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Fattening Up a Runt Puppy: A Guide To Nurturing The Underdog

Photo of Tamsin De La Harpe

Written by Tamsin De La Harpe

fattening up a runt puppy

Ever noticed the smallest, most adorable pup in a litter that seems to be lagging behind its siblings in size? That little trooper is often called the ‘runt’ of the litter. But don’t let their size fool you; with the right care and attention, these little ones can thrive just as well as their bigger siblings. 

In this article, we’ll dive into the world of runt puppies, exploring what makes them the tiny wonders they are and how you can help them grow into healthy, happy dogs. It’s vital the dog breeders and pet parents know hot to take care of a runt, what to feed a puppy to help them gain weight. We will draw vital insights from experts like Dr. Dennis Lawler, who has worked wonders in pediatric care for puppies and Dr. Clare Hemmings in her work on puppy nutrition. Let’s embark on a journey to understand and nurture these little fighters.

Key Takeaways: Fattening Up a Runt Puppy

Caring for the smallest member of a puppy litter, the runt, requires special attention and care. Here are the key takeaways on how to help these tiny fur babies grow into healthy and happy dogs:

how to fatten up a runt puppy

1. Supplemental Feeding is Crucial

  • In the earliest stages, supplementing with a high-quality puppy milk replacer is vital, especially for feedings every two hours.
  • Ensure you’re using a formula specifically designed for puppies, as cow’s milk or alternativ-e-archives can be harmful.

2. Monitoring and Managing Health

  • Keep the runt warm with heat lamps, as they’re more susceptible to hypothermia.
  • Regular deworming and maintaining a clean environment are crucial to prevent infections and parasites.
  • Regular check-ups with a vet can help identify and address any underlying health issues early.

3. Careful Weaning Process

4. Feeding Practices and Routines

  • Feed the runt separately to ensure it gets enough food without competition from siblings.
  • Offer smaller, more frequent meals to help the runt gain weight.

5. Understanding the Runt’s Unique Needs

  • Understand that runts face more challenges and may need more attention and care.
  • Provide a nurturing environment, not just for their physical growth but also for their emotional well-being.

Over the years, I have had many runts in my pack. I have raised them, fostered them, and adopted them. Of course, the first thing that many people want to know is “will the runt grow to full size?” Or will they ever be as big as their bigger siblings. 

The short answer is it’s impossible to know for sure if the runt will grow as big as the bigger pups in the litter. I have certainly seen a handful runts grow as big as their larger siblings once they reached adulthood. I have also seen the largest dog in the litter not turn out to be the largest adult. However, I find that most runts tend to stay on the smaller to average side as they grow up.

Warning: While you can help a runt a lot in the earliest stage of their life by helping them gain weight, it’s vital not to overdo it.  Obesity in puppies can cause a host of lifelong health issues, including joint issues and arthritis. So when fattening up your runt, make sure you don’t get them so fat that they suffer in later life.

Before we delve deeper into the exact steps to take to fatten up the smallest puppy, let’s first have a look at what a runt actually is and what challenges they face.

So, What is a Runt Puppy and What Causes It?

A small skinny runt puppy

In every litter of puppies, there’s often one or two puppies that’s noticeably smaller than the rest – the runts. But what exactly makes a puppy the runt, and why does it happen?

A Matter of Timing and Position

Interestingly, the journey of a runt puppy begins long before they’re born. Female dogs can conceive puppies at different times during their heat cycle, which can give some embryos a head start in development. This means that when they are born, these more developed puppies are already bigger and stronger. It’s a bit of a race from the start!

Moreover, the position in the uterus plays a big role. Some embryos snag the prime spots, getting better access to nutrition. This head start means they’re more robust at birth and can more easily nudge their way to the best nursing spots, leaving the smaller ones struggling to get enough milk.

Beyond Size: Health Factors

But size isn’t the only thing that defines a runt. Several health factors can contribute to a puppy being smaller or weaker. These include:

  • Congenital Birth Defects: Some runts may have underlying genetic conditions that affect their growth and development. This includes issues like enlarged tongues that make it harder for them to nurse.
  • Infections: Respiratory, viral, bacterial, or parasitic infections can hinder a puppy’s growth, making them the runt of the litter.
  • Fading Puppy Syndrome: This is a situation where a puppy, often a runt, fails to thrive after birth. It’s a complex condition, sometimes linked to their struggle to compete with siblings for nutrition and warmth.

Just Naturally Smaller

In some cases, a puppy might simply be born smaller without any underlying health issues. They’re the runts not because of health complications but due to natural variation in size and growth rates within a litter.

The Importance of Helping Runts Gain Weight

A litter of puppies with one smaller runt

When it comes to raising puppies, every little one counts, especially the runts. These tiny members of the litter often face bigger challenges than their siblings, making it crucial for us, as dog owners or caretakers, to give them an extra helping hand. Let’s talk about why helping runt puppies gain weight is so important for their journey to becoming healthy, happy adult dogs.

The Tough Life of a Runt

Being the smallest in the litter isn’t easy. Runts often struggle right from the start. They might be nudged away or even refused food by their mother. And it’s not just mom; their bigger littermates can unwittingly push them aside, hogging all the nourishment and warmth. This battle for food in those critical early days can leave runts weaker and struggling to keep up.

The Risks of Being Underweight

One of the biggest concerns for runt puppies is low body weight, and here’s why it’s a big deal:

  • Risk of Hypothermia

Puppies huddle together for warmth. If a runt is too weak or pushed aside, they miss out on this vital source of heat. This can lead to hypothermia, a dangerous drop in body temperature, which is particularly risky in the early neonatal phase.

  • Vulnerability to Illness

A skinny puppy is a vulnerable one. Their little bodies need all the nutrients they can get to fight off diseases and grow strong. Without proper weight gain, runts are more susceptible to infections and health complications.

  • Social and Behavioral Challenges

The world of puppies is not just about physical growth but also about learning social cues and behaviors. Runts, often bullied or left out by their siblings, can grow up to be more fearful, insecure, or even aggressive as they learn to fend for themselves from a young age.

Nurturing the Littlest Ones: How to Help Runt Puppies Thrive

A litter of Golden puppies with one smaller black runt puppy

When you’re faced with the challenge of caring for a runt puppy, it’s like being on a special mission. These tiny fur babies need extra care and attention to help them catch up with their siblings. Let’s dive into the best ways to support these little fighters and give them the best start in life.

Step 1: Supplement with Puppy Milk Replacer

The journey of helping a runt puppy often starts with ensuring they get enough nutrition. It’s Important to weigh puppies when they are born and again the next day. Dr. Munich from the Referral Centre for Reproduction in Small Animals says that if a puppy loses 10%  or more of their birth weight in the first 24 hours, it’s vital to supplement their food. Here’s where puppy milk replacers come in:

Round-the-Clock Feeding

Be prepared for some sleepless nights. Runts need to be fed every two hours with a warm, suitable puppy milk replacer. Now this key: ideally you want to make sure your runt gets access to the teat and the bigger pups aren’t pushing them aside. But you will also need to supplement their food with bottle feeding.

Dr. Samantha Scully warns that you should never feed a newborn puppy with an eyedropper, saying that “eye droppers can result in aspiration pneumonia.”  So to keep newborns from breathing the liquid into their lungs, you need to persuade them to drink from a special puppy bottle and teat. If the runt is extremely weak, you need to tube feed them. 

The Right Formula

It’s super important to choose a puppy-specific formula. Cow’s milk or other alternativ-e-archives just won’t cut it. They lack the essential nutrients puppies need and can even cause digestive issues like diarrhea that can be deadly in such tiny bodies.

Mother’s Milk is Key

In the first few days, it’s crucial that the runt also nurses from their mom to get colostrum. You can get colostrum replacers on the market, but it’s better that they get this from their mother. This first milk is loaded with antibodies and helps bolster their fragile immune systems. So, watch those nursing sessions and make sure the little one gets its fair share.

Step 2: Keeping Them Warm

  • Heat lamps can be life-savers for runt puppies. These little ones can’t regulate their body temperature well and are prone to getting cold (and hypothermic), which can make them too weak to nurse.
  • Keep a close eye on them, ensuring they are warm enough but not overheated. Puppies who stop huddling with the litter for warmth are in danger.

Step 3: Health Management

  • If you notice a runt or smaller puppies in the litter, it’s a good idea to deworm the mother within two days of birth. This helps reduce the risk of the puppies contracting parasites, which can impede their growth.
  • Ensuring the mother is vaccinated is also crucial. A clean environment for the litter helps prevent infections that can be particularly harmful to runts.
  • Deworm the puppies at 2 weeks, 4 weeks, and 6 weeks. Internal parasites are a common reason why runts struggle to thrive, so keeping on top of this is vital.

Fattening Up for a Healthy Future

Fattening up a runt isn’t just about getting them to a healthy weight; it’s about giving them a fighting chance to thrive. A well-nourished runt can catch up in growth, ward off diseases more effectively, and develop a more confident and secure temperament.

So, as caretakers, it’s our job to monitor their food intake, ensure they’re getting enough warmth, and provide that extra TLC they need. It’s about nurturing not just their bodies but their little spirits too, helping them grow into the wonderful, healthy dogs they’re meant to be. Remember, every puppy deserves a chance to thrive, no matter how small their start in life may be!

Weaning and Beyond: Nourishing the Runt Puppy for Growth

As our little runt puppies start their journey from milk to munching on puppy food, it’s a critical phase that needs our careful attention. Let’s explore how we can best support them during this weaning process and ensure they get the nourishment they need to flourish.

The Weaning Process

Weaning is when puppies transition from mother’s milk to solid food, and it’s a big step for any puppy, especially the runts. For a detailed look at when puppies are typically weaned, check out this insightful article on PawSafe: When are Puppies Weaned.

  • Starting with Gruel: Initially, we introduce them to a thin, watery gruel. This puppy gruel gradually becomes thicker and more solid over about three weeks. It’s a gentle way to shift their tiny tummies to solid food.

Choosing the Right Food

When it comes to what our puppies eat, it’s all about high-quality, high-calorie puppy food to support their rapid growth. For some great insights on what puppies can eat, take a look at this article from PawSafe: What Can Puppies Eat. Additionally, if you’re looking to specifically fatten up a puppy, this article offers valuable guidance: How to Fatten Up a Dog.

  • High-Calorie Puppy Food: The runt of the litter needs all the energy they can get, so a high-calorie puppy food is essential. This gives them the fuel they need to grow and catch up in size and strength.
  • Feeding Separately: It’s super important to feed the runt separately. Their bigger siblings might unintentionally push them away from the food bowl, so giving the runt its own space ensures it gets enough to eat.
  • Extra Meals for Extra Energy: Consider giving the runt a few smaller meals in between the regular feeding times. This extra boost can help them gain the much-needed weight and strength.

A Little Extra Love

Remember, feeding a runt puppy is as much about the physical act of providing food as it is about offering them the care and attention they need. It’s about creating a nurturing environment where they feel safe, loved, and have the opportunity to eat at their own pace. With patience and dedication, you’ll see your little underdog start to thrive, growing stronger and healthier day by day. 


Helping a runt puppy grow and thrive requires a mix of specialized care, close monitoring, and a whole lot of love. By understanding their unique needs and providing the right nutrition and environment, we can give these little ones a fighting chance to develop into strong, healthy dogs. Remember, every puppy, no matter how small, has the potential to live a full and joyful life.


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Tamsin De La Harpe


Tamsin de la Harpe has nearly two decades of experience with dogs in rescue, training, and behavior modification with fearful and aggressive dogs. She has worked closely with veterinarians and various kennels, building up extensive medical knowledge and an understanding of canine health and physiology. She also spent two years in the animal sciences as a canine nutrition researcher, focusing on longevity and holistic healthcare for our four-legged companions. Tamsin currently keeps a busy homestead with an assortment of rescue dogs and three Bullmastiffs.

Tamsin de la Harpe has nearly two decades of experience with dogs in rescue, training, and behavior modification with fearful and aggressive dogs. She has worked closely with veterinarians and various kennels, building up extensive medical knowledge and an understanding of canine health and physiology. She also spent two years in the animal sciences as a canine nutrition researcher, focusing on longevity and holistic healthcare for our four-legged companions. Tamsin currently keeps a busy homestead with an assortment of rescue dogs and three Bullmastiffs.