not the final answer in his nutritional story. As Sinclair returned to high-intensity intellectual work — writing, research, journalism — he found that raw fruit and nuts, while excellent for active physical life, could not sustain sustained mental output over long periods. He eventually concluded that broiled lean beef was the most effective protein source for hard intellectual work, eating it alongside Dr. Salisbury's recommended practice of hot water between meals.

Sinclair wrote directly about this evolution, noting that the post-fast diet mattered as much as the fast itself: of the 277 fasting cases he documented from reader reports, roughly half of those where results were not permanent were attributed to wrong eating after the fast ended.

The 1911 Post-Fast Protocol

Framing Sinclair's work as historical context — written more than a century before modern nutritional biochemistry — his staged approach to post-fast eating was:

1. Days 1–3 post-fast: Orange juice or grape juice only — no solid protein
2. Days 3–7: Warm milk introduced in small amounts, half a glass at a time, building slowly
3. Second week onward: Raw fruits, nuts, or lean broiled meat introduced carefully once the digestive system had restarted
4. Long-term: A sustainable diet built around whatever the individual body responded well to

The principle running through all of this: the digestive tract after a fast is like a clean slate. Introducing protein too quickly, in too large a quantity, or in too heavy a form would overwhelm it. Sinclair (1911) noted that the wrong approach to breaking a fast caused more reported harm than the fast itself — including one case where a person broke a 50-day fast with half a dozen figs and caused intestinal abrasions from eating too quickly.

What Modern Science Adds

Contemporary nutritional science gives us much more precise tools than Sinclair had available. The mechanisms are now well understood.

Quantity: How much protein do you actually need?

Current evidence points to these practical targets:

• After a standard 16:8 fast: Your first meal should include 25–40g of high-quality protein. This is enough to trigger muscle protein synthesis without taxing a gut that has been resting for 16 hours.
• After a 24–72 hour extended fast: Start conservatively — 20–25g of easily digestible protein in your first meal, then build over the following 24–48 hours. Larger amounts too soon cause nausea, bloating, and cramping.
• Daily target for sustained intermittent fasting: Roughly 1.2–1.6g of protein per kilogram of body weight, consumed entirely within the eating window.

The leucine trigger

One of the most important discoveries since Sinclair's time is the role of the amino acid leucine in switching on muscle protein synthesis. Leucine acts as a direct signal to muscle cells to begin rebuilding. For this signal to fire fully, a meal needs approximately 2.5–3g of leucine — which typically means eating at least 25–35g of complete protein.

This is why quantity matters, not just the presence of some protein.

The digestion window after fasting

After a fast, digestive enzyme production is lower than during normal feeding. The stomach maintains its acid production, but the pace at which food moves through the gut slows. This is why starting with easily digestible proteins — eggs, white fish, cooked chicken — is generally easier on the system than beginning with a large steak or a concentrated protein shake.

The body's ability to digest and absorb protein rebuilds within a few hours, which is why the first meal should be moderate and the second meal can be more substantial.

Best protein sources after a fast, ranked by digestibility

1. Eggs — among the highest biological value of any food; easily digestible in any form; two to three eggs provide 14–21g of complete protein with all essential amino acids
2. White fish (cod, haddock, tilapia, sole) — lean, light, high-quality protein with minimal digestive burden; excellent first-meal choice
3. Chicken or turkey (cooked and not processed) — excellent protein density, manageable fat content, tolerated well even a few hours into refeeding
4. Salmon and oily fish — adds omega-3 fatty acids alongside protein; slightly richer, so better as a second meal rather than the very first
5. Full-fat Greek yogurt or fresh cheese (ricotta, cottage cheese) — protein plus casein for sustained amino acid release; gentle on the digestive system
6. Lean beef, broiled or grilled (as Sinclair recommended) — excellent for sustained energy and rich in bioavailable iron and zinc, but best introduced a day or two after breaking a longer fast rather than as the first meal

What to avoid in the first post-fast meal

• Protein powder shakes — highly processed; many brands contain sweeteners, emulsifiers, and artificial flavourings that irritate a recently rested gut. Whole food protein sources are more bioavailable and gentler.
• Very large portions of any protein — even excellent sources like salmon can cause discomfort if eaten in excess after a fast
• Protein combined with large amounts of starch — slows digestion and can create the intestinal fermentation that Salisbury and Sinclair both identified as the root of many post-fast digestive problems

Connection to Modern Fasting Practice

The principle Sinclair articulated in 1911 — that what you eat after the fast determines whether the benefits are sustained — maps directly onto what modern intermittent fasting practitioners report.

Getting post-fast protein right matters for several specific reasons:

Muscle protein synthesis: Adequate leucine-rich protein after each eating window prevents the gradual loss of muscle mass that can occur with sustained caloric restriction without sufficient protein.

Gut repair: Amino acids — particularly glutamine — are used directly by the cells lining the intestine for repair and regeneration after a fasting period. A protein-poor breaking of the fast slows this process.

Satiety and appetite regulation: High-quality protein is the most satiating macronutrient. Eating sufficient protein when breaking a fast reduces the likelihood of overeating during the eating window and erasing the metabolic benefits of the fasting period.

A Practical Post-Fast Protein Plan

After a daily 16:8 fast:

• First meal: 2–3 eggs cooked in butter with leafy greens; or 150g of grilled white fish with salad; or 150g of chicken with roasted vegetables
• Second meal (if eating twice): 100–150g of beef, lamb, salmon, or eggs with non-starchy vegetables and a healthy fat source

After a 24–48 hour fast:

• Hours 1–3: small amount of plain water or herbal tea; no rush to eat
• Hours 3–5: 2 eggs cooked gently; or 100g of white fish; do not eat a large meal
• Hours 6–12: a regular protein-rich meal of 100–150g of any high-quality protein with vegetables
• Day 2: return to normal eating window without restriction

After a 72+ hour extended fast (following Sinclair's staged approach):

• Day 1, hours 1–6: warm water, herbal tea; perhaps a small amount of warm bone broth
• Day 1, hours 6–12: one or two eggs; or 100g of white fish
• Day 2: full protein meals can resume, starting at 100g per meal and building up
• Day 3: return to normal eating with no special restrictions

For the Complete Guide

For the full protocol covering what to eat before, during, and after fasting windows, get Intermittent Fasting in Practice on Amazon → [Amazon link]. Buy the book and claim 3 months free on our fasting app at https://www.fastinginpractice.com/redeem

Frequently Asked Questions

How much protein should I eat when breaking an intermittent fast?

For a standard 16:8 fast, aim for 25–40g of complete protein in your first meal. This is enough to trigger muscle protein synthesis without overwhelming a digestive system that has been resting for 16 hours.

Is protein powder a good choice after fasting?

Generally not the best first choice. Whole food proteins like eggs, fish, and chicken are more bioavailable and gentler on the digestive system. Most protein powders contain processing additives that can irritate the gut coming out of a fast.

What did Upton Sinclair eat after his long fasts?

After his 12-day fasts, Sinclair started with orange juice or grape juice, then moved to warm milk for several weeks, and eventually found broiled lean beef alongside hot water to be the most effective post-fast protein source for sustained intellectual work — a finding that aligns with what modern science knows about the high bioavailability of animal protein.

Can I eat a large steak right after breaking a 72-hour fast?

Not recommended. After an extended fast, digestive enzyme production is lower and the gut needs time to restart. A very large protein meal after 72+ hours of fasting can cause nausea, cramping, and bloating. Start with a smaller portion — one or two eggs or 100g of white fish — and build up over the following day.

What is the single best protein source to break a fast?

Eggs are widely considered the most practical and digestible post-fast protein. They have a high biological value, contain all essential amino acids, are easy to prepare in any quantity, and are gentle enough to eat even early in the refeeding process. White fish is the next best option.

Related Articles

• How to break a fast the right way
• What should your first meal be after fasting?
• Orange juice, grape juice, or broth: the best foods to break your fast

This article draws on historical research from 1911 and is for informational purposes only — not medical advice. Always consult a qualified healthcare professional before making any dietary changes.

Cite as: Sinclair, U. (1911). The Fasting Cure. Mitchell Kennerley.